LTC3855

FEATURES n n n n n n n n n n n n n n n Dual, 180° Phased Controllers Reduce Required Input Capacitance and Power Supply Induced Noise Accurate Multiphase Current Matching RSENSE or DCR Current Sensing ±0.75% 0.6V Output Voltage Accuracy Phase-Lockable Fixed Frequency 250kHz to 780kHz High Efficiency: Up to 95% Dual N-channel MOSFET Synchronous Drive Wide VIN Range: 4V to 38V (40V Max) Operation Wide VOUT Range: 0.6V to 12.5V Operation Adjustable Soft-Start Current Ramping or Tracking Foldback Output Current Limiting Output Overvoltage Protection Power Good Output Voltage Monitor 5V Low Dropout Regulator Small 28-Lead QFN and Narrow SSOP Packages LTC3869/LTC3869-2 Dual, 2-Phase Synchronous Step-Down DC/DC Controllers DESCRIPTION The LTC®3869 is a high performance dual synchronous step-down switching regulator controller that drives all N-channel synchronous power MOSFET stages. A constant frequency current mode architecture allows a phaselockable frequency of up to 780kHz. Power loss and noise due to the ESR of the input capacitors are minimized by operating the two controller output stages out-of-phase. OPTI-LOOP® compensation allows the transient response to be optimized over a wide range of output capacitance and ESR values. The LTC3869 features a precision 0.6V reference and a power good output indicator. A wide 4V to 38V input supply range encompasses most battery chemistries. Independent TK/SS pins for each controller ramp the output voltage during start-up. Current foldback limits MOSFET heat dissipation during short-circuit conditions. The MODE/PLLIN pin selects among Burst Mode® operation, pulse-skipping mode, or continuous inductor current mode and allows the IC to be synchronized to an external clock. The LTC3869 is pin compatible with LTC3850 and is available in both low profile 28-lead QFN and narrow SSOP packages. L, LT, LTC, LTM, Linear Technology, the Linear logo, OPTI-LOOP Burst Mode and PolyPhase , are registered trademarks and No RSENSE is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 5481178, 5705919, 5929620, 6100678, 6144194, 6177787, 6580258, 6498466, 6611131. APPLICATIONS n n n n n Server Systems Telecom Systems Industrial and Medical Instruments High Power Battery-Operated Devices DC Power Distribution Systems TYPICAL APPLICATION High Efficiency Dual 5V/3.3V Step-Down Converter 4.7µF Efficiency and Power Loss 22µF VIN 7V TO 24V 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 VOUT2 3.3V 5A 0 0.01 POWER LOSS 0.1 1 LOAD CURRENT (A) 10 3869 TA01b + VIN 0.1µF TG1 BOOST1 SW1 BG1 fIN 500kHz MODE/PLLIN ILIM SENSE1+ RUN1 SENSE1– VFB1 ITH1 PGOOD INTVCC TG2 BOOST2 SW2 BG2 PGND FREQ SENSE2+ RUN2 SENSE2– VFB2 ITH2 1µF 0.1µF VIN = 12V, VOUT = 3.3V VIN = 12V, VOUT = 5V 1300 1200 1100 POWER LOSS (mW) EFFICIENCY 1000 900 800 700 600 500 400 3.2µH 2.2µH LTC3869 VOUT1 5V 5A 147k 90.9k 470pF 122k 15k 3869 TA01 + 470pF 47µF 20k 15k 0.1µF TK/SS1 SGND TK/SS2 0.1µF 20k + 56µF 3869f 1 LTC3869/LTC3869-2 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage: VIN........................... 40V to –0.3V Top Side Driver Voltages: BOOST1, BOOST2 ...................................... 46V to –0.3V Switch Voltage: SW1, SW2 ........................... 40V to –5V INTVCC , RUN1, RUN2, PGOOD, EXTVCC, BOOST1-SW1, BOOST2-SW2 ...................... 6V to –0.3V SENSE1+, SENSE2+, SENSE1–, SENSE2– Voltages...................................... 13V to –0.3V MODE/PLLIN, ILIM, TK/SS1, TK/SS2, FREQ Voltages ...................................... INTVCC to –0.3V ITH1 , ITH2 , VFB1 , VFB2 Voltages .............. INTVCC to –0.3V INTVCC Peak Output Current ................................100mA Operating Junction Temperature Range (Note 2).................................................. –40°C to 125°C Junction Temperature (Note 3) ............................. 125°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) GN Package ...................................................... 300°C PIN CONFIGURATION TOP VIEW MODE/PLLIN SENSE1+ TOP VIEW RUN1 SW1 TG1 1 2 3 4 5 6 7 8 9 28 FREQ 27 MODE/PLLIN 26 SW1 25 TG1 24 BOOST1 23 BG1 22 VIN 21 INTVCC 20 BG2 19 PGND 18 BOOST2 17 TG2 16 SW2 15 PGOOD RUN1 FREQ SENSE1+ SENSE1– VFB1 TK/SS1 ITH1 SGND ITH2 TK/SS2 28 27 26 25 24 23 SENSE1– 1 TK/SS1 2 ITH1 3 VFB1 4 VFB2 5 ITH2 6 TK/SS2 7 SENSE2– 8 9 10 11 12 13 14 RUN2 SENSE2+ EXTVCC PGOOD SW2 ILIM SGND 29 22 BOOST1 21 BG1 20 VIN 19 INTVCC 18 BG2 17 PGND 16 BOOST2 15 TG2 VFB2 10 SENSE2– 11 SENSE2 + 12 RUN2 13 EXTVCC 14 UFD PACKAGE 28-LEAD (4mm × 5mm) PLASTIC QFN TJMAX = 125°C, θJA = 34°C/W, EXPOSED PAD (PIN 29) IS SGND, MUST BE SOLDERED TO PCB GN PACKAGE 28-LEAD PLASTIC SSOP TJMAX = 125°C, θJA = 80°C/W ORDER INFORMATION LEAD FREE FINISH LTC3869EUFD#PBF LTC3869IUFD#PBF LTC3869IGN-2#PBF TAPE AND REEL LTC3869EUFD#TRPBF LTC3869IUFD#TRPBF LTC3869IGN-2#TRPBF PART MARKING* 3869 3869 LTC3869GN-2 PACKAGE DESCRIPTION 28-Lead (4mm × 5mm) Plastic QFN 28-Lead (4mm × 5mm) Plastic QFN 28-Lead Narrow Plastic SSOP TEMPERATURE RANGE –40°C to 125°C –40°C to 125°C –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 3869f 2 LTC3869/LTC3869-2 ELECTRICAL CHARACTERISTICS SYMBOL VIN VOUT VFB1,2 IFB1,2 VREFLNREG VLOADREG gm1,2 IQ DFMAX UVLO UVLOHYS VOVL ISENSE ITK/SS1,2 VRUN1,2 PARAMETER Input Voltage Range Output Voltage Range Regulated Feedback Voltage (Notes 2, 4) Feedback Current Reference Voltage Line Regulation Output Voltage Load Regulation ITH1,2 Voltage = 1.2V, 0°C to 85°C ITH1,2 Voltage = 1.2V, –40°C to 125°C (Note 4) VIN = 4.0V to 38V (Note 4) (Note 4) Measured in Servo Loop; ∆ITH Voltage = 1.2V to 0.7V l Measured in Servo Loop; ∆ITH Voltage = 1.2V to 1.6V l ITH1,2 = 1.2V; Sink/Source 5µA; (Note 4) (Note 5) VIN = 15V VRUN1,2 = 0V In Dropout VINTVCC Ramping Down Measured at VFB1,2 (Each Channel); VSENSE1,2 = 3.3V VTK/SS1,2 = 0V VRUN1, VRUN2 Rising VFB1,2 = 0.5V, VSENSE1,2 = 3.3V, ILIM = 0V VFB1,2 = 0.5V, VSENSE1,2 = 3.3V, ILIM = Float VFB1,2 = 0.5V, VSENSE1,2 = 3.3V, ILIM = INTVCC VFB1,2 = 0.5V, VSENSE1,2 = 3.3V, ILIM = 0V VFB1,2 = 0.5V, VSENSE1,2 = 3.3V, ILIM = Float VFB1,2 = 0.5V, VSENSE1,2 = 3.3V, ILIM = INTVCC VFB1,2 = 0.5V, VSENSE1,2 = 3.3V, LTC3869IGN-2 ILIM = Float (Note 8) CLOAD = 3300pF CLOAD = 3300pF (Note 8) CLOAD = 3300pF CLOAD = 3300pF 25 25 25 25 30 30 90 l l l The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN1,2 = 5V unless otherwise noted. CONDITIONS MIN 4 0.6 0.5955 0.5940 0.600 0.600 –15 0.002 0.01 –0.01 2 3 30 94 3.0 0.64 1.0 1.1 25 45 68 23 43 68 40 95 3.2 0.6 l l l l TYP MAX 38 12.5 0.6045 0.6060 –50 0.01 0.1 –0.1 UNITS V V V V nA %/V % % mmho mA µA % V V V µA µA V mV mV mV mV mV mV mV mV mV Main Control Loops Transconductance Amplifier gm Input DC Supply Current Normal Mode Shutdown Maximum Duty Factor Undervoltage Lockout UVLO Hysteresis Feedback Overvoltage Lockout Sense Pins Bias Current Soft-Start Charge Current RUN Pin On Threshold 50 3.4 0.68 ±2 1.5 1.35 35 55 82 37 57 82 60 2 0.66 ±1 1.25 1.22 80 30 50 75 30 50 75 50 VRUN1,2(HYS) RUN Pin On Hysteresis VSENSE(MAX) Maximum Current Sense Threshold, 0°C to 85°C (Note 2) Maximum Current Sense Threshold, –40°C to 125°C (Note 2) l l l l l l l VMISMATCH TG1, 2 tr TG1, 2 tf BG1, 2 tr BG1, 2 tf TG/BG t1D BG/TG t2D tON(MIN) Channel to Channel Current Sense Mismatch Voltage of VSENSE(MAX) TG Transition Time: Rise Time Fall Time BG Transition Time: Rise Time Fall Time ns ns ns ns ns ns ns Top Gate Off to Bottom Gate On Delay CLOAD = 3300pF Each Driver (Note 6) Synchronous Switch-On Delay Time Bottom Gate Off to Top Gate On Delay CLOAD = 3300pF Each Driver (Note 6) Top Switch-On Delay Time Minimum On-Time (Note 7) 3869f 3 LTC3869/LTC3869-2 ELECTRICAL CHARACTERISTICS SYMBOL VINTVCC VLDO INT VEXTVCC VLDOHYS VLDO EXT VPGL IPGOOD VPG PARAMETER Internal VCC Voltage INTVCC Load Regulation EXTVCC Switchover Voltage EXTVCC Hysteresis EXTVCC Voltage Drop PGOOD Voltage Low PGOOD Leakage Current PGOOD Trip Level ICC = 20mA, VEXTVCC = 5V IPGOOD = 2mA VPGOOD = 5V VFB with Respect to Set Output Voltage VFB Ramping Negative VFB Ramping Positive VFREQ = 1.2V VFREQ = 0V VFREQ ≥ 2.4V 450 210 700 9 TG High TG Low BG High BG Low –10 10 500 250 780 250 10 2.6 1.5 2.4 1.1 Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formulas: LTC3869UFD: TJ = TA + (PD • 34°C/W) LTC3869GN-2: TJ = TA + (PD • 80°C/W) Note 4: The LTC3869 is tested in a feedback loop that servos VITH1,2 to a specified voltage and measures the resultant VFB1,2. Note 5: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information. Note 6: Delay times are measured using 50% levels. Note 7: The minimum on-time condition is specified for an inductor peak-to-peak ripple current ≥40% of IMAX (see Minimum On-Time Considerations in the Applications Information section). Note 8: Guaranteed by design. 11 550 290 850 INTVCC Linear Regulator 6V < VIN < 38V ICC = 0mA to 20mA EXTVCC Ramping Positive l The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN1,2 = 5V unless otherwise noted. CONDITIONS MIN 4.8 4.5 TYP 5 0.5 4.7 200 50 0.1 100 0.3 ±2 MAX 5.2 2 UNITS V % V mV mV V µA % % kHz kHz kHz kΩ µA Ω Ω Ω Ω PGOOD Output Oscillator and Phase-Locked Loop fNOM fLOW fHIGH IFREQ TG RUP TG RDOWN BG RUP BG RDOWN Nominal Frequency Lowest Frequency Highest Frequency Frequency Setting Current TG Pull-Up RDS(ON) TG Pull-Down RDS(ON) BG Pull-Up RDS(ON) BG Pull-Down RDS(ON) RMODE/PLLIN MODE/PLLIN Input Resistance On Chip Driver Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3869 is tested under pulsed load conditions such that TJ ≈ TA. The LTC3869E is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3869I is guaranteed to meet performance specifications over the full –40°C to 125°C operating junction temperature range. The maximum ambient temperature consistent with these specifications is determined by specific operating conditions in conjunction with board layout, the package thermal impedence and other environmental factors. 