LT3781EG

LT3781 “Bootstrap” Start Dual Transistor Synchronous Forward Controller FEATURES s s DESCRIPTIO s s s s s s s s s High Voltage Operation up to 72V Synchronizable Operating Frequency and Output Switch Phase for Multiple Controller Systems Synchronous Switch Output Undervoltage Lockout Protection with 6V Hysteresis for Self-Biased Power Fixed Frequency Operation to 350kHz Local ±1% Voltage Reference Input Overvoltage Protection Low Start-Up Current Programmable Start Inhibit for Power Supply Sequencing and Protection Optocoupler Support Soft-Start Control Isolated Telecommunication Systems Personal Computers and Peripherals Distributed Power Step-Down Converters Lead Acid Battery Backup Systems Automotive and Heavy Equipment The LT®3781 controller simplifies the design of high power synchronous dual transistor forward DC/DC converters. The part employs fixed frequency current mode control and supports both isolated and nonisolated topologies. The IC drives external N-channel power MOSFETs and operates with input voltages up to 72V. The LT3781 is ideal for output derived power schemes, through the use of a large undervoltage lockout hysteresis range. The part is also equipped with an 18V VCC shunt regulator, which prevents exceeding absolute maximum ratings while in trickle start applications. The LT3781’s operating frequency is programmable and can be synchronized up to 350kHz. Switch phase is also controlled during synchronized operation to accommodate multiple-converter systems. Internal logic guarantees 50% maximum duty cycle operation to prevent transformer saturation. The LT3781 is available in a 20-lead SSOP package. , LTC and LT are registered trademarks of Linear Technology Corporation. APPLICATIO S s s s s s TYPICAL APPLICATIO VIN+ L1 4.7µH 36V-72V to 5V/7A DC/DC Synchronous Forward Converter (Quarter-Brick Footprint) T1 6 Q1 MURS120T3 7 5 MURS120T3 R9 0.025Ω 1/2W 8 4 9 Q5 ZVN3310F • + C2 22µF 100V C3 1.5µF 100V C4 1.5µF 100V • • 1 2 3 Q3 10 11 12 1nF 100V 1nF 100V 10Ω 0.25W • VIN– BAT54 1OV BIAS MMBD914LT1 270k 0.25W 73.2k 1% 20k 13 2 1 0.1µF 68µF 20V 1nF 1.24k 1% 10k 1µF BAS21 20 VCC VBST OVLO 10k 0.1µF 100V 330pF 5V OUT 3.3k 0.047µF 51Ω 6.8k 1k 1% LTC1693-2 6 VCC1 VCC2 5 IN2 OUT2 7 1 IN1 OUT1 2 4 GND2 GND1 8 3 1OV BIAS FZT690 4.7µF 16V 19 18 LT3781 15 11 14 PGND 12 SG 9 TG BSTREF BG SENSE SHDN 5VREF FSET THERM SYNC SGND SS VC VFB 3 7 4 8 10 6 5 52.3k 150pF + 100Ω 4.7nF 0.01µF 3.01k 1% C2:SANYO 100MV22AX C3, C4: VITRAMON VJ1825Y155MXB C5: 4X KEMET T510X337KO10AS L1: COILCRAFT DO1608C-472 U L2 4.7µH 10Ω 0.25W VOUT = 5V IOUT = 7A VOUT+ MBR0540T1 Q6 U U + C5 330µF 10V 4.7Ω VOUT– 100Ω CMPZ5242B 12V 2k 0.22µF 50V L2: PANASONIC ETQP6F4R1LF4 Q1,Q3:100V SILICONIX SUD40N10-25 Q5,Q6: SILICONIX Si4450 T1:COILTRONICS VP5-1200 3781f 1 LT3781 ABSOLUTE (Note 1) AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW SHDN OVLO THERM SGND 5VREF FSET SYNC SS VFB 1 2 3 4 5 6 7 8 9 20 VBST 19 TG 18 BSTREF 17 NC 16 NC 15 BG 14 PWRGND 13 VCC 12 SG 11 SENSE Power Supply (VCC) Low Impedance Source Voltage ............. –0.3V to 20V Shutdown Mode: (Supply Self-Regulates to 18V) Maximum Input Current ............................... 20mA Topside Supply (VBST) .................................................... VBSTREF – 0.3V to VBSTREF +20V (VBST(MAX) = 90V) Topside Reference Pin (VBSTREF) ............... –0.6V to 75V SHDN Pin Voltage ........................... –0.3V to VCC + 0.3V All Other Input Voltages .............. –0.3V to 5VREF + 0.3V 5VREF Pin Sink Current ......................................... 10mA FSET Pin Current ...................................... –2mA to 5mA All Other Input Pin Currents ...................... –2mA to 2mA Operating Ambient Temperature Range (Note 4) ...............–40°C to 85°C Operating Junction Temperature Range ...............................–40°C to 125°C Storage Temperature Range ..................