MAX15046

19-4719; Rev 0; 7/09 40V, High-Performance, Synchronous Buck Controller General Description The MAX15046 synchronous step-down controller operates from a 4.5V to 40V input voltage range and generates an adjustable output voltage from 85% of the input voltage down to 0.6V, supporting loads up to 25A. The device allows monotonic startup into a prebiased bus without discharging the output and features adaptive internal digital soft-start. The MAX15046 offers the ability to adjust the switching frequency from 100kHz to 1MHz with an external resistor. The MAX15046’s adaptive synchronous rectification eliminates the need for an external freewheeling Schottky diode. The device also utilizes the external low-side MOSFET’s on-resistance as a current-sense element, eliminating the need for a current-sense resistor. This protects the DC-DC components from damage during output overloaded conditions or output shortcircuit faults without requiring a current-sense resistor. Hiccup-mode current limit reduces power dissipation during short-circuit conditions. The MAX15046 includes a power-good output and an enable input with precise turn-on/turn-off threshold, which can be used for input supply monitoring and for power sequencing. Additional protection features include sink-mode current limit, and thermal shutdown. Sink-mode current limit prevents reverse inductor current from reaching dangerous levels when the device is sinking current from the output. The MAX15046 is available in a 16-pin QSOP or 16-pin QSOP-EP package and operates over the -40NC to +125NC temperature range. Pin Configurations appear at end of data sheet. S S S S S Features S Input Voltage Ranges from 4.5V to 40V or 5V MAX15046 S S S S S S S Q10% Adjustable Outputs from 0.85 x VIN Down to 0.6V Adjustable Switching Frequency (100kHz to 1MHz) with Q10% (1MHz) Accuracy Adaptive Internal Digital Soft-Start Up to 25A Output Capability Cycle-by-Cycle Valley-Mode Current Limit with Adjustable, Temperature-Compensated Threshold (30mV to 300mV) Monotonic Startup into Prebiased Output Q1% Accurate Voltage Reference 3A-Peak Gate Drivers Hiccup-Mode Short-Circuit Protection (PatentPending Architecture) Overtemperature Shutdown Power-Good (PGOOD) Output and Enable Input (EN) with Q5% Accurate Threshold Thermally Enhanced 16-Pin QSOP Package Applications Industrial Power Supplies (PLC, Industrial Computers, Fieldbus Components, Fieldbus Couplers) Telecom Power Supplies Base Stations Ordering Information PART MAX15046AAEE+ TEMP RANGE -40°C to +125°C PIN-PACKAGE 16 QSOP MAX15046BAEE+ -40°C to +125°C 16 QSOP-EP* +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. Typical Operating Circuit 4.5V TO 40V VIN C1 IN VCC ON OFF PGOOD EN LIM R4 C5 R3 C6 C7 R1 R3 COMP FB RT MAX15046 CSP DH L1 LX C2 BST DL D1 DRV C4 PGND GND R5 Q2 C3 Q1 0.6V TO 0.85V x VIN VOUT R2 _______________________________________________________________ Maxim Integrated Products 1 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 40V, High-Performance, Synchronous Buck Controller MAX15046 ABSOLUTE MAXIMUM RATINGS IN to GND .............................................................. -0.3V to +45V VCC to GND..................... -0.3V to lower of (VIN + 0.6V) and 6V EN, DRV to GND ..................................................... -0.3V to +6V PGOOD to GND .................................................... -0.3V to +45V PGND to GND ...................................................... -0.3V to +0.3V DL to PGND............................................. -0.3V to (VDRV + 0.3V) BST to PGND ....................................................... -0.3V to +50V LX and CSP to PGND............................................... -1V to +45V LX and CSP to PGND............................-2V (50ns max) to +45V BST to LX................................................................. -0.3V to +6V CSP to LX ............................................................. -0.3V to +0.3V DH to LX .................................................. -0.3V to (VBST + 0.3V) All Other Pins to GND .............................. -0.3V to (VCC + 0.3V) VCC Short Circuit to GND.......................................... Continuous PGOOD Maximum Sink Current .........................................20mA Continuous Power Dissipation (TA = +70NC): 16-Pin QSOP (derate 9.6mW/NC above +70NC) ....... 771.5mW 16-Pin QSOP-EP (derate 22.7mW/NC above +70NC) 1818.2mW Junction-to-Case Thermal Resistance (θJC) (Note 1) 16-Pin QSOP ................................................................ 37NC/W 16-Pin QSOP-EP ............................................................ 6NC/W Junction-to-Ambient Thermal Resistance (θJA) (Note 1) 16-Pin QSOP ........................................................... 103.7NC/W 16-Pin QSOP-EP .......................................................... 44NC/W Operating Temperature Range ........................ -40NC to +125NC Junction Temperature .....................................................+150NC Storage Temperature Range............................ -65NC to +150NC Lead Temperature (soldering, 10s) ................................+300NC Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer board. For detailed information on package thermal considerations, refer to http://www.maxim-ic.com/thermal-tutorial. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VIN = 24V, VEN = 5V, VGND = VPGND = 0V, CIN = 1FF, CVCC = 4.7FF, RRT = 49.9kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER SYSTEM SPECIFICATIONS Input-Voltage Range Quiescent Supply Current Shutdown Supply Current VCC REGULATOR Output Voltage VCC Regulator Dropout VCC Short-Circuit Output Current VCC Undervoltage Lockout VCC Undervoltage Lockout Hysteresis ERROR AMPLIFIER (FB, COMP) FB Input-Voltage Set Point FB Input Bias Current FB to COMP Transconductance Open-Loop Gain Unity-Gain Bandwidth Capacitor from COMP to GND = 47pF VFB IFB gM VFB = 0.6V ICOMP = Q20FA 584 -250 600 1200 80 5 590 596 +250 1800 mV nA FS dB MHz VCCUVLO VVCC 6V ≤ VIN ≤ 40V, ILOAD = 6mA VIN = 4.5V, ILOAD = 25mA VIN = 5V VVCC rising 30 3.8 5 5.25 0.18 55 4 400 5.5 0.45 90 4.2 V V mA V mV VIN IIN_Q IIN_SBY 4.5 VIN = VCC = VDRV VIN = 24V, VFB = 0.9V, no switching VIN = 24V, VEN = 0V, IVCC = 0, PGOOD = unconnected 4.5 2 0.35 40 5.5 3 0.55 V mA mA SYMBOL CONDITIONS MIN TYP MAX UNITS 2 ______________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller ELECTRICAL CHARACTERISTICS (continued) (VIN = 24V, VEN = 5V, VGND = VPGND = 0V, CIN = 1FF, CVCC = 4.7FF, RRT = 49.9kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER VCOMP-RAMP Minimum Voltage COMP Source/Sink Current ENABLE (EN) EN Input High EN Input Low EN Input Leakage Current OSCILLATOR Switching Frequency (100kHz) Switching Frequency (300kHz) Switching Frequency (1MHz) Switching Frequency Adjustment Range RT Voltage PWM MODULATOR PWM Ramp Peak-to-Peak Amplitude PWM Ramp Valley Minimum Controllable On-Time Maximum Duty Cycle Minimum Low-Side On-Time OUTPUT DRIVERS/DRIVERS SUPPLY (VDRV) Undervoltage Lockout DRV Undervoltage Lockout Hysteresis Low, sinking 100mA, VBST - VLX = 5V High, sourcing 100mA, VBST - VLX = 5V Low, sinking 100mA, VDRV = VCC = 5.25V High, sourcing 100mA, VDRV = VCC = 5.25V Sinking, VBST - VLX = 5V Sourcing, VBST - VLX = 5V VDRV_UVLO fSW = 300kHz (RRT = 49.9kI) fSW = 1MHz (RRT = 49.9kI) VDRV rising 85 VRAMP VVALLEY 1.5 1.5 70 87.5 110 4.0 4.2 400 1 1.5 1 1.5 3 A 2 3 4 I 3 4 4.4 125 V V ns % ns V mV VRT fSW fSW fSW RRT = 150kI RRT = 49.9kI RRT = 14.3kI (Note 3) RRT = 49.9kI 80 270 0.9 100 1.15 1.2 100 300 1 120 330 1.1 1000 1.25 kHz kHz MHz kHz V VEN_H VEN_L IEN VEN rising VEN falling VEN = 5.5V -1 1.14 1.20 1.05 +1 1.26 V V FA ICOMP VCOMP = 1.4V 50 SYMBOL CONDITIONS MIN TYP 200 80 110 MAX UNITS mV FA MAX15046 DH On-Resistance DL On-Resistance DH Peak Current CLOAD = 10nF _______________________________________________________________________________________ 3 40V, High-Performance Synchronous Buck Controller MAX15046 ELECTRICAL CHARACTERISTICS (continued) (VIN = 24V, VEN = 5V, VGND = VPGND = 0V, CIN = 1FF, CVCC = 4.7FF, RRT = 49.9kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER SYMBOL CONDITIONS Sinking, VDRV = VCC = 5.25V Sourcing, VDRV = VCC = 5.25V MIN TYP 3 A 2 10 ns MAX UNITS DL Peak Current CLOAD = 10nF DH, DL Break-Before-Make Time (Dead Time) SOFT-START Soft-Start Duration Reference Voltage Steps CURRENT LIMIT/HICCUP Cycle-by-Cycle Valley CurrentLimit Threshold Adjustment Range LIM Reference Current LIM Reference Current Temperature Coefficient CSP Input Bias Current Number of Consecutive CurrentLimit Events to Hiccup Hiccup Timeout Peak Low-Side Sink Current-Limit Threshold POWER-GOOD (PGOOD) PGOOD Threshold PGOOD Threshold Hysteresis PGOOD Output Low Voltage PGOOD Output Leakage Current THERMAL SHUTDOWN Thermal Shutdown Threshold Thermal Shutdown Hysteresis Note 3: Select RRT as: RRT = 17.3 × 10 9 2048 64 Switching Cycles Steps VCSP - VPGND, valley limit = VLIM/10 ILIM VLIM = 0.3V VLIM = 3V 45 30 mV 300 50 2300 55 FA ppm/NC +1 7 4096 FA Events Switching Cycles mV VLIM = 0.3V to 3V, TA = +25NC VCSP = 40V -1 VCSP - VPGND, sink limit = VLIM/20, RILIM = 30kI, VLIM = 1.5V, TA = +25NC VFB rising VFB falling VPGOOD_L ILEAK_PGOOD IPGOOD = 2mA, VEN = 0V VPGOOD = 40V, VEN = 5V, VFB = 1V Temperature rising -1 90 75 94 2.65 97.5 0.4 +1 %VFB %VFB V FA NC NC +150 20 Note 2: All devices are 100% tested at room temperature and guaranteed by design over the specified temperature range. fSW + (1 x 10 -7 )(fSW 2 ) , where fSW is in Hertz. 4 ______________________________________________________________________________________ 40V, High-Performance Synchronous Buck Controller Typical Operating Characteristics (VIN = 24V, TA = +25NC, unless otherwise noted.) MAX15046 EFFICIENCY vs. LOAD CURRENT (VIN = 24V) MAX15046 toc01 EFFICIENCY vs. LOAD CURRENT (VIN = 12V) MAX15046 toc02 VOUT vs. LOAD CURRENT 0.45 % OUTPUT FROM NORMAL 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0 MAX15046 toc03 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 3 6 9 12 VOUT = 3.3V V OUT = 1.8V VOUT = 5V VOUT = 1.2V 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 VOUT = 1.8V VOUT = 1.2V VOUT = 3.3V 0.50 VOUT = 5V 15 0 3 6 9 12 15 0 2 4 6 8 10 12 LOAD CURRENT (A) LOAD CURRENT (A) LOAD CURRENT (A) VCC vs. LOAD CURRENT MAX15046 toc04 VCC LINE REGULATION MAX15046 toc05 VCC vs. TEMPERATURE ILOAD = 5mA MAX15046 toc06 5.256 5.254 5.252 5.266 5.265 5.264 VCC (V) 5.2 5.0 4.8 4.6 4.4 IVCC = 40mA 4.2 4.0 IVCC = 5mA, 10mA, 20mA, 30mA VCC (V) VCC (V) 5.250 5.248 5.246 5.244 5.242 0 5 10 15 20 25 30 35 40 45 LOAD CURRENT (mA) 5.263 5.262 5.261 5.260 0 5 10 15 20 VIN (V) 25 30 35 40 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (NC) SWITCHING FREQUENCY vs. RRT MAX15046 toc07 SWITCHING FREQUENCY vs. TEMPERATURE MAX15046 toc08 IIN vs. SWITCHING FREQUENCY CDH = CDL = 0 2.7 2.6 IIN (mA) 2.5 2.4 2.3 2.2 MAX15046 toc09 1000 900 800 FREQUENCY (kHz) 700 600 500 400 300 200 100 0 0 40 80 RRT (kI) 120 1200 1000 FREQUENCY (kHz) 800 600 400 200 0 RT = 49.9kI RT = 150kI RT = 14.3kI RT = 25.5kI 2.8 160 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (NC) 100 FREQUENCY (kHz) 1000 _______________________________________________________________________________________ 5 40V, High-Performance, Synchronous Buck Controller MAX15046 Typical Operating Characteristics (continued) (VIN = 24V, TA = +25NC, unless otherwise noted.) LIM REFERENCE CURRENT vs. TEMPERATURE MAX15046 toc10 75 70 65 60 55 50 45 40 0.15 CURRENT-LIMIT THRESHOLD (V) 0.10 0.05 