3869f 4 LTC3869/LTC3869-2 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency vs Output Current and Mode 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 CIRCUIT OF FIGURE 16 1 10 LOAD CURRENT (A) 100 3869 G01 TA = 25°C, unless otherwise noted. Full Load Efficiency and Power Loss vs Input Voltage 90 1.8V 5 EFFICIENCY 85 1.2V 4 POWER LOSS (W) Efficiency vs Output Current and Mode 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 CIRCUIT OF FIGURE 16 1 10 LOAD CURRENT (A) 100 3869 G02 Burst Mode OPERATION DCM VIN = 12V VOUT = 1.8V CCM Burst Mode OPERATION EFFICIENCY (%) DCM CCM VIN = 12V VOUT = 1.2V 1.8V 80 POWER LOSS 1.2V 3 75 CIRCUIT OF FIGURE 16 5 10 15 INPUT VOLTAGE (V) 2 20 3869 G03 Load Step (Burst Mode Operation) ILOAD 5A/DIV 300mA TO 5A IL 5A/DIV VOUT 100mV/DIV AC-COUPLED 50µs/DIV 3869 G04 Load Step (Forced Continuous Mode) ILOAD 5A/DIV 300mA TO 5A IL 5A/DIV VOUT 100mV/DIV AC-COUPLED 50µs/DIV 3869 G05 Load Step (Pulse-Skipping Mode) ILOAD 5A/DIV 300mA TO 5A IL 5A/DIV VOUT 100mV/DIV AC-COUPLED 50µs/DIV 3869 G06 VIN = 12V VOUT = 1.8V VIN = 12V VOUT = 1.8V VIN = 12V VOUT = 1.8V Inductor Current at Light Load FORCED CONTINUOUS MODE 5A/DIV Burst Mode OPERATION 5A/DIV PULSESKIPPING MODE 5A/DIV VIN = 12V VOUT = 1.8V ILOAD = 400mA 1µs/DIV 3869 G07 Prebiased Output at 2V VOUT 2V/DIV Coincident Tracking RUN 2V/DIV VOUT1 VFB 500mV/DIV TK/SS 500mV/DIV VIN = 12V VOUT = 3.3V 2ms/DIV 3869 G08 VOUT1 VOUT2 1V/DIV 5ms/DIV VOUT1 = 1.8V, 1.5 LOAD VOUT2 = 1.2V, 1 LOAD VOUT2 3869 G09 3869f 5 LTC3869/LTC3869-2 TYPICAL PERFORMANCE CHARACTERISTICS Tracking Up and Down with External Ramp TK/SS1 TK/SS2 2V/DIV 4.0 3.5 QUIESCENT CURRENT (mA) INTVCC VOLTAGE (V) –25 0 50 25 75 TEMPERATURE (°C) 100 125 VOUT1 3.0 2.5 2.0 1.5 1.0 0.5 0 –50 TA = 25°C, unless otherwise noted. Quiescent Current without EXTVCC vs Temperature 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 INTVCC Line Regulation VOUT2 VOUT1 VOUT2 500mA/DIV 10ms/DIV VIN = 12V VOUT1 = 1.8V, 1.5 LOAD VOUT2 = 1.2V, 1 LOAD 3869 G10 0 10 3869 G11 30 20 INPUT VOLTAGE (V) 40 3869 G12 Current Sense Threshold vs ITH Voltage 80 ILIM = INTVCC CURRENT SENSE THRESHOLD (mV) 60 40 20 ILIM = GND 0 –20 –40 ILIM = FLOAT 80 70 60 50 40 30 20 10 0 Maximum Current Sense Threshold vs Common Mode Voltage 80 CURRENT SENSE THRESHOLD (mV) ILIM = INTVCC 70 60 50 40 30 20 10 0 Maximum Current Sense Threshold vs Duty Cycle ILIM = INTVCC VSENSE (mV) ILIM = FLOAT ILIM = FLOAT ILIM = GND ILIM = GND 0 0.5 1 VITH (V) 1.5 2 3869 G13 0 2 4 6 8 10 12 3869 G14 0 20 VSENSE COMMON MODE VOLTAGE (V) 40 60 DUTY CYCLE (%) 80 100 3869 G15 Maximum Current Sense Voltage vs Feedback Voltage (Current Foldback) MAXIMUM CURRENT SENSE THRESHOLD (mV) 90 80 70 TK/SS CURRENT (µA) 60 50 40 30 20 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 3869 G16 TK/SS Pull-Up Current vs Temperature 1.6 ILIM = INTVCC ILIM = FLOAT 1.4 ILIM = GND 1.2 1.0 –50 –25 FEEDBACK VOLTAGE (V) 0 50 25 75 TEMPERATURE (°C) 100 125 3869 G17 3869f 6 LTC3869/LTC3869-2 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown (RUN) Threshold vs Temperature 1.24 REGULATED FEEDBACK VOLTAGE (mV) 1.22 RUN PIN THRESHOLD (V) 1.20 1.18 1.16 1.14 1.12 1.10 1.08 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 OFF ON 604 602 FREQUENCY (kHz) 600 598 596 594 592 590 –40 –15 60 35 85 10 TEMPERATURE (°C) 110 125 3869 G19 TA = 25°C, unless otherwise noted. Oscillator Frequency vs Temperature 900 800 700 600 500 400 300 200 100 0 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 VFREQ = GND VFREQ = 1.2V VFREQ = INTVCC Regulated Feedback Voltage vs Temperature 3869 G18 3869 G20 Undervoltage Lockout Threshold (INTVCC) vs Temperature 4.1 3.9 UVLO THRESHOLD (V) 3.7 3.5 3.3 3.1 2.9 2.7 2.5 –40 –20 40 20 60 0 TEMPERATURE (°C) 80 100 480 FALLING FREQUENCY (kHz) RISING 510 520 Oscillator Frequency vs Input Voltage 60 SHUTDOWN INPUT CURRENT (µA) 5 10 25 15 20 30 INPUT VOLTAGE (V) 35 40 50 40 30 20 10 0 Shutdown Current vs Input Voltage 500 490 5 10 15 20 30 25 INPUT VOLTAGE (V) 35 40 3869 G21 3869 G22 3869 G23 Shutdown Current vs Temperature 45 40 SHUTDOWN CURRENT (µA) Quiescent Current vs Input Voltage without EXTVCC 3.8 3.6 3.4 SUPPLY CURRENT (mA) 3.2 3.0 2.8 2.6 2.4 2.2 2.0 35 30 25 20 15 10 5 0 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 1.8 5 10 3869 G24 15 20 30 25 INPUT VOLTAGE (V) 35 40 3869 G25 3869f 7 LTC3869/LTC3869-2 PIN FUNCTIONS (UFD/GN) RUN1, RUN2 (Pin 27, Pin 10/Pin 1, Pin 13): Run Control Inputs. A voltage above 1.2V on either pin turns on the IC. However, forcing either of these pins below 1.2V causes the IC to shut down the circuitry required for that particular channel. There are 1µA pull-up currents for these pins. Once the RUN pin raises above 1.2V, an additional 4.5µA pull-up current is added to the pin. VFB1, VFB2 (Pin 4, Pin 5/Pin 4, Pin 10): Error Amplifier Feedback Inputs. These pins receive the remotely sensed feedback voltages for each channel from external resistive dividers across the outputs. ITH1, ITH2 (Pin 3, Pin 6/Pin 6, Pin 8): Current Control Thresholds and Error Amplifier Compensation Points. Each associated channels’ current comparator tripping threshold increases with its ITH control voltage. SGND (Pin 29/Pin 7): Signal Ground. All small-signal components and compensation components should connect to this ground, which in turn connects to PGND at one point. Pin 29 is the exposed pad, only available for the UFD package. The exposed pad must be soldered to PCB ground for electrical connection and rated thermal performance. TK/SS1, TK/SS2 (Pin 2, Pin 7/Pin 5, Pin 9): Output Voltage Tracking and Soft-Start Inputs. When one particular channel is configured to be the master of two channels, a capacitor to ground at this pin sets the ramp rate for the master channel’s output voltage. When the channel is configured to be the slave of two channels, the VFB voltage of the master channel is reproduced by a resistor divider and applied to this pin. Internal soft-start currents of 1.2µA are charging these pins. MODE/PLLIN (Pin 25/Pin 27): Forced Continuous Mode, Burst Mode Operation, or Pulse-Skipping Mode Selection Pin and External Synchronization Input to Phase Detector Pin. Connect this pin to SGND to force both channels in continuous mode of operation. Connect to INTVCC to enable pulse-skipping mode of operation. Leave the pin floating will enable Burst Mode operation. A clock on the pin will force the controller into continuous mode of operation and synchronize the internal oscillator with the clock on this pin. The PLL compensation components are integrated inside the IC. FREQ (Pin 26/Pin 28): There is a precision 10µA current flowing out of this pin. Connect a resistor to ground set the controllers’ operating frequency. Alternatively, this pin can be driven with a DC voltage to vary the frequency of the internal oscillator. ILIM (Pin 11/NA): Current Comparator Sense Voltage Range Inputs. This pin is to be programmed to SGND, FLOAT or INTVCC to set the maximum current sense threshold to three different levels for each comparator. The current limit default value is set to be 50mV for LTC3869GN-2. EXTVCC (Pin 12/Pin 14): External Power Input to an Internal Switch Connected to INTVCC. This switch closes and supplies the IC power, bypassing the internal low dropout regulator, whenever EXTVCC is higher than 4.7V. Do not exceed 6V on this pin. VIN (Pin 20/Pin 22): Main Input Supply. Decouple this pin to PGND with a capacitor (0.1µF to 1µF). BOOST1, BOOST2 (Pin 22, Pin 16/Pin 24, Pin 18): Boosted Floating Driver Supplies. The (+) terminal of the booststrap capacitors connect to these pins. These pins swing from a diode voltage drop below INTVCC up to VIN + INTVCC. TG1, TG2 (Pin 23, Pin 15/Pin 25, Pin 17): Top Gate Driver Outputs. These are the outputs of floating drivers with a voltage swing equal to INTVCC superimposed on the switch nodes voltages. SW1, SW2 (Pin 24, Pin 14/Pin 26, Pin 16): Switch Node Connections to Inductors. Voltage swing at these pins is from a Schottky diode (external) voltage drop below ground to VIN. SENSE1+, SENSE2+ (Pin 28, Pin 9/Pin 2, Pin 12): Current Sense Comparator Inputs. The (+) inputs to the current comparators are normally connected to DCR sensing networks or current sensing resistors. SENSE1–, SENSE2– (Pin 1, Pin 8/Pin 3, Pin 11): Current Sense Comparator Inputs. The (–) inputs to the current comparators are connected to the outputs. PGND (Pin 17/Pin 19): Power Ground Pin. Co nnect this pin closely to the sources of the bottom N-channel MOSFETs, the (–) terminal of CVCC and the (–) terminal of CIN. BG1, BG2 (Pin 21, Pin 18/Pin 23, Pin 20): Bottom Gate Driver Outputs. These pins drive the gates of the bottom N-channel MOSFETs between PGND and INTVCC. 3869f 8 LTC3869/LTC3869-2 PIN FUNCTIONS (UFD/GN) INTVCC (Pin 19/Pin 21): Internal 5V Regulator Output. The control circuits are powered from this voltage. Decouple this pin to PGND with a minimum of 4.7µF low ESR tantalum or ceramic capacitor. PGOOD (Pin 13/Pin 15): Power Good Indicator Output. Open drain logic out that is pulled to ground when either channel output exceeds ±10% regulation windows, after the internal 20µs power bad mask timer expires. FUNCTIONAL BLOCK DIAGRAM FREQ MODE/PLLIN EXTVCC 4.7V 10µA MODE/SYNC DETECT VIN VIN + 5V REG – 0.6V F + CIN – PLL-SYNC F + INTVCC INTVCC BOOST OSC S R Q FCNT ON BURSTEN TG SW SENSE+ SENSE– CB M1 L1 DB + ICMP 3k – + IREV – SWITCH LOGIC AND ANTISHOOT THROUGH VOUT RUN BG M2 CVCC PGND PGOOD UVLO + COUT OV ILIM LTC3869UFD ONLY SLOPE COMPENSATION INTVCC + 1 51k ITHB SLOPE RECOVERY ACTIVE CLAMP 0.54V VFB UV R2 – + OV SS RUN VIN SLEEP R1 0.66V SGND – + 1.2µA EA 0.5V 1.2V 1µA 3869 FD 0.5V CC1 CSS ITH RC – – + 0.6V REF –++ – + RUN TK/SS 3869f 9 LTC3869/LTC3869-2 OPERATION Main Control Loop The LTC3869 is a constant-frequency, current mode stepdown controller with two channels operating 180 degrees out-of-phase. During normal operation, each top MOSFET is turned on when the clock for that channel sets the RS latch, and turned off when the main current comparator, ICMP, resets the RS latch. The peak inductor current at which ICMP resets the RS latch is controlled by the voltage on the ITH pin, which is the output of each error amplifier EA. The VFB pin receives the voltage feedback signal, which is compared to the internal reference voltage by the EA. When the load current increases, it causes a slight decrease in VFB relative to the 0.6V reference, which in turn causes the ITH voltage to increase until the average inductor current matches the new load current. After the top MOSFET has turned off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the reverse current comparator IREV, or the beginning of the next cycle. INTVCC/EXTVCC Power Power for the top and bottom MOSFET drivers and most other internal circuitry is derived from the INTVCC pin. When the EXTVCC pin is left open or tied to a voltage less than 4.7V, an internal 5V linear regulator supplies INTVCC power from VIN. If EXTVCC is taken above 4.7V, the 5V regulator is turned off and an internal switch is turned on connecting EXTVCC. Using the EXTVCC pin allows the INTVCC power to be derived from a high efficiency external source such as one of the LTC3869 switching