–65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ORDER PART NUMBER LT3781EG LT3781IG VC 10 G PACKAGE 20-LEAD PLASTIC SSOP TJMAX = 125°C, θJA = 90°C/W Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS SYMBOL PARAMETER Supply and Protection VCCUVLO Undervoltage Lockout Threshold VCCSHDN ICC Shutdown Mode Shunt Regulator DC Active Supply Current The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF. CONDITIONS Falling Edge Rising Edge 100µA < IVCC ≤ 10mA (Note 2) VSHDN = 1.35V, VCC = 8V VSHDN < 0.3V TG Logic High (Note 2) VSHDN < 0.3V q q q q MIN 8.0 13 16.5 TYP 8.4 14.5 18 17 800 16 5.0 0.1 1.25 150 – 10 225 6.4 7.0 0.6 5.0 45 1 MAX 8.6 16 19.9 22 25 1200 30 8.5 1.35 200 –6 7.1 7.5 UNITS V V V mA mA µA µA mA µA V mV µA mV V V V V V mA Ω 3781f DC Active UVLO Supply Current DC Standby Supply Current VBST DC Active Supply Current DC Standby Supply Current VSHDN Shutdown Rising Threshold Shutdown Threshold Hysteresis ISS Soft-Start Charge Current VSS Soft-Start Reset Threshold VBSTUVLO Boost Undervoltage Lockout (VBST-BSTREF) Boost UVLO Hysteresis 5V External Reference V5VREF 5V Reference Voltage I5VREFSC R5VREF Short-Circuit Current Output Impedance q q q q VSS = 2V Falling Edge Rising Edge q q q q 1.15 100 –14 5.7 6.5 0.3 4.85 4.80 20 0 ≤ (I5VREF – IVC) < 20mA q 5.10 5.15 Source, IVC = 0 0 ≤ (I5VREF – IVC) < 20mA q 2 U W U U WW W LT3781 ELECTRICAL CHARACTERISTICS SYMBOL Error Amp VFB IFB AV IVC VVC GBW AV ISENSE VSENSE tD tMIN VTHERM/ VOVLO tD fOSC IFSET VSYNC fSYNC tH, L Error Amplifier Reference Voltage Feedback Input Current Error Amplifier Voltage Gain Error Amplifier Current Limit Zero Current Output Voltage Maximum Output Voltage Gain Bandwidth Product Amplifier DC Gain Input Bias Current Current Limit Threshold Current Sense to Switch Delay Switch Minimum On Time Threshold (Rising Edge) Threshold Hysteresis Fault Delay to Output Disable Oscillator Frequency, Free Run Frequency Programming Error FSET Input Bias Current SYNC Logic High Input Threshold SYNC Logic Low Input Threshold SYNC Frequency Maximum SYNC Pulse Width (Logic High or Logic Low) TG On Voltage TG Off Voltage TG Rise/Fall Time BG On Voltage BG Off Voltage BG Rise/Fall Time SG On Voltage SG Off Voltage SG Rise/Fall Time SG to BG Enable Lag Time TG to BG Enable Lag Time PARAMETER The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VTS = 0V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF. CONDITIONS Measured at Feedback Pin q MIN 1.242 1.225 TYP 1.250 –50 72 MAX 1.258 1.265 UNITS V V nA dB mA mA V V MHz V/V µA VFB = VREF Source Sink q q 10 0.5 25 1 1.4 3.2 (Note 3) 1 12 – 275 Current Sense Measured at SENSE Pin q 135 130 150 175 165 170 mV mV ns ns Measured at BG Output q q 250 1.2 20 1.25 40 650 700 q THERM and OVLO Fault Detectors 1.3 60 V mV ns kHz % nA 2 350 1/fOSC V V kHz s >50mV Overdrive Measured at FSET Pin fOSC ≤ 500kHz (Note 3) FSET Charging, VFSET = 2V Positive-Going Edge Negative-Going Edge fOSC = Oscillator Free-Run Frequency q q q Oscillator and Synchronization Decoder –10 50 0.8 fOSC/2 1.4 1.4 5 Output Drivers VTG tTGr/f VBG tBGr/f VSG tSGr/f tSG-BG tTG-BG q q 11 11.5 0.1 35 11.5 0.1 35 11.5 0.1 35 150 100 0.5 12 0.5 12 0.5 300 V V ns V V ns V V ns ns ns 3781f 10% to 90%/90% to 10% q q 11 10% to 90%/90% to 10% q q 11 10% to 90%/90% to 10% 4V On/Off Thresholds 4V On/Off Thresholds q 80 3 LT3781 ELECTRICAL CHARACTERISTICS Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Supply current specification does not include external FET gate charge currents. Actual supply currents will be higher and vary with operating frequency, operating voltages, and the type of external switch elements used. See Applications Information. Note 3: Guaranteed but not tested. Note 4: The LT3781E is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating ambient temperature range are assured by design, characterization and correlation with statistical process controls. For guaranteed performance to specifications over the –40°C to 85°C operating ambient temperature range, the LT3781I is available. TYPICAL PERFOR A CE CHARACTERISTICS Shutdown Mode: VCC vs ICC 18.4 TA = 25°C 18.20 18.2 VCC (V) 18.0 17.8 17.6 100µ 300µ 1m ICC (A) VCC (V) ICC Supply Current vs Temperature 20 VCC = 12V ICC SUPPLY CURRENT (mA) ICC SUPPLY CURRENT (mA) 19 18 17 16 15 –55 –40 40 80 TEMPERATURE (°C) 4 UW 0 Shutdown Mode: VCC vs Temperature (ICC = 1mA) 18.15 18.10 18.05 3m 10m 3781 • G01a 18.00 –55 –40 40 80 0 TEMPERATURE (°C) 125 3781 • G01b ICC Supply Current vs VCC Supply Voltage 18 TA = 25°C 17 16 125 3781 G01 15 9 10 12 14 16 SUPPLY VOLTAGE (V) 18 3781 G03 3781f LT3781 TYPICAL PERFOR A CE CHARACTERISTICS IBST Boost Supply Current vs Temperature 5.2 IBST BOOST SUPPLY CURRENT (mA) 5.1 40 UVLO ICC SUPPLY CURRENT (mA) ICC SUPPLY CURRENT (µA) 5.0 4.9 4.8 –55 –40 40 80 TEMPERATURE (°C) 0 5VREF Voltage vs Temperature 5VREF SHORT-CIRCUIT CURRENT LIMIT (mA) 5.10 60 5.05 ERROR AMP REFERENCE (V) 5VREF VOLTAGE (V) 5.00 4.95 4.90 –55 –40 40 80 TEMPERATURE (°C) 0 VC Pin Short-Circuit Current Limit vs Temperature VC PIN SHORT-CIRCUIT CURRENT LIMIT (mA) 25 12 SOFT-START OUTPUT CURRENT (µA) 11 20 SOFT-START OUTPUT CURRENT (µA) 15 10 –55 –40 40 80 TEMPERATURE (°C) 0 UW 3781 G04 3781 G07 3781 G10 ICC Supply Current vs SHDN Pin Voltage 60 TA = 25°C 1 UVLO ICC Supply Current vs Temperature 0.8 20 0.6 125 0 0 0.2 0.4 0.6 0.8 1.0 SHDN PIN VOLTAGE (V) 1.2 3781 G05 0.5 –55 –40 0 40 80 TEMPERATURE (°C) 125 3781 G06 5VREF Short-Circuit Current Limit vs Temperature 1.260 Error Amp Reference vs Temperature 1.255 50 1.250 40 1.245 125 30 –55 –40 40 80 TEMPERATURE (°C) 0 125 3781 G08 1.240 –55 –40 0 40 80 TEMPERATURE (°C) 125 3781 G09 Soft-Start Output Current vs Temperature 60 Soft-Start Output Current vs Soft-Start Pin Voltage TA = 25°C VSS = 2V 40 10 20 9 125 8 –55 –40 0 40 80 TEMPERATURE (°C) 0 125 3781 G11 0 100 200 300 400 SOFT-START PIN VOLTAGE (mV) 500 3781 G12 3781f 5 LT3781 TYPICAL PERFOR A CE CHARACTERISTICS Soft-Start Output Current vs Soft-Start Pin Voltage 60 CURRENT SENSE AMP BANDWIDTH (MHz) 5 SOFT-START OUTPUT CURRENT (µA) 40 20 0 0 1 2 3 4 SOFT-START PIN VOLTAGE (V) PI FU CTIO S SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding positive going threshold of 1.25V enables the LT3781. 150mV of input hysteresis resists mode switching instability. The SHDN pin can be controlled by either a logic level input or with an analog signal. This shutdown feature is typically used for input supply undervoltage protection. A resistor divider from the converter input supply to the SHDN pin monitors that supply for control of system power-up sequencing, etc. An 18V clamp on the VCC pin is enabled during shutdown mode, preventing a trickle start circuit from pulling that pin above maximum operational levels. All other internal functions are disabled during shutdown. OVLO (Pin 2): Overvoltage Shutdown Sense. Typically connected to input supply through a resistor divider. If pin voltage exceeds 1.25V, LT3781 switching function is disabled to protect boosted circuitry from exceeding absolute maximum voltage. 40mV of input hysteresis resists mode switching instability. Exceeding the OVLO threshold also triggers soft-start reset, resulting in a graceful recovery from an input transient event. THERM (Pin 3): System Thermal Shutdown. Auxiliary shutdown pin that is typically used for system thermal protection. If pin voltage exceeds 1.25V, LT3781 switching function is disabled. 40mV of input hysteresis resists mode switching instability. Exceeding the THERM threshold also triggers soft-start reset, resulting in a graceful recovery. SGND (Pin 4): Signal Ground Reference. Careful board layout techniques must be used to prevent corruption of signal ground reference. High-current switching paths must be oriented on the converter ground plane such that currents to/from the switches do not affect the integrity of the LT3781 signal ground reference. 5VREF (Pin 5): 5V Local Reference. Allows connection of external loads up to 20mA DC. Typically bypassed with 1µF ceramic capacitor to SGND. Reference output is current limit protected to a typical value of 45mA. If the load on the 5V reference exceeds the current limit value, LT3781 switching function is disabled and the soft-start function is reset. FSET (Pin 6): Oscillator Timing Pin. Connect a resistor (RFSET) from the 5VREF pin to this pin and a capacitor (CFSET) from this pin to ground. 