0 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -0.35 0 10 20 30 40 50 60 SOURCE CURRENT LIMIT SINK CURRENT LIMIT MAX15046 toc11 80 SINK AND SOURCE CURRENT LIMIT THRESHOLDS vs. RESISTANCE (RILIM) 0.20 LOAD TRANSIENT ON OUT (1A TO 10A) MAX15046 toc12a VOUT 200mV/div CURRENT (FA) IOUT 5A/div -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (NC) 70 200Fs/div RESISTANCE (kI) LOAD TRANSIENT ON OUT (1A TO 15A) MAX15046 toc12b LOAD TRANSIENT ON OUT (1A TO 6A) MAX15046 toc12c STARTUP DISABLE FROM EN (RLOAD = 3.3I) MAX15046 toc13 VOUT 200mV/div VOUT 100mV/div IN 20V/div EN 5V/div IOUT 5A/div IOUT 5A/div VOUT 2V/div PGOOD 5V/div 4ms/div 200Fs/div 200Fs/div STARTUP AND DISABLE FROM IN (RLOAD = 3.3kI) STARTUP WITH PREBIASED OUTPUT (4.0V) MAX15046 toc15a MAX15046 toc14 STARTUP WITH PREBIASED OUTPUT (1.0V) MAX15046 toc15b 24V IN 10V/div 4V VOUT 2V/div PGOOD 5V/div 24V IN 10V/div 1V VOUT 2V/div PGOOD 5V/div IN 10V/div VOUT 2V/div PGOOD 5V/div 20ms/div 4ms/div 4ms/div 6 ______________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller Typical Operating Characteristics (continued) (VIN = 24V, TA = +25NC, unless otherwise noted.) MAX15046 STARTUP WITH PREBIASED OUTPUT (2V) MAX15046 toc15c STARTUP WITH PREBIASED OUTPUT (0.5V) MAX15046 toc15d SINK CURRENT-LIMIT WAVEFORMS (5V PREBIASED) MAX15046 toc16 24V IN 10V/div 2V VOUT 2V/div PGOOD 5V/div VOUT = 3.3V 4ms/div 4ms/div VOUT = 3.3V 24V VIN 10V/div 0.5V VOUT 2V/div PGOOD 5V/div VLX 50V/div ILX 5A/div VOUT 500mV/div 200Fs/div DH_ AND DL_ DISOVERLAP MAX15046 toc17a DH_ AND DL_ DISOVERLAP MAX15046 toc17b VDH 20V/div VDL 5V/div VDH 20V/div VDL 5V/div VLX 20V/div VLX 20V/div 40ns/div 40ns/div OUTPUT SHORT-CIRCUIT BEHAVIOUR MONITOR OUTPUT VOLTAGE AND CURRENT MAX15046 toc18 LINE-TRANSIENT RESPONSE MAX15046 toc19 24V VOUT 200mV/div VIN 200mV/div 5V IOUT 10A/div VOUT (AC-COUPLED) 200mV/div 10ms/div 10ms/div _______________________________________________________________________________________ 7 40V, High-Performance, Synchronous Buck Controller MAX15046 Pin Description PIN 1 NAME IN FUNCTION Regulator Input. Connect to the input rail of the buck converter. Bypass IN to PGND with a 100nF minimum ceramic capacitor. When operating in the 5V Q10% range, connect IN to VCC. 5.25V Linear Regulator Output. Bypass VCC to PGND with a ceramic capacitor of at least 4.7FF when VCC supplies MOSFET gate-driver current at DRV or 2.2FF when VCC is not used to power DRV. Open-Drain Power-Good Output. Pull up PGOOD to an external power supply or output with an external resistor. Active-High Enable Input. Pull EN to GND to disable the buck converter output. Connect to VCC for always-on operation. EN can be used for power sequencing and as a UVLO adjustment input. Current-Limit Input. Connect a resistor from LIM to GND to program the current-limit threshold from 30mV (RLIM = 6kI) to 300mV (RLIM = 60kI). Error-Amplifier Output. Connect compensation network from COMP to FB or from COMP to GND. Feedback Input (Inverting Input of Error Amplifier). Connect FB to a resistive divider between the buck converter output and GND to adjust the output voltage from 0.6V up to 0.85 x IN. Oscillator-Timing Resistor Input. Connect a resistor from RT to GND to set the oscillator frequency from 100kHz to 1MHz. Analog Ground. Connect PGND and AGND together at a single point. Power Ground. Use PGND as a return path for the low-side MOSFET gate driver. Gate-Driver Supply Voltage. DRV is internally connected to the low-side driver supply. Bypass DRV to PGND with a 2.2FF minimum ceramic capacitor (see the Typical Application Circuits). Low-Side External MOSFET Gate-Driver Output. DL swings from DRV to PGND. Boost Flying Capacitor Connection. Internally connected to the high-side driver supply. Connect a ceramic capacitor of at least 100nF between BST and LX and a diode between BST and DRV for the high-side MOSFET gate-driver supply. Inductor Connection. Also serves as a return terminal for the high-side MOSFET driver current. Connect LX to the switching side of the inductor. High-Side External MOSFET Gate-Driver Output. DH swings from BST to LX. Current-Sense Positive Input. Connect to the drain of low-side MOSFET with Kelvin connection. Exposed Pad. EP is internally connected to ground. Connect EP to a large copper ground plane to maximize thermal performance. 2 VCC 3 4 5 6 7 8 9 10 11 12 13 PGOOD EN LIM COMP FB RT GND PGND DRV DL BST 14 15 16 — LX DH CSP EP 8 ______________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller Functional Diagram MAX15046 VREF RT OSCILLATOR CK HICCUP VREF SOFT-START/STOP AND HICCUP LOGIC FB GM DAC_VREF HICCUP TIMEOUT DH_DL_ENABLE COMP ENABLE COMPARATOR EN EN_INT VREF BGAP_OK BANDGAP OK GENERATOR BGAP_OK OSC ENABLE CK ENABLE PWM COMPARATOR VREF CK RAMP GENERATOR RAMP PWM V_BGAP BST HIGHSIDE DRIVER HICCUP DH VIN_OK V_BGAP INTERNAL VOLTAGE REGULATOR DC-DC OSCILLATOR AND ENABLE LOGIC VL_OK CK DH_DL_ENABLE HICCUP TIMEOUT PWM CONTROL LOGIC LX VCC BGAP_OK VCC UVLO CSP DRV BGAP_OK V_DRV DRV UVLO VDRV_OK SINK CURRENT-LIMIT COMPARATOR SHUTDOWN LIM/20 LOWSIDE DRIVER DL PGND FB VIN_OK LIM THERMAL SHUTDOWN AND ILIM CURRENT GENERATOR VALLEY CURRENT-LIMIT COMPARATOR VIN_OK VIN_OK IBIAS MAIN BIAS CURRENT GENERATOR LIM/10 VREF ENABLE PGOOD IN IN UVLO PGOOD COMPARATOR GND VREF = 0.6V MAX15046 VBGAP = 1.24V BANDGAP REFERENCE _______________________________________________________________________________________ 9 40V, High-Performance, Synchronous Buck Controller MAX15046 Detailed Description The MAX15046 synchronous step-down controller operates from a 4.5V to 40V input-voltage range and generates an adjustable output voltage from 85% of the inputvoltage down to 0.6V while supporting loads up to 25A. As long as the device supply voltage is within 5.0V to 