regulator outputs. Each top MOSFET driver is biased from the floating bootstrap capacitor CB, which normally recharges during each off cycle through an external diode when the top MOSFET turns off. If the input voltage VIN decreases to a voltage close to VOUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout detector detects this and forces the top MOSFET off for about one-twelfth of the clock period plus 100ns every third cycle to allow CB to recharge. However, it is recommended that a load be present or the IC operates at low frequency during the drop-out transition to ensure CB is recharged. Shutdown and Start-Up (RUN1, RUN2 and TK/SS1, TK/SS2 Pins) The two channels of the LTC3869 can be independently shut down using the RUN1 and RUN2 pins. Pulling either of these pins below 1.2V shuts down the main control loop for that controller. Pulling both pins low disables both controllers and most internal circuits, including the INTVCC regulator. Releasing either RUN pin allows an internal 1µA current to pull up the pin and enable that controller. Alternatively, the RUN pin may be externally pulled up or driven directly by logic. Be careful not to exceed the Absolute Maximum Rating of 6V on this pin. The start-up of each controller’s output voltage VOUT is controlled by the voltage on the TK/SS1 and TK/SS2 pins. When the voltage on the TK/SS pin is less than the 0.6V internal reference, the LTC3869 regulates the VFB voltage to the TK/SS pin voltage instead of the 0.6V reference. This allows the TK/SS pin to be used to program the soft-start period by connecting an external capacitor from the TK/SS pin to SGND. An internal 1.2µA pull-up current charges this capacitor, creating a voltage ramp on the TK/SS pin. As the TK/SS voltage rises linearly from 0V to 0.6V (and beyond), the output voltage VOUT rises smoothly from zero to its final value. Alternatively the TK/SS pin can be used to cause the start-up of VOUT to “track” that of another supply. Typically, this requires connecting to the TK/SS pin an external resistor divider from the other supply to ground (see the Applications Information section). When the corresponding RUN pin is pulled low to disable a controller, or when INTVCC drops below its undervoltage lockout threshold of 3.2V, the TK/SS pin is pulled low by an internal MOSFET. When in undervoltage lockout, both controllers are disabled and the external MOSFETs are held off. 3869f 10 LTC3869/LTC3869-2 OPERATION Light Load Current Operation (Burst Mode Operation, Pulse-Skipping, or Continuous Conduction) The LTC3869 can be enabled to enter high efficiency Burst Mode operation, constant-frequency pulse-skipping mode, or forced continuous conduction mode. To select forced continuous operation, tie the MODE/PLLIN pin to a DC voltage below 0.6V (e.g., SGND). To select pulse-skipping mode of operation, tie the MODE/PLLIN pin to INTVCC. To select Burst Mode operation, float the MODE/PLLIN pin. When a controller is enabled for Burst Mode operation, the peak current in the inductor is set to approximately one-third of the maximum sense voltage even though the voltage on the ITH pin indicates a lower value. If the average inductor current is higher than the load current, the error amplifier EA will decrease the voltage on the ITH pin. When the ITH voltage drops below 0.5V, the internal sleep signal goes high (enabling sleep mode) and the top MOSFET is turned off immediately, but the bottom MOSFET is turned off when the inductor current reaches zero. In sleep mode, the load current is supplied by the output capacitor. As the output voltage decreases, the EA’s output begins to rise. When the output voltage drops enough, the sleep signal goes low, and the controller resumes normal operation by turning on the top external MOSFET on the next cycle of the internal oscillator. When a controller is enabled for Burst Mode operation, the inductor current is not allowed to reverse. The reverse current comparator (IREV) turns off the bottom external MOSFET just before the inductor current reaches zero, preventing it from reversing and going negative. Thus, the controller operates in discontinuous operation. In forced continuous operation, the inductor current is allowed to reverse at light loads or under large transient conditions. The peak inductor current is determined by the voltage on the ITH pin. In this mode, the efficiency at light loads is lower than in Burst Mode operation. However, continuous mode has the advantages of lower output ripple and less interference with audio circuitry. When the MODE/PLLIN pin is connected to INTVCC, the LTC3869 operates in PWM pulse-skipping mode at light loads. At very light loads, the current comparator ICMP may remain tripped for several cycles and force the external top MOSFET to stay off for the same number of cycles (i.e., skipping pulses). The inductor current is not allowed to reverse (discontinuous operation). This mode, like forced continuous operation, exhibits low output ripple as well as low audio noise and reduced RF interference as compared to Burst Mode operation. It provides higher low current efficiency than forced continuous mode, but not nearly as high as Burst Mode operation. Single Output Multiphase Operation The LTC3869 can be used for single output multiphase converters by making these connections • Tie all of the ITH pins together. • Tie all of the VFB pins together. • Tie all of the TK/SS pins together. • Tie all of the RUN pins together. LTC3869 has excellent current matching performance between channels to ensure that there are equal thermal stress for both channels. Frequency Selection and Phase-Locked Loop (FREQ and MODE/PLLIN Pins) The selection of switching frequency is a trade-off between efficiency and component size. Low frequency operation increases efficiency by reducing MOSFET switching losses, but requires larger inductance and/or capacitance to maintain low output ripple voltage. The switching frequency of the LTC3869 controller can be selected using the FREQ pin. If the MODE/PLLIN pin is not being driven by an external clock source, the FREQ pin can be used to program the controller’s operating frequency from 250kHz to 780kHz. There is a precision 10µA current flowing out of the FREQ pin, so the user can program the controller’s switching frequency with a single resistor to SGND. A curve is provided later in the application section showing the relationship between the voltage on the FREQ pin and switching frequency. 3869f 11 LTC3869/LTC3869-2 OPERATION A phase-locked loop (PLL) is integrated on the LTC3869 to synchronize the internal oscillator to an external clock source that is connected to the MODE/PLLIN pin. The controller is operating in forced continuous mode when it is synchronized. The PLL loop filter network is integrated inside the LTC3869. The phase-locked loop is capable of locking any frequency within the range of 250kHz to 770kHz. The frequency setting resistor should always be present to set the controller’s initial switching frequency before locking to the external clock. Power Good (PGOOD Pin) When VFB pin voltage is not within ±10% of the 0.6V reference voltage, the PGOOD pin is pulled low. The PGOOD pin is also pulled low when the RUN pin is below 1.2V or when the LTC3869 is in the soft-start or tracking phase. The PGOOD pin will flag power good immediately when both VFB pins are within the ±10% of the reference window. However, there is an internal 20µs power bad mask when VFB goes out the ±10% window. The PGOOD pin is allowed to be pulled up by an external resistor to a source of up to 6V. Output Overvoltage Protection An overvoltage comparator, OV, guards against transient overshoots (>10%) as well as other more serious conditions that may overvoltage the output. In such cases, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. 3869f 12 LTC3869/LTC3869-2 APPLICATIONS INFORMATION The Typical Application on the first page is a basic LTC3869 application circuit. LTC3869 can be configured to use either DCR (inductor resistance) sensing or low value resistor sensing. The choice between the two current sensing schemes is largely a design trade-off between cost, power consumption, and accuracy. DCR sensing is becoming popular because it saves expensive current sensing resistors and is more power efficient, especially in high current applications. However, current sensing resistors provide the most accurate current limits for the controller. Other external component selection is driven by the load requirement, and begins with the selection of RSENSE (if RSENSE is used) and inductor value. Next, the power MOSFETs are selected. Finally, input and output capacitors are selected. Current Limit Programming The ILIM pin is a tri-level logic input which sets the maximum current limit of the controller. When ILIM is either grounded, floated or tied to INTVCC, the typical value for the maximum current sense threshold will be 30mV, 50mV or 75mV, respectively. Which setting should be used? For the best current limit accuracy, use the 75mV setting. The 30mV setting will allow for the use of very low DCR inductors or sense resistors, but at the expense of current limit accuracy. The 50mV setting is a good balance between the two. For single output dual phase applications, use the 50mV or 75mV setting for optimal current sharing. SENSE+ and SENSE– Pins The SENSE+ and SENSE– pins are the inputs to the current comparators. The common mode input voltage range of the current comparators is 0V to 12.5V. Both SENSE pins are high impedance inputs with small base currents of less than 1µA. When the SENSE pins ramp up from 0V to 1.4V, the small base currents flow out of the SENSE pins. When the SENSE pins ramp down from 12.5V to 1.1V, the small base currents flow into the SENSE pins. The high impedance inputs to the current comparators allow accurate DCR sensing. However, care must be taken not to float these pins during normal operation. The LTC3869GN-2 defaults to 50mV current limit value. COUT RSENSE 3869 F01 Filter components mutual to the sense lines should be placed close to the LTC3869, and the sense lines should run close together to a Kelvin connection underneath the current sense element (shown in Figure 1). Sensing current elsewhere can effectively add parasitic inductance and capacitance to the current sense element, degrading the information at the sense terminals and making the programmed current limit unpredictable. If DCR sensing is used (Figure 2b), sense resistor R1 should be placed close to the switching node, to prevent noise from coupling into sensitive small-signal nodes. The capacitor C1 should be placed close to the IC pins. TO SENSE FILTER, NEXT TO THE CONTROLLER Figure 1. Sense Lines Placement with Sense Resistor Low Value Resistors Current Sensing A typical sensing circuit using a discrete resistor is shown in Figure 2a. RSENSE is chosen based on the required output current. The current comparator has a maximum threshold VSENSE(MAX) determined by the ILIM setting. The input common mode range of the current comparator is 0V to 12.5V. The current comparator threshold sets the peak of the inductor current, yielding a maximum average output current IMAX equal to the peak value less half the peak-topeak ripple current, ∆IL. To calculate the sense resistor value, use the equation: RSENSE = VSENSE(MAX) ΔI IMAX + L 2 Because of possible PCB noise in the current sensing loop, the AC current sensing ripple of ∆VSENSE = ∆IL • RSENSE also needs to be checked in the design to get a good signal-tonoise ratio. In general, for a reasonably good PCB layout, a 10mV ∆VSENSE voltage is recommended as a conservative number to start with, either for RSENSE or DCR sensing applications, for duty cycles less than 40%. 