3781f 6 UW Current Sense Amplifier Bandwidth vs Temperature TA = 25°C 8 7 6 5 4 3 2 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 3781 G14 3781 G13 U U U LT3781 PI FU CTIO S The LT3781 oscillator operates by monitoring the voltage on CFSET as it is charged via RFSET. When the voltage on the FSET pin reaches 2.5V, the oscillator rapidly discharges the capacitor with an average current of about 0.8mA. Once the voltage on the pin is reduced to 1.5V, the pin becomes high-impedance and the charging cycle repeats. The oscillator operates at twice the switching frequency of the controller. Oscillator frequency fOSC can be approximated by the relation: –1   R  2    ≅ 0.5 • 10–6 + C FSET  FSET +  8 • 10– 4 +    RFSET    3     –1 fOSC SYNC (Pin 7): Oscillator Synchronization Input Pin with TTL-Level Compatible Input. The SYNC input signal (at the desired synchronized operating frequency) controls both the internal oscillator (running at twice the SYNC frequency) and the output switch phase. If synchronization function is not desired, this pin may be floated or shorted to ground. The LT3781 internal oscillator drives a toggle flip-flop that assures a ≤ 50% duty-cycle condition during oscillator free-run. The oscillator, therefore, runs at twice the operating frequency of the controller. The SYNC input decoder incorporates a frequency doubling circuit for oscillator synchronization, resetting the internal oscillator on both the rising and falling edges of the input signal. The SYNC input decoder also differentiates transition phase and forces the toggle flip-flop to phase-lock with the SYNC input. A transition to logic high on the SYNC input signal corresponds to the initiation of a new switching cycle (primary switches turning on pending current control) and a transition to logic low forces a primary switch off state. As such, the maximum operating duty cycle is equal to the duty cycle of the SYNC signal. The SYNC input can therefore be used to reduce the maximum duty cycle of the controller by reducing the duty cycle of the SYNC input. U U U SS (Pin 8): Soft-Start. Connect a capacitor (CSS) from this pin to ground. The output voltage of the LT3781 error amplifier corresponds to the peak current sense amplifier output detected before resetting the switch outputs. The soft-start circuit forces the error amplifier output to a zero sense current for start-up. A 10µA current is forced from this pin onto an external capacitor. As the SS pin voltage ramps up, so does the LT3781 internally sensed current limit. This effectively forces the internal current limit to ramp from zero, allowing overall converter current to slowly increase until normal output regulation is achieved. This function reduces output overshoot on converter start-up. The softstart functions incorporate a 1VBE “dead zone” such that a zero-current condition is maintained on the VC pin until the SS pin rises to 1VBE above ground. The SS pin voltage is reset to start-up condition during shutdown, undervoltage lockout, and overvoltage or overcurrent events, yielding a graceful converter output recovery from these events. VFB (Pin 9): Error Amplifier Inverting Input. Typically connected to a resistor divider from the output and compensation components to the VC pin. The VFB pin is the converter output voltage feedback node. Input bias current of ~50nA forces pin high in the event of an open feedback path condition. The error amplifier is internally referenced to 1.25V. Values for the VOUT to VFB feedback resistor (RFB1) and the VFB to ground resistor (RFB2) can be calculated to program converter output voltage (VOUT) via the following relation: VOUT = 1.25 • (RFB1 + RFB2)/RFB2 VC (Pin 10): Error Amplifier Output. The LT3781 error amplifier is a low impedance output inverting gain stage. The amplifier has ample current source capability to allow easy integration of isolation optocouplers that require bias currents up to 10mA. External DC loading of the VC pin reduces the external current sourcing capacity of the 5VREF pin by the same amount as the load on the VC pin. 3781f 