5.5V, the input power bus (VIN) can be as low as 3.3V. The MAX15046 offers adjustable switching frequency from 100kHz to 1MHz with an external resistor. The adjustable switching frequency provides design flexibility in selecting passive components. The MAX15046 adopts an adaptive synchronous rectification to eliminate external freewheeling Schottky diodes and improve efficiency. The device utilizes the on-resistance of the external low-side MOSFET as a current-sense element. The current-limit threshold voltage is resistor-adjustable from 30mV to 300mV and is temperature-compensated, so that the effects of the MOSFET RDS(ON) variation over temperature are reduced. This current-sensing scheme protects the external components from damage during output overloaded conditions or output shortcircuit faults without requiring a current-sense resistor. Hiccup-mode current limit reduces power dissipation during short-circuit conditions. The MAX15046 includes a power-good output and an enable input with precise turn-on/-off threshold to be used for monitoring and for power sequencing. The MAX15046 features internal digital soft-start that allows prebias startup without discharging the output. The digital soft-start function employs sink current limiting to prevent the regulator from sinking excessive current when the prebias voltage exceeds the programmed steadystate regulation level. The digital soft-start feature prevents the synchronous rectifier MOSFET and the body diode of the high-side MOSFET from experiencing dangerous levels of current while the regulator is sinking current from the output. The MAX15046 shuts down at a +150NC junction temperature to prevent damage to the device. The MAX15046 step-down controller uses a PWM voltage-mode control scheme (see the Functional Diagram). Control-loop compensation is external for providing maximum flexibility in choosing the operating frequency and output LC filter components. An internal transconductance error amplifier produces an integrated error voltage at COMP that helps to provide higher DC accuracy. The voltage at COMP sets the duty cycle using a PWM comparator and a ramp generator. On the rising edge of an internal clock, the high-side n-channel MOSFET turns on and remains on until either the appropriate duty cycle or the maximum duty cycle is reached. During the on-time of the high-side MOSFET, the inductor current ramps up. During the second-half of the switching cycle, the high-side MOSFET turns off and the low-side n-channel MOSFET turns on. The inductor releases the stored energy as the inductor current ramps down, providing current to the output. Under overload conditions, when the inductor current exceeds the selected valley current-limit threshold (see the Current-Limit Circuit (LIM) section), the high-side MOSFET does not turn on at the subsequent clock rising edge and the low-side MOSFET remains on to let the inductor current ramp down. An internal linear regulator (VCC) provides a 5.25V nominal supply to power the internal functions and to drive the low-side MOSFET. Connect IN and VCC together when using an external 5V Q10% power supply. The maximum regulator input voltage (VIN) is 40V. Bypass IN to GND with a 1FF ceramic capacitor. Bypass the output of the linear regulator (VCC) with a 4.7FF ceramic capacitor to GND. The VCC dropout voltage is typically 180mV. When VIN is higher than 5.5V, VVCC is typically 5.25V. The MAX15046 also employs an undervoltage lockout circuit that disables the internal linear regulator when VVCC falls below 3.6V (typical). The 400mV UVLO hysteresis prevents chattering on power-up/power-down. DH and DL are optimized for driving large-size n-channel power MOSFETs. Under normal operating conditions and after startup, the DL low-side drive waveform is always the complement of the DH high-side drive waveform, with controlled dead time to prevent crossconduction or “shoot-through.” An adaptive dead-time circuit monitors the DH and DL outputs and prevents the opposite-side MOSFET from turning on until the MOSFET is fully off. Thus, the circuit allows the high-side driver to turn on only when the DL gate driver has turned off preventing the low side (DL) from turning on until the DH gate driver has turned off. The adaptive driver dead time allows operation without shoot-through with a wide range of MOSFETs, minimizing delays and maintaining efficiency. There must be a low-resistance, low-inductance path from DL and DH to the MOSFET gates for the adaptive dead-time circuits Internal 5.25V Linear Regulator MOSFET Gate Drivers (DH, DL) DC-DC PWM Controller 10 _____________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller to function properly. The stray impedance in the gate discharge path can cause the sense circuitry to interpret the MOSFET gate as “off” while the VGS of the MOSFET is still high. To minimize stray impedance, use very short, wide traces. Synchronous rectification reduces conduction losses in the rectifier by replacing the normal low-side Schottky catch diode with a low-resistance MOSFET switch. The MAX15046 features a robust internal pulldown transistor with a typical 1I RDS(ON) to drive DL low. This low onresistance prevents DL from being pulled up during the fast rise time of the LX node, due to capacitive coupling from the drain to the gate of the low-side synchronous rectifier MOSFET. An external Schottky diode between BST and DH is required to boost the gate voltage above LX providing the necessary gate-to-source voltage to turn on the highside MOSFET. The boost capacitor connected between BST and LX holds up the voltage across the floating gate driver during the high-side MOSFET on-time. The charge lost in the boost capacitor for delivering the gate charge is replenished