3869f 13 LTC3869/LTC3869-2 APPLICATIONS INFORMATION For previous generation current mode controllers, the maximum sense voltage was high enough (e.g., 75mV for the LTC1628 / LTC3728 family) that the voltage drop across the parasitic inductance of the sense resistor represented a relatively small error. For today’s highest current density solutions, however, the value of the sense resistor can be less than 1mΩ and the peak sense voltage can be as low as 20mV. In addition, inductor ripple currents greater than 50% with operation up to 1MHz are becoming more common. Under these conditions the voltage drop across the sense resistor’s parasitic inductance is no longer negligible. A typical sensing circuit using a discrete resistor is shown in Figure 2a. In previous generations of controllers, a small RC filter placed near the IC was commonly used to reduce the effects of capacitive and inductive noise coupled inthe sense traces on the PCB. A typical filter consists of two series 10Ω resistors connected to a parallel 1000pF capacitor, resulting in a time constant of 20ns. This same RC filter, with minor modifications, can be used to extract the resistive component of the current sense signal in the presence of parasitic inductance. For example, Figure 3 illustrates the voltage waveform across a 2mΩ sense resistor with a 2010 footprint for the 1.2V/15A converter operating at 100% load. The waveform is the superposition of a purely resistive component and a purely inductive component. It was measured using two scope probes and waveform math to obtain a differential measurement. Based on additional measurements of the inductor ripple current and the on-time and off-time of the top switch, the value of the parasitic inductance was determined to be 0.5nH using the equation: ESL = VESL(STEP) tON • tOFF ΔIL tON + tOFF VIN INTVCC BOOST TG LTC3869 SW BG PGND SENSE + VIN VIN INTVCC BOOST TG VOUT SW LTC3869 BG PGND SENSE+ C1* R2 SENSE– R1** VIN SENSE RESISTOR PLUS PARASITIC INDUCTANCE RS ESL INDUCTOR L DCR VOUT RF CF RF CF • 2RF ≤ ESL/RS POLE-ZERO CANCELLATION SENSE– SGND 3869 F02a SGND FILTER COMPONENTS PLACED NEAR SENSE PINS L R2 R = DCR *PLACE C1 NEAR SENSE+, R1||R2 × C1 = DCR SENSE(EQ) R1 + R2 SENSE– PINS **PLACE R1 NEXT TO INDUCTOR 3869 F02b (2a) Using a Resistor to Sense Current (2b) Using the Inductor DCR to Sense Current Figure 2. Two Different Methods of Sensing Current 3869f 14 LTC3869/LTC3869-2 APPLICATIONS INFORMATION The filter components need to be placed close to the IC. The positive and negative sense traces need to be routed as a differential pair and Kelvin connected to the sense resistor. VESL(STEP) VSENSE 20mV/DIV Inductor DCR Sensing For applications requiring the highest possible efficiency at high load currents, the LTC3869 is capable of sensing the voltage drop across the inductor DCR, as shown in Figure 2b. The DCR of the inductor represents the small amount of DC winding resistance of the copper, which can be less than 1mΩ for today’s low value, high current inductors. In a high current application requiring such an inductor, conduction loss through a sense resistor would cost several points of efficiency compared to DCR sensing. If the external R1|| R2 • C1 time constant is chosen to be exactly equal to the L/DCR time constant, the voltage drop across the external capacitor is equal to the drop across the inductor DCR multiplied by R2/(R1 + R2). R2 scales the voltage across the sense terminals for applications where the DCR is greater than the target sense resistor value. To properly dimension the external filter components, the DCR of the inductor must be known. It can be measured using a good RLC meter, but the DCR tolerance is not always the same and varies with temperature; consult the manufacturers’ data sheets for detailed information. Using the inductor ripple current value from the Inductor Value Calculation section, the target sense resistor value is: RSENSE(EQUIV) = VSENSE(MAX) ΔI IMAX + L 2 500ns/DIV 3869 F03 Figure 3. Voltage Waveform Measured Directly Across the Sense Resistor VSENSE 20mV/DIV 500ns/DIV 3869 F04 Figure 4. Voltage Waveform Measured After the Sense Resistor Filter. CF = 1000pF RF = 100Ω , If the RC time constant is chosen to be close to the parasitic inductance divided by the sense resistor (L/R), the resulting waveform looks resistive again, as shown in Figure 4. For applications using low maximum sense voltages, check the sense resistor manufacturer’s data sheet for information about parasitic inductance. In the absence of data, measure the voltage drop directly across the sense resistor to extract the magnitude of the ESL step and use the equation above to determine the ESL. However, do not over-filter. Keep the RC time constant less than or equal to the inductor time constant to maintain a high enough ripple voltage on VRSENSE. The above generally applies to high density/high current applications where IMAX >10A and low values of inductors are used. For applications where IMAX 40%. However, the LTC3869 uses a scheme that counteracts this compensating ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles. Inductor Value Calculation Given the desired input and output voltages, the inductor value and operating frequency fOSC directly determine the inductor’s peak-to-peak ripple current: C1 is usually selected to be in the range of 0.047µF to 0.47µF This forces R1|| R2 to around 2kΩ, reducing error . that might have been caused by the SENSE pins’ ±1µA current. TL(MAX) is the maximum inductor temperature. The equivalent resistance R1|| R2 is scaled to the room temperature inductance and maximum DCR: R1|| R2 = L (DCR at 20°C) • C1 The sense resistor values are: R1 = R1|| R2 R1 • RD ; R2 = RD 1 − RD IRIPPLE = VOUT ⎛ VIN – VOUT ⎞ ⎜ ⎟ VIN ⎝ fOSC • L ⎠ The maximum power loss in R1 is related to duty cycle, and will occur in continuous mode at the maximum input voltage: PLOSS R1 = ( VIN(MAX) − VOUT ) • VOUT R1 Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors, and output voltage ripple. Thus, highest efficiency operation is obtained at low frequency with a small ripple current. Achieving this, however, requires a large inductor. A reasonable starting point is to choose a ripple current that is about 40% of IOUT(MAX) for a duty cycle less than 40%. Note that the largest ripple current occurs at the highest input voltage. To guarantee that ripple current does not exceed a specified maximum, the inductor should be chosen according to: L≥ VIN – VOUT V • OUT fOSC • IRIPPLE VIN Ensure that R1 has a power rating higher than this value. If high efficiency is necessary at light loads, consider this power loss when deciding whether to use DCR sensing or sense resistors. Light load power loss can be modestly higher with a DCR network than with a sense resistor, due to the extra switching losses incurred through R1. However, DCR sensing eliminates a sense resistor, reduces conduction losses and provides higher efficiency at heavy loads. Peak efficiency is about the same with either method. To maintain a good signal to noise ratio for the current sense signal, use a minimum ∆VSENSE of 10mV for duty cycles less than 40%. For a DCR sensing application, the actual ripple voltage will be determined by the equation: ΔVSENSE = VIN − VOUT VOUT R1 • C1 VIN • fOSC For duty cycles greater than 40%, the 10mV current sense ripple voltage requirement is relaxed because the slope compensation signal aids the signal-to-noise ratio and because a lower limit is placed on the inductor value to avoid subharmonic oscillations. To ensure stability for 3869f 16 LTC3869/LTC3869-2 APPLICATIONS INFORMATION duty cycles up to the maximum of 95%, use the following equation to find the minimum inductance. L MIN > fSW VOUT • 1.4 • ILOAD(MAX) where LMIN is in units of µH fSW is in units of MHz Inductor Core Selection Once the inductance value is determined, the type of inductor must be selected. Core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Power MOSFET and Schottky Diode (Optional) Selection Two external power MOSFETs must be selected for each controller in the LTC3869: one N-channel MOSFET for the top (main) switch, and one N-channel MOSFET for the bottom (synchronous) switch. The peak-to-peak drive levels are set by the INTVCC voltage. This voltage is typically 5V during start-up (see EXTVCC Pin Connection). Consequently, logic-level threshold MOSFETs must be used in most applications. The only exception is if low input voltage is expected (VIN < 5V); then, sub-logic level threshold MOSFETs (VGS(TH) < 3V) should be used. Pay close attention to the BVDSS specification for the MOSFETs as well; most of the logic level MOSFETs are limited to 30V or less. Selection criteria for the power MOSFETs include the on-resistance RDS(ON) , Miller capacitance CMILLER, input voltage and maximum output current. Miller capacitance, CMILLER, can be approximated from the gate charge curve usually provided on the MOSFET manufacturers’ data sheet. CMILLER is equal to the increase in gate charge along the horizontal axis while the curve is approximately flat divided by the specified change in VDS. This result is then multiplied by the ratio of the application applied VDS to the gate charge curve specified VDS. When the IC is operating in continuous mode the duty cycles for the top and bottom MOSFETs are given by: Main Switch Duty Cycle = VOUT VIN VIN – VOUT VIN Synchronous Switch Duty Cycle = The MOSFET power dissipations at maximum output current are given by: V 2 PMAIN = OUT (IMAX ) (1 + d ) RDS(ON) + VIN ( VIN ) 2 ⎛ IMAX ⎞ ⎜ ⎟ (RDR ) (CMILLER ) • ⎝2⎠ ⎡ ⎤ 1 1 ⎢ ⎥ • fOSC + ⎢ VINTVCC – VTH(MIN) VTH(MIN) ⎥ ⎣ ⎦ V – VOUT 2 PSYNC = IN (IMAX ) (1 + d ) RDS(ON) VIN where d is the temperature dependency of RDS(ON) and RDR (approximately 2Ω) is the effective driver resistance at the MOSFET’s Miller threshold voltage. VTH(MIN) is the typical MOSFET minimum threshold voltage. Both MOSFETs have I2R losses while the topside N-channel equation includes an additional term for transition losses, which are highest at high input voltages. For VIN < 20V the high current efficiency generally improves with larger MOSFETs, while for VIN > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CMILLER actually provides higher efficiency. 