7 LT3781 PI FU CTIO S The error amplifier is typically configured using a feedback RC network to realize an integrator circuit. This circuit creates the dominant pole for the converter regulation feedback loop. Integrator characteristics are dominated by the value of the capacitor connected from the VC pin to the VFB pin and the feedback resistor connected to the VFB pin. Specific integrator characteristics can be configured to optimize transient response. The error amplifier can also be configured as a transimpedance amplifier for use in secondary-side controller applications. (See the Applications Information section for configuration and compensation details) SENSE (Pin 11): C urrent Sense Amplifier (CSA) Noninverting Input. Current is monitored via a ground referenced current sense resistor, typically in series with the source of the bottom side switch FET. Internal current limit circuitry provides for a maximum peak value of 150mV across the sense resistor during normal operation. SG (Pin 12): Synchronous Switch Output Driver. This pin can be connected directly to gate of synchronous switch if small FETs are used (CGATE < 5000pF), however, the use of a gate drive buffer is recommended for peak efficiencies. The SG pin output is synchronized and out-of-phase with the BG output. The control timing of the SG output cause it to “lead” the primary switch path during turn-on by 150nS. VCC (Pin 13): IC Local Power Supply Input. Bypass with at a capacitor at least 10 times greater than C5VREF. LT3781 incorporates undervoltage lockout that disables switching functions if VCC is below 8.4V. The LT3781 supports operational VCC power supply voltages from 9V to 18V (20V absolute maximum). An 18V clamp on the VCC pin is enabled during shutdown mode, preventing a trickle start circuit from pulling that pin above maximum operational levels during IC shutdown. PWRGND (Pin 14): Output Driver Ground Reference. Connect through low impedance trace to VIN decoupling capacitor. BG (Pin 15): Bottom Side Primary Switch/Forward Switch Output Driver. This pin can be connected directly to gate(s) of primary bottom side and forward switches if small FETs are used (CGATE total < 5000pF), however, the use of a gate drive buffer is recommended for peak efficiencies. The BG output is enabled at the start of each oscillator cycle in phase with the TG pin but is timed to “lag” the TG output during turn-on and “lead” the TG output during turn-off. These delays force the concentration of transitional losses onto the bottom side primary switch. An adaptive blanking circuit disables the current sense function (via the SENSE pin) while the BG pin is below 5V. BSTREF (Pin 18): VBST Supply Reference. Typically connects to source of topside external power FET switch. TG (Pin 19): Topside (Boosted) Primary Output Driver. This pin can be connected directly to gate of primary topside switch if small FETs are used (CGATE < 5000pF), however, the use of a gate drive buffer is recommended for peak efficiencies. VBST (Pin 20): Topside Primary Driver Bootstrapped Supply. This “boosted” supply rail is referenced to the BSTREF pin. Supply voltage is maintained by a bootstrap capacitor tied from the VBST pin to the boosted supply reference (BSTREF) pin. The charge on the capacitor is refreshed each switch cycle through a Schottky diode connected from the VCC supply (cathode) to the VBST pin (anode). The bootstrap capacitor (CBOOST) must be at least 100 times greater than the total load capacitance on the TG pin. A capacitor in the range of 0.1µF to 1.0µF is generally adequate for most applications. The bootstrap diode must have a reverse breakdown voltage greater than the converter VIN. The LT3781 supports operational VBST supply voltages up to 90V (absolute maximum) referenced to ground. Undervoltage Lockout disables the topside switch until VBST – BSTREF > 7.0V for start-up protection of the topside switch. 8 U U U 3781f 20 VBST 19 TG 18 BSTREF NOL LOGIC FSET 6 f = ×2 T Q S R Q PHASE DETECT S 12 SG VC 10 SENSE 11 5VREF BLANKING VFB 9 ERROR AMP VCC 13 1.25V 18V SHDN 1 + UVLO (