when the high-side MOSFET turns off and the LX node goes to ground. When LX is low, the external diode between VDRV and BST recharges the boost capacitor. See the Boost Capacitor and Diode Selection section in the Applications Information to choose the right boost capacitor and diode. Drive EN high to turn on the MAX15046. A soft-start sequence starts to increase (step-wise) the reference voltage of the error amplifier. The duration of the softstart ramp is 2048 switching cycles and the resolution is 1/64th of the steady-state regulation voltage allowing a smooth increase of the output voltage. A logic-low on EN initiates a soft-stop sequence by stepping down the reference voltage of the error amplifier. After the soft-stop sequence is completed, the MOSFET drivers are both turned off. See Figure 1. Connect EN to VCC for always-on operation. Owing to the accurate turn-on/-off thresholds, EN can be used as a UVLO adjustment input, and for power sequencing together with the PGOOD outputs. F G H I MAX15046 Enable Input (EN), Soft-Start, and Soft-Stop High-Side Gate-Drive Supply (BST) UVLO VCC A B C D E EN VOUT 2048 CLOCK CYCLES DAC_VREF 2048 CLOCK CYCLES DH DL SYMBOL UVLO VCC EN VOUT DAC_VREF DH DL A DEFINITION Undervoltage threshold value is provided in the Electrical Characteristics table. Internal 5.25V linear regulator output. Active-high enable input. Regulator output voltage. Regulator internal soft-start and soft-stop signal. Regulator high-side gate-driver output. Regulator low-side gate-driver output. VCC rising while below the UVLO threshold. EN is low. SYMBOL B C D E F G H I DEFINITION VCC is higher than the UVLO threshold. EN is low. EN is pulled high. DH and DL start switching. Normal operation. VCC drops below UVLO. VCC goes above the UVLO threshold. DH and DL start switching. Normal operation. EN is pulled low. VOUT enters soft-stop. EN is pulled high. DH and DL start switching. Normal operation. VCC drops below UVLO. Figure 1. Power On-Off Sequencing ______________________________________________________________________________________ 11 40V, High-Performance, Synchronous Buck Controller When the valley current limit is reached during soft-start, the MAX15046 regulates to the output impedance times the limited inductor current and turns off after 4096 clock cycles. When starting up into a large capacitive load (for example), the inrush current will not exceed the currentlimit value. If the soft-start is not completed before 4096 clock cycles, the device turns off. The device remains off for 8192 clock cycles before trying to soft-start again. This implementation allows the soft-start time to be automatically adapted to the time necessary to keep the inductor current below the limit while charging the output capacitor. The MAX15046 includes a power-good comparator to monitor the output voltage and detect the power-good threshold, fixed at 93% of the nominal FB voltage. The open-drain PGOOD output requires an external pullup resistor. PGOOD sinks up to 2mA of current while low. PGOOD goes high (high-Z) when the regulator output increases above 93% of the designed nominal regulated voltage. PGOOD goes low when the regulator output voltage drops to below 90% of the nominal regulated voltage. PGOOD asserts low during the hiccup timeout period. When the MAX15046 starts into a prebiased output, DH and DL are off so that the converter does not sink current from the output. DH and DL do not start switching until the PWM comparator commands the first PWM pulse. The first PWM pulse occurs when the ramping reference voltage increases above the FB voltage. When the output voltage is biased above the output set point, the controller tries to pull the output down to the set point once the internal soft-start is complete. This pulldown is controlled by the sink current limit, which is slowly increased to its normal value to minimize output undershoot. The current-limit circuit employs a ‘valley’ and sink current-sensing algorithm that uses the on-resistance of the low-side MOSFET as a current-sensing element, to eliminate costly sense resistors. The current-limit circuit is also temperature compensated to track the on-resistance variation of the MOSFET overtemperature. The current limit is adjustable with an external resistor at LIM and accommodates MOSFETs with a wide range of on-resistance characteristics (see the Setting the Valley Current Limit section). The adjustment range is from 0.3V MAX15046 to 3V for the valley current limit, corresponding to resistor values of 6kI to 60kI. The valley current-limit threshold across the low-side MOSFET is precisely 1/10th of the voltage at LIM, while the sink current-limit threshold is 1/20th of the voltage at LIM. Valley current limit acts when the inductor current flows towards the load, and CSP is more negative than PGND during the low-side MOSFET on-time. If the magnitude of the current-sense signal exceeds the valley current-limit threshold at the end of the low-side MOSFET on-time, the MAX15046 does not initiate a new PWM cycle and lets the inductor current decay in the next cycle. The controller also ‘rolls back’ the internal reference voltage so that the controller finds a regulation point determined by the current-limit value and the resistance of the short. In this manner, the controller acts as a constant current source. This method greatly reduces inductor ripple current during the short event, which reduces inductor sizing restrictions and reduces the possibility for audible noise. After 4096 clock cycles, the device goes into hiccup mode. Once the short is removed, the internal reference voltage soft-starts back up to the normal reference voltage and regulation continues. Sink current limit is implemented by monitoring