3869f 17 LTC3869/LTC3869-2 APPLICATIONS INFORMATION The synchronous MOSFET losses are greatest at high input voltage when the top switch duty factor is low or during a short-circuit when the synchronous switch is on close to 100% of the period. The term (1 + d) is generally given for a MOSFET in the form of a normalized RDS(ON) vs Temperature curve, but d = 0.005/°C can be used as an approximation for low voltage MOSFETs. The optional Schottky diodes conduct during the dead time between the conduction of the two power MOSFETs. These prevent the body diodes of the bottom MOSFETs from turning on, storing charge during the dead time and requiring a reverse recovery period that could cost as much as 3% in efficiency at high VIN. A 1A to 3A Schottky is generally a good compromise for both regions of operation due to the relatively small average current. Larger diodes result in additional transition losses due to their larger junction capacitance. A Schottky diode in parallel with the bottom FET may also provide a modest improvement in Burst Mode efficiency. Soft-Start and Tracking The LTC3869 has the ability to either soft-start by itself with a capacitor or track the output of another channel or external supply. When one particular channel is configured to soft-start by itself, a capacitor should be connected to its TK/SS pin. This channel is in the shutdown state if its RUN pin voltage is below 1.2V. Its TK/SS pin is actively pulled to ground in this shutdown state. Once the RUN pin voltage is above 1.2V, the channel powers up. A soft-start current of 1.2µA then starts to charge its soft-start capacitor. Note that soft-start or tracking is achieved not by limiting the maximum output current of the controller but by controlling the output ramp voltage according to the ramp rate on the TK/SS pin. Current foldback is disabled during this phase to ensure smooth soft-start or tracking. The soft-start or tracking range is defined to be the voltage range from 0V to 0.6V on the TK/SS pin. The total soft-start time can be calculated as: tSOFTSTART = 0.6 • CSS 1.2µA Regardless of the mode selected by the MODE/PLLIN pin, the regulator will always start in pulse-skipping mode up to TK/SS = 0.5V. Between TK/SS = 0.5V and 0.54V, it will operate in forced continuous mode and revert to the selected mode once TK/SS > 0.54V. The output ripple is minimized during the 40mV forced continuous mode window ensuring a clean PGOOD signal. When the channel is configured to track another supply, the feedback voltage of the other supply is duplicated by a resistor divider and applied to the TK/SS pin. Therefore, the voltage ramp rate on this pin is determined by the ramp rate of the other supply’s voltage. Note that the small soft-start capacitor charging current is always flowing, producing a small offset error. To minimize this error, select the tracking resistive divider value to be small enough to make this error negligible. In order to track down another channel or supply after the soft-start phase expires, the LTC3869 is forced into continuous mode of operation as soon as VFB is below the undervoltage threshold of 0.54V regardless of the setting on the MODE/PLLIN pin. However, the LTC3869 should always be set in force continuous mode tracking down when there is no load. After TK/SS drops below 0.1V, its channel will operate in discontinuous mode. 3869f 18 LTC3869/LTC3869-2 APPLICATIONS INFORMATION Output Voltage Tracking The LTC3869 allows the user to program how its output ramps up and down by means of the TK/SS pins. Through these pins, the output can be set up to either coincidentally or ratiometrically track another supply’s output, as shown in Figure 5. In the following discussions, VOUT1 refers to the LTC3869’s output 1 as a master channel and VOUT2 refers to the LTC3869’s output 2 as a slave channel. In practice, though, either phase can be used as the master. To implement the coincident tracking in Figure 5a, connect an additional resistive divider to VOUT1 and connect its midpoint to the TK/SS pin of the slave channel. The ratio of this divider should be the same as that of the slave channel’s feedback divider shown in Figure 6a. In this tracking mode, VOUT1 must be set higher than VOUT2. To implement the ratiometric tracking in Figure 6b, the ratio of the VOUT2 divider should be exactly the same as the master channel’s feedback divider shown in Figure 6b. By selecting different resistors, the LTC3869 can achieve different modes of tracking including the two in Figure 5. So which mode should be programmed? While either mode in Figure 5 satisfies most practical applications, some tradeoffs exist. The ratiometric mode saves a pair of resistors, but the coincident mode offers better output regulation. When the master channel’s output experiences dynamic excursion (under load transient, for example), the slave channel output will be affected as well. For better output regulation, use the coincident tracking mode instead of ratiometric. VOUT1 OUTPUT VOLTAGE OUTPUT VOLTAGE VOUT1 VOUT2 VOUT2 TIME 3869 F05a TIME 3869 F08b (5a) Coincident Tracking (5b) Ratiometric Tracking Figure 5. Two Different Modes of Output Voltage Tracking VOUT1 TO TK/SS2 PIN R3 R1 TO VFB1 PIN TO VFB2 PIN R3 VOUT2 VOUT1 TO TK/SS2 PIN R1 TO VFB1 PIN TO VFB2 PIN R3 VOUT2 R4 R2 R4 R2 R4 3869 F09 (6a) Coincident Tracking Setup (6b) Ratiometric Tracking Setup Figure 6. Setup for Coincident and Ratiometric Tracking 3869f 19 LTC3869/LTC3869-2 APPLICATIONS INFORMATION INTVCC Regulators and EXTVCC The LTC3869 features a true PMOS LDO that supplies power to INTVCC from the VIN supply. INTVCC powers the gate drivers and much of the LTC3869’s internal circuitry. The linear regulator regulates the voltage at the INTVCC pin to 5V when VIN is greater than 5.5V. EXTVCC connects to INTVCC through a P-channel MOSFET and can supply the needed power when its voltage is higher than 4.7V. Each of these can supply a peak current of 100mA and must be bypassed to ground with a minimum of 4.7µF ceramic capacitor or low ESR electrolytic capacitor. No matter what type of bulk capacitor is used, an additional 0.1µF ceramic capacitor placed directly adjacent to the INTVCC and PGND pins is highly recommended. Good bypassing is needed to supply the high transient currents required by the MOSFET gate drivers and to prevent interaction between the channels. High input voltage applications in which large MOSFETs are being driven at high frequencies may cause the maximum junction temperature rating for the LTC3869 to be exceeded. The INTVCC current, which is dominated by the gate charge current, may be supplied by either the 5V linear regulator or EXTVCC. When the voltage on the EXTVCC pin is less than 4.7V, the linear regulator is enabled. Power dissipation for the IC in this case is highest and is equal to VIN • IINTVCC. The gate charge current is dependent on operating frequency as discussed in the Efficiency Considerations section. The junction temperature can be estimated by using the equations given in Note 3 of the Electrical Characteristics. For example, the LTC3869 INTVCC current is limited to less than 42mA from a 38V supply in the UFD package and not using the EXTVCC supply: TJ = 70°C + (42mA)(38V)(34°C/W) = 125°C To prevent the maximum junction temperature from being exceeded, the input supply current must be checked while operating in continuous conduction mode (MODE/PLLIN = SGND) at maximum VIN. When the voltage applied to EXTVCC rises above 4.7V, the INTVCC linear regulator is turned off and the EXTVCC is connected to the INTVCC. The EXTVCC remains on as long as the voltage applied to EXTVCC remains above 4.5V. Using the EXTVCC allows the MOSFET driver and control power to be derived from one of the LTC3869’s switching regulator outputs during normal operation and from the INTVCC when the output is out of regulation (e.g., start-up, short-circuit). If more current is required through the EXTVCC than is specified, an external Schottky diode can be added between the EXTVCC and INTVCC pins. Do not apply more than 6V to the EXTVCC pin and make sure that EXTVCC < VIN at all times. Significant efficiency and thermal gains can be realized by powering INTVCC from the output, since the VIN current resulting from the driver and control currents will be scaled by a factor of (Duty Cycle)/(Switcher Efficiency). Tying the EXTVCC pin to a 5V supply reduces the junction temperature in the previous example from 125°C to: TJ = 70°C + (42mA)(5V)(34°C/W) = 77°C However, for 3.3V and other low voltage outputs, additional circuitry is required to derive INTVCC power from the output. The following list summarizes the four possible connections for EXTVCC: 1. EXTVCC left open (or grounded). This will cause INTVCC to be powered from the internal 5V regulator resulting in an efficiency penalty of up to 10% at high input voltages. 2. EXTVCC connected directly to VOUT. This is the normal connection for a 5V regulator and provides the highest efficiency. 3. EXTVCC connected to an external supply. If a 5V external supply is available, it may be used to power EXTVCC providing it is compatible with the MOSFET gate drive requirements. 