the voltage drop across the low-side MOSFET when CSP is more positive than PGND. When the voltage drop across the low-side MOSFET exceeds 1/20th of the voltage at LIM at any time during the low-side MOSFET on-time, the low-side MOSFET turns off and the inductor current flows from the output through the body diode of the high-side MOSFET. When the sink current limit activates, the DH/ DL switching sequence is no longer complementary and both MOSFETs are turned off. Carefully observe the PCB layout guidelines to ensure that noise and DC errors do not corrupt the currentsense signals at CSP and PGND. Mount the MAX15046 close to the low-side MOSFET with short, direct traces making a Kelvin-sense connection so that trace resistance does not add to RDS(ON) of the low-side MOSFET. Hiccup mode overcurrent protection reduces power dissipation during prolonged short-circuit or severe overload conditions. An internal 3-bit counter counts up on each switching cycle when the valley current-limit threshold is reached. The counter counts down on each switching cycle when the threshold is not reached, and stops at zero (000). When the current-limit condition persists and the counter reaches 111 (= 7 events), the Power-Good Output (PGOOD) Startup into a Prebiased Output Current-Limit Circuit (LIM) Hiccup Mode Overcurrent Protection 12 _____________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller MAX15046 stops both DL and DH drivers and waits for 4096 switching cycles (hiccup timeout delay) before attempting a new soft-start sequence. The hiccup-mode protection remains active during the soft-start time. The MAX15046 provides an internal undervoltage lockout (UVLO) circuit to monitor the voltage on VCC. The UVLO circuit prevents the MAX15046 from operating when VCC is lower than VUVLO. The UVLO threshold is 4V, with 400mV hysteresis to prevent chattering on the rising/falling edge of the supply voltage. DL and DH stay low to inhibit switching when the device is in undervoltage lockout. Thermal-overload protection limits total power dissipation in the MAX15046. When the junction temperature of the device exceeds +150NC, an on-chip thermal sensor shuts down the device, forcing DL and DH low, which allows the device to cool. The thermal sensor turns the device on again after the junction temperature cools by 20NC. The regulator shuts down and soft-start resets during thermal shutdown. Power dissipation in the LDO regulator and excessive driving losses at DH/DL trigger thermal-overload protection. Carefully evaluate the total power dissipation (see the Power Dissipation section) to avoid unwanted triggering of the thermal-overload protection in normal operation. The maximum voltage conversion ratio is limited by the maximum duty cycle (Dmax): VOUT D × VDROP2 + (1-D max ) × VDROP1 < D max - max VIN VIN where VDROP1 is the sum of the parasitic voltage drops in the inductor discharge path, including synchronous rectifier, inductor, and PCB resistance. VDROP2 is the sum of the resistance in the charging path, including high-side switch, inductor, and PCB resistance. In practice, provide adequate margin to the above conditions for good load-transient response. Set the MAX15046 output voltage by connecting a resistive divider from the output to FB to GND (Figure 2). Select R2 from between 4kI and 16kI. Calculate R1 with the following equation:  V  R1 = R 2  OUT  -1 VFB      where VFB = 0.59V (see the Electrical Characteristics table) and VOUT can range from 0.6V to (0.85 O VIN). Resistor R1 also plays a role in the design of the Type III compensation network. Review the values of R1 and R2 when using a Type III compensation network (see the Type III Compensation Network (Figure 4) section). MAX15046 Undervoltage Lockout Thermal-Overload Protection Setting the Output Voltage Applications Information The MAX15046 operates from 4.5V to 40V input supplies and regulates output down to 0.6V. The minimum voltage conversion ratio (VOUT/VIN) is limited by the minimum controllable on-time. For proper fixed-frequency PWM operation, the voltage conversion ratio must obey the following condition: VOUT > t ON(MIN) × fSW VIN Effective Input-Voltage Range OUT R1 FB R2 MAX15046 where tON(MIN) is 125ns and fSW is the switching frequency in Hertz. Pulse skipping occurs to decrease the effective duty cycle when the desired voltage conversion does not meet the above condition. Decrease the switching frequency or lower the input voltage VIN to avoid pulse skipping. Figure 2. Adjustable Output Voltage ______________________________________________________________________________________ 13 40V, High-Performance, Synchronous Buck Controller MAX15046 An external resistor connecting RT to GND sets the switching frequency (fSW). The relationship between fSW and RRT is: 17.3 × 10 9 R RT = fSW + (1x10 -7 ) x (fSW 2 ) Setting the Switching Frequency saturation current rating (ISAT) must be high enough to ensure that saturation cannot occur below the maximum current-limit value (ICL(MAX)), given the tolerance of the on-resistance of the low-side MOSFET and of the LIM reference current (ILIM). Combining these conditions, select an inductor with a saturation current (ISAT) of: I SAT ≥ 1.35 × I CL(TYP) where ICL(TYP) is the typical current-limit set point. The factor 1.35 includes RDS(ON) variation of 25% and 10% for the LIM reference current error. A variety of inductors from different manufacturers are available to meet this requirement (for example, Vishay IHLP-4040DZ-1-5 and other inductors from the same series). The minimum current-limit threshold must be high enough to support the maximum expected load current with the worst-case low-side MOSFET on-resistance value as the RDS(ON) of the low-side MOSFET is used as the currentsense element. The inductor’s valley current occurs at ILOAD(MAX) minus one half of the ripple current. The minimum value of the current-limit threshold voltage (VITH) must be higher than the voltage on the low-side MOSFET during the ripple-current valley,  LIR  VITH > R DS(ON,MAX) × ILOAD(MAX) × 1 − 2   where RDS(ON,MAX) in I is the maximum on-resistance of the low-side MOSFET at maximum load current ILOAD(MAX) and is calculated from the following equation: R DS(ON,MAX) = R DS(ON) × [1 + TC MOSFET × (TMAX - TAMB )] where fSW is in kHz and RRT is in kI. For example, a 300kHz switching frequency is set with RRT = 49.9kI. Higher frequencies allow designs with lower inductor values and less output capacitance. Peak currents and I2R losses are lower at higher switching frequencies, but core losses, gate-charge currents, and switching losses increase. Three key inductor parameters must be specified for operation with the MAX15046: inductance value (L), inductor saturation current (ISAT), and DC resistance (RDC). To determine the inductance, select the ratio of inductor peak-to-peak AC current to DC average current (LIR) first. For LIR values that are too high, the RMS currents are high, and therefore I2R losses are high. Use high-valued inductors to achieve low LIR values. Typically, inductor resistance is proportional to inductance for a given package type, which again makes I2R losses high for very low LIR values. A good compromise between size and loss is a 30% peak-to-peak ripple current to average-current ratio (LIR = 0.3). The switching frequency, input voltage, output voltage, and selected LIR determine the inductor value as follows: L= VOUT (VIN - VOUT ) VIN × fSW × I OUT × LIR Inductor Selection Setting the Valley Current Limit where VIN, VOUT, and IOUT are typical values. The switching frequency is set by RT (see Setting the Switching Frequency section). The exact inductor value is not critical and can be adjusted to make trade-offs among size, cost, and efficiency. Lower inductor values minimize size and cost, but also improve transient response and reduce efficiency due to higher peak currents. On the other hand, higher inductance increases efficiency by reducing the RMS current. Find a low-loss inductor with the lowest possible DC resistance that fits in the allotted dimensions. The where RDS(ON) (in I is the on-resistance of the lowside MOSFET at ambient temperature TAMB (in degrees Celsius), TCMOSFET is the temperature coefficient of the low-side MOSFET in ppm/NC, and TMAX (in degrees Celsius) is the temperature at maximum load current ILOAD(MAX). Obtain the RDS(ON) and TCMOSFET from the MOSFET data sheet. 14 _____________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller Connect an external resistor (RLIM) from LIM to GND to adjust the current-limit threshold, which is temperature-compensated with a temperature coefficient of -2300ppm/NC. The relationship between the current-limit threshold (VITH) and RLIM is: R LIM = 50 × 10 −6 The output-voltage ripple as a consequence of the ESR and the output capacitance is: ∆VESR = IP-P × ESR ∆VQ = 8 × C OUT × fSW IP-P MAX15046 10 × VITH × 1 + 2300 × (TMAX - TAMB)     V - VOUT   VOUT  IP-P =  IN ×   fSW × L   VIN  where RLIM is in I, VITH is in V, TMAX and TAMB are in NC. An RLIM resistance range of 6kI to 60kI corresponds to a current-limit threshold of 30mV to 300mV. Use 1% tolerance resistors when adjusting the current limit to minimize error in the current-limit threshold. The input filter capacitor reduces peak current drawn from the power source and reduces noise and voltage ripple on the input caused by the switching circuitry. The input capacitor must meet the ripple current requirement (IRMS) imposed by the switching currents as defined by the following equation: IRMS = ILOAD(MAX) VOUT (VIN - VOUT ) VIN where IP-P is the peak-to-peak inductor current ripple (see the Inductor Selection section). Use these equations for initial capacitor selection. Decide on the final values by testing a prototype or an evaluation circuit. Check the output capacitor against load-transient response requirements. The allowable deviation of the output voltage during fast load transients determines the capacitor output capacitance, ESR, and equivalent series inductance (ESL). The output capacitor supplies the load current during a load step until the controller responds with a higher duty cycle. The response time (tRESPONSE) depends on the closed-loop bandwidth of the converter (see the Compensation Design section). The resistive drop across the ESR of the output capacitor, the voltage drop across the ESL (DVESL) of the capacitor, and the capacitor discharge, cause a voltage droop during the load step. Use a combination of low-ESR tantalum/aluminum electrolytic and ceramic capacitors for improved transient load and voltage ripple performance. Nonleaded capacitors and capacitors in parallel help reduce the ESL. Keep the maximum output-voltage deviation below the tolerable limits of the load. Use the following equations to calculate the required ESR, ESL, and capacitance value during a load step: ESR = ∆VESR I STEP Input Capacitor IRMS attains a maximum value when the input voltage equals twice the output voltage (VIN = 2VOUT), so IRMS(MAX) = ILOAD(MAX)/2. For most applications, nontantalum capacitors (ceramic, aluminum, polymer, or OS-CON) are preferred at the inputs due to the robustness of nontantalum capacitors to accommodate high inrush currents of systems being powered from very low impedance sources. Additionally, two (or more) smallervalue low-ESR capacitors should be connected in parallel to reduce high-frequency noise. The key selection parameters for the output capacitor are capacitance value, ESR, and voltage