4. EXTVCC connected to an output-derived boost network. For 3.3V and other low voltage regulators, efficiency gains can still be realized by connecting EXTVCC to an output-derived voltage that has been boosted to greater than 4.7V. 3869f 20 LTC3869/LTC3869-2 APPLICATIONS INFORMATION For applications where the main input power is below 5V, tie the VIN and INTVCC pins together and tie the combined pins to the 5V input with a 1Ω or 2.2Ω resistor as shown in Figure 7 to minimize the voltage drop caused by the gate charge current. This will override the INTVCC linear regulator and will prevent INTVCC from dropping too low due to the dropout voltage. Make sure the INTVCC voltage is at or exceeds the RDS(ON) test voltage for the MOSFET which is typically 4.5V for logic level devices. UVLO comparator constantly monitors the INTVCC voltage to ensure that an adequate gate-drive voltage is present. It locks out the switching action when INTVCC is below 3.2V. To prevent oscillation when there is a disturbance on the INTVCC, the UVLO comparator has 600mV of precision hysteresis. Another way to detect an undervoltage condition is to monitor the VIN supply. Because the RUN pins have a precision turn-on reference of 1.2V, one can use a resistor divider to VIN to turn on the IC when VIN is high enough. An extra 4.5µA of current flows out of the RUN pin once the RUN pin voltage passes 1.2V. One can program the hysteresis of the run comparator by adjusting the values of the resistive divider. For accurate VIN undervoltage detection, VIN needs to be higher than 4.5V. CIN and COUT Selection The selection of CIN is simplified by the 2-phase architecture and its impact on the worst-case RMS current drawn through the input network (battery/fuse/capacitor). It can be shown that the worst-case capacitor RMS current occurs when only one controller is operating. The controller with the highest (VOUT)(IOUT) product needs to be used in the formula below to determine the maximum RMS capacitor current requirement. Increasing the output current drawn from the other controller will actually decrease the input RMS ripple current from its maximum value. The out-ofphase technique typically reduces the input capacitor’s RMS ripple current by a factor of 30% to 70% when compared to a single phase power supply solution. In continuous mode, the source current of the top MOSFET is a square wave of duty cycle (VOUT)/(VIN). To prevent large voltage transients, a low ESR capacitor sized for the maximum RMS current of one channel must be used. The maximum RMS capacitor current is given by: CIN Required IRMS ≈ IMAX ⎡ 1/ 2 ⎣( VOUT ) ( VIN – VOUT )⎤ ⎦ VIN LTC3869 VIN CINTVCC 4.7µF INTVCC RVIN 1Ω 5V + CIN 3869 F07 Figure 7. Setup for a 5V Input Topside MOSFET Driver Supply (CB, DB) External bootstrap capacitors CB connected to the BOOST pins supply the gate drive voltages for the topside MOSFETs. Capacitor CB in the Functional Diagram is charged though external diode DB from INTVCC when the SW pin is low. When one of the topside MOSFETs is to be turned on, the driver places the CB voltage across the gate source of the desired MOSFET. This enhances the MOSFET and turns on the topside switch. The switch node voltage, SW, rises to VIN and the BOOST pin follows. With the topside MOSFET on, the boost voltage is above the input supply: VBOOST = VIN + VINTVCC. The value of the boost capacitor CB needs to be 100 times that of the total input capacitance of the topside MOSFET(s). The reverse breakdown of the external Schottky diode must be greater than VIN(MAX). Make sure the diode is a low leakage diode even at hot temperature to prevent leakage current feeding INTVCC. When adjusting the gate drive level, the final arbiter is the total input current for the regulator. If a change is made and the input current decreases, then the efficiency has improved. If there is no change in input current, then there is no change in efficiency. Undervoltage Lockout The LTC3869 has two functions that help protect the controller in case of undervoltage conditions. A precision This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturers’ ripple current ratings are often based on only 2000 hours of life. 3869f 21 LTC3869/LTC3869-2 APPLICATIONS INFORMATION This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Several capacitors may be paralleled to meet size or height requirements in the design. Due to the high operating frequency of the LTC3869, ceramic capacitors can also be used for CIN. Always consult the manufacturer if there is any question. The benefit of the LTC3869 2-phase operation can be calculated by using the equation above for the higher power controller and then calculating the loss that would have resulted if both controller channels switched on at the same time. The total RMS power lost is lower when both controllers are operating due to the reduced overlap of current pulses required through the input capacitor’s ESR. This is why the input capacitor’s requirement calculated above for the worst-case controller is adequate for the dual controller design. Also, the input protection fuse resistance, battery resistance, and PC board trace resistance losses are also reduced due to the reduced peak currents in a 2-phase system. The overall benefit of a multiphase design will only be fully realized when the source impedance of the power supply/battery is included in the efficiency testing. The sources of the top MOSFETs should be placed within 1cm of each other and share a common CIN(s). Separating the sources and CIN may produce undesirable voltage and current resonances at VIN. A small (0.1µF to 1µF) bypass capacitor between the chip VIN pin and ground, placed close to the LTC3869, is also suggested. A 2.2Ω to 10Ω resistor placed between CIN (C1) and the VIN pin provides further isolation between the two channels. The selection of COUT is driven by the effective series resistance (ESR). Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering. The output ripple (∆VOUT) is approximated by: ⎛ 1⎞ ΔVOUT ≈ IRIPPLE ⎜ESR + ⎟ 8fCOUT ⎠ ⎝ where f is the operating frequency, COUT is the output capacitance and IRIPPLE is the ripple current in the inductor. The output ripple is highest at maximum input voltage since IRIPPLE increases with input voltage. Setting Output Voltage The LTC3869 output voltages are each set by an external feedback resistive divider carefully placed across the output, as shown in Figure 8. The regulated output voltage is determined by: ⎛ R⎞ VOUT = 0.6V • ⎜1+ B ⎟ ⎝ RA ⎠ To improve the frequency response, a feed-forward capacitor, CFF , may be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW line. VOUT RB CFF 1/2 LTC3869 VFB RA 3869 F08 Figure 8. Setting Output Voltage Fault Conditions: Current Limit and Current Foldback The LTC3869 includes current foldback to help limit load current when the output is shorted to ground. If the output falls below 50% of its nominal output level, then the maximum sense voltage is progressively lowered from its maximum programmed value to one-third of the maximum value. Foldback current limiting is disabled during the soft-start or tracking up. Under short-circuit conditions with very low duty cycles, the LTC3869 will begin cycle skipping in order to limit the short-circuit current. In this situation the bottom MOSFET will be dissipating most of the power but less than in normal operation. The shortcircuit ripple current is determined by the minimum ontime tON(MIN) of the LTC3869 (≈ 90ns), the input voltage and inductor value: ΔIL(SC) = tON(MIN) • VIN L The resulting short-circuit current is: ISC = 1/3 VSENSE(MAX) RSENSE 1 – ΔIL(SC) 2 3869f 22 LTC3869/LTC3869-2 APPLICATIONS INFORMATION Phase-Locked Loop and Frequency Synchronization The LTC3869 has a phase-locked loop (PLL) comprised of an internal voltage-controlled oscillator (VCO) and a phase detector. This allows the turn-on of the top MOSFET of controller 1 to be locked to the rising edge of an external clock signal applied to the MODE/PLLIN pin. The turn-on of controller 2’s top MOSFET is thus 180 degrees outof-phase with the external clock. The phase detector is an edge sensitive digital type that provides zero degrees phase shift between the external and internal oscillators. This type of phase detector does not exhibit false lock to harmonics of the external clock. The output of the phase detector is a pair of complementary current sources that charge or discharge the internal filter network. There is a precision 10µA of current flowing out of FREQ pin. This allows the user to use a single resistor to SGND to set the switching frequency when no external clock is applied to the MODE/PLLIN pin. The internal switch between FREQ pin and the integrated PLL filter network is ON, allowing the filter network to be pre-charged to the same voltage potential as the FREQ pin. The relationship between the voltage on the FREQ pin and the operating frequency is shown in Figure 9 and specified in the Electrical Characteristics table. If an external clock is detected on the MODE/PLLIN pin, the internal switch mentioned above will turn off and isolate the influence of FREQ pin. Note that the LTC3869 can only be synchronized to an external clock whose frequency is within range of the LTC3869’s internal VCO. This is guaranteed to be between 250kHz and 780kHz. A simplified block diagram is shown in Figure 10. If the external clock frequency is greater than the internal oscillator’s frequency, fOSC , then current is sourced continuously from the phase detector output, pulling up the filter network. When the external clock frequency is less than fOSC , current is sunk continuously, pulling down the filter network. If the external and internal frequencies are the same but exhibit a phase difference, the current sources turn on for an amount of time corresponding to the phase difference. The voltage on the filter network is adjusted until the phase and frequency of the internal and external oscillators are identical. At the stable operating point, the phase detector output is high impedance and the filter capacitor holds the voltage. 900 800 700 FREQUENCY (kHz) 600 500 400 300 200 100 0 0 0.5 1 1.5 FREQ PIN VOLTAGE (V) 2 2.5 3869 F09 Figure 9. Relationship Between Oscillator Frequency and Voltage at the FREQ Pin 2.4V 5V RSET 10µA FREQ EXTERNAL OSCILLATOR MODE/ PLLIN DIGITAL SYNC PHASE/ FREQUENCY DETECTOR VCO 3869 F10 Figure 10. Phase-Locked Loop Block Diagram Typically, the external clock (on MODE/PLLIN pin) input high threshold is 1.6V, while the input low threshold is 1V. It is not recommended to apply the external clock when IC is in shutdown. Minimum On-Time Considerations Minimum on-time tON(MIN) is the smallest time duration that the LTC3869 is capable of turning on the top MOSFET. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum on-time limit and care should be taken to ensure that tON(MIN) < VOUT VIN (ƒ) 3869f 23 LTC3869/LTC3869-2 APPLICATIONS INFORMATION If the duty cycle falls below what can be accommodated by the minimum on-time, the controller will begin to skip cycles. The output voltage will continue to be regulated, but the ripple voltage and current will increase. The minimum on-time for the LTC3869 is approximately 90ns, with reasonably good PCB layout, minimum 40% inductor current ripple and at least 10mV – 15mV ripple on the current sense signal. The minimum on-time can be affected by PCB switching noise in the voltage and current loop. As the peak sense voltage decreases the minimum on-time gradually increases to 130ns. This is of particular concern in forced continuous applications with low ripple current at light loads. If the duty cycle drops below the minimum on-time limit in this situation, a significant amount of cycle skipping can occur with correspondingly larger current and voltage ripple. Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Percent efficiency can be expressed as: %Efficiency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC3869 circuits: 1) IC VIN current, 2) INTVCC regulator current, 3) I2R losses, 4) Topside MOSFET transition losses. 1. The VIN current is the DC supply current given in the Electrical Characteristics table, which excludes MOSFET driver and control currents. VIN current typically results in a small (1µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT , causing a rapid drop in VOUT . No regulator can alter its delivery of current quickly enough to prevent this sudden step change in output voltage if the load switch resistance is low and it is driven quickly. If the ratio of CLOAD to COUT is greater than 1:50, the switch rise time should be controlled so that the load rise time is limited to approximately 25 • CLOAD . Thus a 10µF capacitor would require a 250µs rise time, limiting the charging current to about 200mA. 3869f 25 LTC3869/LTC3869-2 APPLICATIONS INFORMATION PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the IC. These items are also illustrated graphically in the layout diagram of Figure 11. Figure 12 illustrates the current waveforms present in the various branches of the 2-phase synchronous regulators operating in the continuous mode. Check the following in your layout: 1. Are the top N-channel MOSFETs M1 and M3 located within 1 cm of each other with a common drain connection at CIN? Do not attempt to split the input decoupling for the two channels as it can cause a large resonant loop. 2. Are the signal and power grounds kept separate? The combined IC signal ground pin and the ground return of CINTVCC must return to the combined COUT (–) terminals. The VFB and ITH traces should be as short as possible. The path formed by the top N-channel MOSFET, Schottky diode and the CIN capacitor should have short leads and PC trace lengths. The output capacitor (–) terminals should be connected as close as possible to the (–) terminals of the input capacitor by placing the capacitors next to each other and away from the Schottky loop described above. 3. Do the LTC3869 VFB pins’ resistive dividers connect to the (+) terminals of COUT? The resistive divider must be connected between the (+) terminal of COUT and signal ground. The feedback resistor connections should not be along the high current input feeds from the input capacitor(s). 4. Are the SENSE+ and SENSE– leads routed together with minimum PC trace spacing? The filter capacitor between SENSE+ and SENSE– should be as close as possible to the IC. Ensure accurate current sensing with Kelvin connections at the sense resistor or inductor, whichever is used for current sensing. 5. Is the INTVCC decoupling capacitor connected close to the IC, between the INTVCC and the power ground pins? This capacitor carries the MOSFET drivers current peaks. An additional 1µF ceramic capacitor placed immediately next to the INTVCC and PGND pins can help improve noise performance substantially. 6. Keep the switching nodes (SW1, SW2), top gate nodes (TG1, TG2), and boost nodes (BOOST1, BOOST2) away from sensitive small-signal nodes, especially from the opposite channel’s voltage and current sensing feedback pins. All of these nodes have very large and fast moving signals and therefore should be kept on the “output side” of the LTC3869 and occupy minimum PC trace area. If DCR sensing is used, place the top resistor (Figure 2b, R1) close to the switching node. 7. Use a modified “star ground” technique: a low impedance, large copper area central grounding point on the same side of the PC board as the input and output capacitors with tie-ins for the bottom of the INTVCC decoupling capacitor, the bottom of the voltage feedback resistive divider and the SGND pin of the IC. PC Board Layout Debugging Start with one controller at a time. It is helpful to use a DC-50MHz current probe to monitor the current in the inductor while testing the circuit. Monitor the output switching node (SW pin) to synchronize the oscilloscope to the internal oscillator and probe the actual output voltage as well. Check for proper performance over the operating voltage and current range expected in the application. The frequency of operation should be maintained over the input voltage range down to dropout and until the output load drops below the low current operation threshold—typically 10% of the maximum designed current level in Burst Mode operation. The duty cycle percentage should be maintained from cycle to cycle in a well-designed, low noise PCB implementation. Variation in the duty cycle at a subharmonic rate can suggest noise pickup at the current or voltage sensing inputs or inadequate loop compensation. Overcompensation of the loop can be used to tame a poor PC layout if regulator bandwidth optimization is not required. Only after each controller is checked for its individual performance should both controllers be turned on at the same time. A particularly difficult region of operation is when one controller channel is nearing its current comparator trip point when the other channel is turning on its top MOSFET. This occurs around 50% duty cycle on either channel due to the phasing of the internal clocks and may cause minor duty cycle jitter. 3869f 26 LTC3869/LTC3869-2 APPLICATIONS INFORMATION TK/SS1 ITH1 VFB1 SENSE1+ SENSE1– FREQ ILIM fIN MODE/PLLIN RUN1 RUN2 SGND SENSE2 – RPU2 PGOOD VPULL-UP PGOOD LTC3869 L1 TG1 SW1 CB1 BOOST1 BG1 VIN PGND EXTVCC INTVCC BG2 BOOST2 SW2 TG2 L2 CB2 RIN CVIN VIN CINTVCC 1µF CERAMIC M3 M4 M1 M2 1µF CERAMIC RSENSE VOUT1 D1 COUT1 + GND + CIN SENSE2+ VFB2 ITH2 TK/SS2 + COUT2 D2 RSENSE 3869 F11 + VOUT2 Figure 11. Recommended Printed Circuit Layout Diagram 3869f 27 LTC3869/LTC3869-2 APPLICATIONS INFORMATION SW1 L1 RSENSE1 VOUT1 D1 COUT1 RL1 VIN RIN CIN SW2 L2 RSENSE2 VOUT2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH. D2 COUT2 RL2 3869 F12 Figure 12. Branch Current Waveforms 3869f 28 LTC3869/LTC3869-2 APPLICATIONS INFORMATION Reduce VIN from its nominal level to verify operation of the regulator in dropout. Check the operation of the undervoltage lockout circuit by further lowering VIN while monitoring the outputs to verify operation. Investigate whether any problems exist only at higher output currents or only at higher input voltages. If problems coincide with high input voltages and low output currents, look for capacitive coupling between the BOOST, SW, TG, and possibly BG connections and the sensitive voltage and current pins. The capacitor placed across the current sensing pins needs to be placed immediately adjacent to the pins of the IC. This capacitor helps to minimize the effects of differential noise injection due to high frequency capacitive coupling. If problems are encountered with high current output loading at lower input voltages, look for inductive coupling between CIN, Schottky and the top MOSFET components to the sensitive current and voltage sensing traces. In addition, investigate common ground path voltage pickup between these components and the SGND pin of the IC. Design Example As a design example for a two channel high current regulator, assume VIN = 12V(nominal), VIN = 20V(maximum), VOUT1 = 1.8V, VOUT2 = 1.2V, IMAX1,2 = 15A, and f = 400kHz (see Figure 13). The regulated output voltages are determined by: ⎛ R⎞ VOUT = 0.6V • ⎜1+ B ⎟ ⎝ RA ⎠ Using 20k 1% resistors from both VFB nodes to ground, the top feedback resistors are (to the nearest 1% standard value) 40.2k and 20k. The frequency is set by biasing the FREQ pin to 1V (see Figure 9). The inductance values are based on a 35% maximum ripple current assumption (5.25A for each channel). The highest value of ripple current occurs at the maximum input voltage: VOUT L= ƒ • ΔIL(MAX) ⎛ ⎞ ⎜1 − VOUT ⎟ ⎜V ⎟ IN(MAX) ⎠ ⎝ Channel 1 will require 0.78µH, and channel 2 will require 0.54µH. The Vishay IHLP4040DZ-01, 0.56µH inductor is chosen for both rails. At the nominal input voltage (12V), the ripple current will be: ΔIL(NOM) = VOUT ƒ•L ⎞ ⎛ ⎜1 − VOUT ⎟ ⎟ ⎜V IN(NOM) ⎠ ⎝ Channel 1 will have 6.8A (46%) ripple, and channel 2 will have 4.8A (32%) ripple. The peak inductor current will be the maximum DC value plus one-half the ripple current, or 18.4A for channel 1 and 17.4A for channel 2. The minimum on-time occurs on channel 2 at the maximum VIN, and should not be less than 90ns: tON(MIN) = VOUT VIN(MAX) ƒ = 1.2V = 150ns 20V(400kHz) With ILIM floating, the equivalent RSENSE resistor value can be calculated by using the minimum value for the maximum current sense threshold (43mV). RSENSE(EQUIV) = VSENSE(MIN) ΔIL(NOM) ILOAD(MAX) + 2 The equivalent required RSENSE value is 2.4mΩ for channel 1 and 2.5mΩ for channel 2. The DCR of the 0.56µH inductor is 1.7mΩ typical and 1.8mΩ maximum for a 25°C ambient. At 100°C, the estimated maximum DCR value is 2.3mΩ. The maximum DCR value is just slightly under the equivalent RSENSE values. Therefore, R2 is not required to divide down the signal. 3869f 29 LTC3869/LTC3869-2 APPLICATIONS INFORMATION 2.2Ω 4.7µF M1 L1 0.56µH M2 0.1µF 1µF 10µF 25V ×2 D4 0.1µF M3 L2 0.56µH M4 VIN 4.5V TO 20V + 82µF 25V D3 VIN PGOOD EXTVCC INTVCC TG1 BOOST1 SW1 BG1 ILIM SENSE1+ TG2 BOOST2 SW2 BG2 PGND FREQ SENSE2+ SENSE2– LTC3869 3.09k 1% MODE/PLLIN 3.09k 1% 0.1µF SENSE1– RUN2 RUN1 VFB1 ITH1 150pF 0.1µF TK/SS1 SGND 0.1µF VOUT1 1.8V 15A 40.2k 1% 20k 1% VFB2 ITH2 TK/SS2 0.1µF 100k 1% + 1nF COUT1 330µF ×2 20k 1% 12.1k 1% 1nF 4.99k 1% VOUT2 1.2V 15A 20k 1% 3869 F13 150pF + COUT2 330µF ×2 L1, L2: VISHAY IHLP4040DZ-01, 0.56µH M1, M3: RENESAS RJK0305DPB M2, M4: RENESAS RJK0330DPB Figure 13. High Efficiency Dual 400kHz 1.8V/1.2V Step-Down Converter 95 VIN = 12V MODE = CCM 1.8V RSENSE 1.8V DCR SENSE 5 90 EFFICIENCY (%) EFFICIENCY 4 POWER LOSS (W) 85 3 80 POWER LOSS 75 1.2V RSENSE 1.2V DCR SENSE 0 2 4 6 8 10 12 LOAD CURRENT (A) 14 16 2 1 70 0 DCR SENSE APP: SEE FIGURE 16 RSENSE APP: SEE FIGURE 19 3869 F14 Figure 14. DCR Sense Efficiency vs RSENSE Efficiency 3869f 30 LTC3869/LTC3869-2 APPLICATIONS INFORMATION For each channel, 0.1µF is selected for C1. R1 = (DCRMAX L 0.56µH = = 3.11k at 25°C) • C1 1.8mΩ • 0.1µF For a 2mΩ sense resistor, a short-circuit to ground will result in a folded back current of: ISC = (1/ 3) 50mV – 1 ⎛ 90ns(20V) ⎞ = 6.7A ⎜ 0.002Ω ⎟ 2 ⎝ 0.56µH ⎠ Choose R1 = 3.09k The power loss in R1 at the maximum input voltage is: PLOSS R1 = (VIN(MAX) − VOUT ) • VOUT R1 A Renesas RJK0330DPB, RDS(ON) = 3.9mΩ, is chosen for the bottom FET. The resulting power loss is: 20V – 1.8V (15A )2 • 20V ⎡1+ (0.005) • (75°C – 25°C)⎤ • 0.0039Ω ⎣ ⎦ PSYNC = PSYNC = 1W CIN is chosen for an RMS current rating of at least 7.5A at temperature assuming only channel 1 or 2 is on. COUT is chosen with an equivalent ESR of 4.5mΩ for low output ripple. The output ripple in continuous mode will be highest at the maximum input voltage. The output voltage ripple due to ESR is approximately: VORIPPLE = RESR (∆IL) = 0.0045Ω • 6.8A = 31mVP–P Further reductions in output voltage ripple can be made by placing a 100µF ceramic across COUT. The resulting power loss for R1 is 11mW for channel 1 and 7mW for channel 2. The sum of the sense resistor and DCR is 2.5mΩ (max) for the RSENSE application whereas the inductor DCR for the DCR sense application is 1.8mΩ (max). As a result of the lower conduction losses from the switch node to VOUT, the DCR sensing application has higher efficiency. The power dissipation on the topside MOSFET can be easily estimated. Choosing a Renesas RJK0305DPB MOSFET results in: RDS(ON) = 13mΩ (max), VMILLER = 2.6V, CMILLER ≅ 150pF At maximum input voltage with TJ . (estimated) = 75°C: 1.8V (15A )2 [1+ (0.005)(75°C – 25°C)] • 20V ⎛ 15A ⎞ (0.013Ω) + (20V )2 ⎜ ⎟ (2Ω) (150pF ) • ⎝2⎠ ⎡ 1⎤ 1 + (400kHz ) ⎢ ⎣ 5V – 2.6V 2.6V ⎥ ⎦ = 329mW + 288mW PMAIN = = 617mW 3869f 31 RUN1 FREQ SW1 SENSE1– SENSE1+ TYPICAL APPLICATIONS MODE/PLLIN LTC3869/LTC3869-2 RUN2 ILIM PGOOD SW2 TG2 32 0.1µF 86.6k 10µF ×2 24.9k 3.01k L1 0.68µH CMDSH-3 2.2 M2 RJK0330DPB M1 RJK0305DPB TG1 BOOST1 PGND BG1 LTC3869 INTVCC EXTVCC BG2 PGND BOOST2 CMDSH-3 0.1µF 4.7µF VIN 0.1µF 0.1µF + 82µF 25V ×2 VIN 4.5V TO 20V 20k 1nF 63.4k 100pF TK/SS1 ITH1 VFB1 SGND VFB2 ITH2 TK/SS2 SENSE2+ SENSE2– + COUT1 100µF 6.3V 20k VOUT1 2.5V COUT2 15A 330µF 4V ×2 100pF 20k 15k 1nF 0.1µF 0.1µF 40.2k 0.1µF M3 RJK0305DPB L2 0.68µH + M4 RJK0330DPB 3.01k 24.9k COUT3 100µF 6.3V PGOOD 100k VOUT2 1.8V COUT4 15A 330µF 4V ×2 L1, L2: VISHAY IHLP5050CE-01, 0.68µH COUT1, COUT3: MURATA GRM32ER60J107ME20 COUT2, COUT4: KEMET T520V337M004ATE009 RNTC1, RNTC2: MURATA NCP18WF104J03RB 3869 F15 Figure 15. 2.5V, 15A and 1.8V, 15A Supply with DCR Sensing, fSW = 350kHz 3869f 100Ω 100Ω 86.6k 10µF ×2 RUN1 FREQ SW1 M1 RJK0305DPB TG1 BOOST1 PGND BG1 LTC3869 INTVCC EXTVCC BG2 PGND BOOST2 RUN2 ILIM PGOOD SW2 TG2 0.1µF CMDSH-3 M3 RJK0305DPB L2 0.4µH 0.002Ω 0.1µF 4.7µF VIN 2.2 M2 RJK0330DPB CMDSH-3 L1 0.4µH 0.1µF 0.002Ω SENSE1– SENSE1+ 0.1µF 0.1µF + 82µF 25V ×2 VIN 4.5V TO 20V TYPICAL APPLICATIONS 18k 1nF 63.4k MODE/PLLIN 150pF TK/SS1 ITH1 VFB1 SGND VFB2 ITH2 TK/SS2 SENSE2+ SENSE2– + COUT1 100µF 6.3V 20k COUT2 330µF 4V ×2 VOUT1 1.8V 15A 150pF 20k 15k 1.5nF 0.1µF 0.1µF 20k 100Ω 100Ω + M4 RJK0330DPB COUT3 100µF 6.3V PGOOD 100k COUT4 330µF 4V ×2 VOUT2 1.2V 15A L1, L2: VITEC 59PR9875 COUT1, COUT3: MURATA GRM31CR60J107ME39L COUT2, COUT4: SANYO 2R5TPE330M9 3869 F16 Figure 16. 1.8V, 15A and 1.2V, 15A Supply, fSW = 400kHz LTC3869/LTC3869-2 33 3869f RUN1 FREQ SW1 SENSE1– SENSE1+ TYPICAL APPLICATIONS MODE/PLLIN LTC3869/LTC3869-2 RUN ILIM PGOOD SW2 TG2 34 100Ω 100Ω 86.6k 10µF ×4 M1 RJK0305DPB TG1 BOOST1 PGND BG1 LTC3869 INTVCC EXTVCC BG2 PGND BOOST2 0.1µF CMDSH-3 M3 RJK0305DPB L2 0.44µH 0.001Ω 1% 0.1µF 4.7µF VIN 2.2 M2 RJK0330DPB ×2 CMDSH-3 L1 0.44µH 0.1µF 0.001Ω 1% 0.1µF + 270µF 16V VIN 4.5V TO 14V TK/SS1 ITH1 VFB1 SGND VFB2 ITH2 TK/SS2 SENSE2+ 0.1µF SENSE2– 0.1µF 20k COUT1 100µF 6.3V ×4 + VOUT 1.2V COUT2 40A 330µF 2.5V ×4 2200pF 100pF 20k 5.9k 100Ω PGOOD 100k 100Ω M4 RJK0330DPB L1, L2: PULSE PA0513.441NLT COUT1: MURATA GRM31CR60J107ME39L COUT2: SANYO 2R5TPE330M9 3869 F17 Figure 17. High Efficiency Dual Phase 1.2V, 40A Supply, fSW = 250kHz 3869f 0.1µF 10µF ×4 RUN1 FREQ SW1 SENSE1– SENSE1+ M1 RJK0305DPB TG1 BOOST1 PGND BG1 LTC3869 INTVCC EXTVCC BG2 PGND BOOST2 RUN ILIM PGOOD SW2 TG2 0.1µF CMDSH-3 M3 RJK0305DPB L2 0.47µH 4.7µF 1µF VIN 2.2 M2 RJK0330DPB ×2 CMDSH-3 0.1µF L1 0.47µH 3.92k + 270µF 16V VIN 4.5V TO 14V MODE/PLLIN TYPICAL APPLICATIONS TK/SS1 ITH1 VFB1 SGND VFB2 ITH2 TK/SS2 SENSE2+ 0.1µF SENSE2– 0.1µF 20k COUT1 100µF 6.3V ×4 + COUT2 330µF 2.5V ×4 VOUT 1.2V 40A 3300pF 330pF 20k 10k PGOOD 100k M4 RJK0330DPB ×2 3.92k L1, L2: VISHAY IHLP5050FD-01, 0.47µH COUT1: MURATA GRM31CR60J107ME39L COUT2: SANYO 2R5TPE330M9 3869 F18 Figure 18. High Efficiency Dual Phase 1.2V, 40A Supply with DCR Sensing, fSW = 250kHz LTC3869/LTC3869-2 35 3869f SENSE1– SENSE1+ RUN1 FREQ SW1 MODE/PLLIN TYPICAL APPLICATIONS LTC3869/LTC3869-2 RUN ILIM PGOOD SW2 TG2 36 100Ω 100Ω 400kHz 100k 10µF ×4 M1 RJK0305DPB ×2 0.001Ω 1% L1 0.23µH CMDSH-3 2.2 M2 RJK0330DPB ×2 0.1µF + 270µF 16V VIN 4.5V TO 14V TK/SS1 ITH1 VFB1 SGND VFB2 ITH2 TK/SS2 SENSE2+ BG2 PGND BOOST2 0.1µF CMDSH-3 M3 RJK0305DPB ×2 L2 0.23µH 100Ω PGOOD 100k M4 RJK0330DPB ×2 0.001Ω 1% 1nF SENSE2– EXTVCC 4.7µF 1µF LTC3869 INTVCC VIN BG1 PGND1 BOOST1 TG1 0.1µF 0.1µF 10k COUT1 100µF 6.3V ×2 + VOUT 0.9V COUT2 50A 330µF 2.5V ×4 2700pF 220pF 20k 5.1k 100Ω L1, L2: PULSE PA0513.441NLT COUT1: MURATA GRM31CR60J107ME39L COUT2: SANYO 2R5TPE330M9 3869 F19 Figure 19. Small Size, Dual Phase 0.9V, 50A Supply, fSW = 400kHz 3869f 0.1µF 0.1µF 4.7µF ×6 24k 18k L1 13µH SDM10K45 2.2 M2 BSC093N040LS RUN1 FREQ SW1 SENSE1– SENSE1+ M1 BSC093N040LS TG1 BOOST PGND1 BG1 LTC3869 INTVCC EXTVCC 0.1µF BG2 PGND BOOST2 RUN2 ILIM PGOOD SW2 TG2 0.1µF SDM10K45 M3 BSC093N040LS L2 3.7µH 4.7µF VIN 0.1µF + TYPICAL APPLICATIONS VIN 13V TO 38V 100µF 50V 10k 5.6nF 383k MODE/PLLIN 47pF TK/SS1 ITH1 VFB1 SGND VFB2 ITH2 TK/SS2 SENSE2+ SENSE2– + VOUT1 12V 6A COUT2 39µF 16V ×2 20k 47pF 20k 4.99k 5.6nF 0.1µF 0.1µF 147k PGOOD 100k M4 BSC093N040LS + 8.2k 24k 3869 F20 COUT2 39µF 16V ×2 VOUT2 5V 10A L1: WURTH 7443551131 L2: WURTH 7443551370 COUT1, COUT2: SANYO 16SVPC39MV Figure 20. 12V, 6A and 5V, 10A Supply with DCR Sensing, fSW = 250kHz LTC3869/LTC3869-2 37 3869f LTC3869/LTC3869-2 PACKAGE DESCRIPTION (Reference LTC DWG # 05-08-1712 Rev B) UFD Package 28-Lead Plastic QFN (4mm × 5mm) 0.70 ± 0.05 4.50 ± 0.05 3.10 ± 0.05 2.50 REF 2.65 ± 0.05 3.65 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 3.50 REF 4.10 ± 0.05 5.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 (2 SIDES) PIN 1 TOP MARK (NOTE 6) 0.75 ± 0.05 R = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER 27 28 0.40 ± 0.10 1 2 5.00 ± 0.10 (2 SIDES) 2.50 REF R = 0.115 TYP 3.50 REF 3.65 ± 0.10 2.65 ± 0.10 (UFD28) QFN 0506 REV B 0.200 REF 0.00 – 0.05 0.25 ± 0.05 0.50 BSC BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X). 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 3869f 38 LTC3869/LTC3869-2 PACKAGE DESCRIPTION GN Package 28-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .045 ± .005 .386 – .393* (9.804 – 9.982) 28 27 26 25 24 23 22 21 20 19 18 17 1615 .033 (0.838) REF .254 MIN .150 – .165 .229 – .244 (5.817 – 6.198) .150 – .157** (3.810 – 3.988) .0165 ± .0015 RECOMMENDED SOLDER PAD LAYOUT .0250 BSC 1 .0532 – .0688 (1.35 – 1.75) 23 4 56 7 8 9 10 11 12 13 14 .004 – .0098 (0.102 – 0.249) .015 ± .004 × 45° (0.38 ± 0.10) .0075 – .0098 (0.19 – 0.25) 0° – 8° TYP .016 – .050 (0.406 – 1.270) NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) .008 – .012 (0.203 – 0.305) TYP .0250 (0.635) BSC GN28 (SSOP) 0204 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 3869f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 39 LTC3869/LTC3869-2 TYPICAL APPLICATIONS 3.3V/5A, 5V/5A Converter Using Sense Resistors 2.2Ω 1µF Si4816BDY M1 L2 2.2µH 0.1µF 4.7µF Si4816BDY M2 0.1µF L2 3.3µH 22µF 50V VIN 7V TO 24V D3 VIN PGOOD INTVCC TG1 BOOST1 SW1 BG1 TG2 BOOST2 SW2 BG2 PGND FREQ SENSE2+ SENSE2– RUN2 EXTVCC VFB2 ITH2 SGND TK/SS2 0.1µF D4 LTC3869 10Ω 8mΩ 1000pF 10Ω 15pF VOUT1 3.3V 5A MODE/PLLIN ILIM SENSE1+ SENSE1– RUN1 VFB1 ITH1 TK/SS1 0.1µF 10Ω 1000pF 10Ω 10pF VOUT2 5V 5A 8mΩ + 90.9k 1% COUT1 220µF 20k 1% 1000pF 100pF 10k 1% 1000pF 122k 1% 15k 1% 100pF 147k 1% 20k 1% + COUT2 150µF 3869 TA02 L1: TDK RLF 7030T-2R2M5R4 L2: TDK ULF10045T-3R3N6R9 COUT1: SANYO 4TPE220MF COUT2: SANYO 6TPE150MI RELATED PARTS PART NUMBER DESCRIPTION LTC3850/ LTC3850-1/ LTC3850-2 LTC3860 LTC3855 LTC3890 LTC3856 LTC3853 LTC3851A/ LTC3851A-1 LTC3833 Dual 2-Phase, High Efficiency Synchronous Step-Down DC/DC Controller, RSENSE or DCR Current Sensing and Tracking Dual, Multiphase, Synchronous Step-Down DC/DC Controller with Differential Amplifier and Tri-State Output Drive Dual, Multiphase, Synchronous Step-Down DC/DC Controller with Differential Amplifier and DCR Temperature Compensation Dual, High VIN Low IQ Synchronous Step-Down DC/DC Controller 2-Phase, Single Output Synchronous Step-Down DC/DC Controller with Differential Amplifier and DCR Temperature Compensation COMMENTS Phase-Lockable Fixed 250kHz to 780kHz Frequency, 4V ≤ VIN ≤ 30V, 0.8V ≤ VOUT ≤ 5.25V Operates with Power Blocks, DRMOS Devices or External Drivers/ MOSFETs, 3V ≤ VIN ≤ 24V, tON(MIN) = 20ns Phase-Lockable Fixed Frequency 250kHz to 770kHz, 4.5V ≤ VIN ≤ 38V, 0.8V ≤ VOUT ≤ 12V PLL Capable Fixed Frequency 50kHz to 900kHz, 4V ≤ VIN ≤ 60V, 0.8V ≤ VOUT ≤ 24V, IQ = 50µA Phase-Lockable Fixed 250kHz to 770kHz Frequency, 4.5V ≤ VIN ≤ 38V, 0.8V ≤ VOUT ≤ 5V Triple Output, Multiphase Synchronous Step-Down DC/DC Controller, Phase-Lockable Fixed 250kHz to 750kHz Frequency, RSENSE or DCR Current Sensing and Tracking 4V ≤ VIN ≤ 24V, VOUT3 Up to 13.5V No RSENSE™ Wide VIN Range Synchronous Step-Down DC/DC Controller Fast Controlled On-Time, High Frequency Synchronous Step-Down Controller with Differential Amplifier PLL Fixed Frequency 250kHz to 750kHz, 4V ≤ VIN ≤ 38V, 0.8V ≤ VOUT ≤ 5.25V, MSOP-16E, 3mm × 3mm QFN-16, SSOP-16 Up to 2MHz Operating Frequency, 4V ≤ VIN ≤ 38V, 0.8V ≤ VOUT ≤ 5.5V, 3mm × 4mm QFN-20, TSSOP-20E 3869f 40 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● LT 0211 • PRINTED IN USA www.linear.com  LINEAR TECHNOLOGY CORPORATION 2011