rating. These parameters affect the overall stability, output ripple voltage, and transient response. The output ripple has two components: variations in the charge stored in the output capacitor, and the voltage drop across the capacitor’s ESR caused by the current flowing into and out of the capacitor: DVRIPPLE = DVESR + DVQ Output Capacitor I ×t C OUT = STEP RESPONSE ∆VQ ESL = ∆VESL × t STEP I STEP 1 3 × fO t RESPONSE ≅ where ISTEP is the load step, tSTEP is the rise time of the load step, tRESPONSE is the response time of the controller, and fO is the closed-loop crossover frequency. ______________________________________________________________________________________ 15 40V, High-Performance, Synchronous Buck Controller MAX15046 The MAX15046 provides an internal transconductance amplifier with the inverting input and the output available for external frequency compensation. The flexibility of external compensation offers wide selection of output filtering components, especially the output capacitor. Use high-ESR aluminum electrolytic capacitors for costsensitive applications. Use low-ESR tantalum or ceramic capacitors at the output for size-sensitive applications. The high switching frequency of the MAX15046 allows the use of ceramic capacitors at the output. Choose all passive power components to meet the output ripple, component size, and component cost requirements. Choose the compensation components for the error amplifier to achieve the desired closed-loop bandwidth and phase margin. To choose the appropriate compensation network type, the power-supply poles and zeros, the zero-crossover frequency, and the type of the output capacitor must be determined first. In a buck converter, the LC filter in the output stage introduces a pair of complex poles at the following frequency: fPO = 1 2π × L OUT × C OUT Compensation Design 2) The gain at the frequency where the phase shift is -180N (gain margin) must be less than 1. Maintain a phase margin of around 60N to achieve a robust loop stability and well-behaved transient response. When using an electrolytic or large-ESR tantalum output capacitor, the capacitor ESR zero fZO typically occurs between the LC poles and the crossover frequency fO (fPO < fZO < fO). Choose the Type II (PI-Proportional, Integral) compensation network. When using a ceramic or low-ESR tantalum output capacitor, the capacitor ESR zero typically occurs above the desired crossover frequency fO, that is fPO < fO < fZO. Choose the Type III (PID- Proportional, Integral, and Derivative) compensation network. If fZO is lower than fO and close to fPO, the phase lead of the capacitor ESR zero almost cancels the phase loss of one of the complex poles of the LC filter around the crossover frequency. Use a Type II compensation network with a midband zero and a high-frequency pole to stabilize the loop. In Figure 3, RF and CF introduce a midband zero (fZ1). RF and CCF in the Type II compensation network provide a high-frequency pole (fP1), which mitigates the effects of the output high-frequency ripple. Use the following steps to calculate the component values for Type II compensation network as shown in Figure 3: 1) Calculate the gain of the modulator (GAINMOD), comprised of the regulator’s pulse-width modulator, LC filter, feedback divider, and associated circuitry at crossover frequency: GAINMOD = VIN V ESR × × FB VRAMP (2π × fO × L OUT ) VOUT Type II Compensation Network (Figure 3) The output capacitor introduces a zero at: 1 fZO = 2π × ESR × C OUT where ESR is the equivalent series resistance of the output capacitor. The loop-gain crossover frequency (fO), where the loop gain equals 1 (0dB) should be set below 1/10th of the switching frequency: f fO ≤ SW 10 Choosing a lower crossover frequency reduces the effects of noise pickup into the feedback loop, such as jittery duty cycle. To maintain a stable system, two stability criteria must be met: 1) The phase shift at the crossover frequency, fO, must be less than 180N. In other words, the phase margin of the loop must be greater than zero. where VIN is the input voltage of the regulator, VRAMP is the amplitude of the ramp in the pulse-width modulator, VFB is the FB input voltage set point (0.6V typically, see the Electrical Characteristics table), and VOUT is the desired output voltage. The gain of the error amplifier (GAINEA) in midband frequencies is: GAINEA = gM x RF where gM is the transconductance of the error amplifier. 16 _____________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller The total loop gain, which is the product of the modulator gain and the error-amplifier gain at fO, is: 1) GAINMOD × GAINEA = 1 So : VIN V ESR × × FB × g M × R F = 1 VOSC (2π × fO × L OUT ) VOUT Solving for R F : RF = VOSC × (2π × fO × L OUT ) × VOUT VFB × VIN × g M × ESR When using a low-ESR tantalum or ceramic type, the ESR-induced zero frequency is usually above the targeted zero crossover frequency (fO). Use Type III compensation. Type III compensation provides two zeros and three poles at the following frequencies: fZ1 = fZ2 = 1 2π × R F × C F 1 2π × CI × (R1 + RI ) Type III Compensation Network (See Figure 4) MAX15046 2) Set a midband zero (fZ1) at 0.75 x fPO (to cancel one of the LC poles): fZ1 = Solving for CF: CF = 1 2π × R F × fPO × 0.75 1 = 0.75 × fPO 2π × R F × C F Two midband zeros (fZ1 and fZ2) cancel the pair of complex poles introduced by the LC filter: fP1 = 0 fP1 introduces a pole at zero frequency (integrator) for nulling DC output-voltage errors: fP2 = 1 2 π × RI × C I 3) Place a high-frequency pole at fP1 = 0.5 x fSW (to attenuate the ripple at the switching frequency fSW) and calculate CCF using the following equation: C CF = 1 π × R F × fSW 1 CF Depending on the location of the ESR zero (fZO), use fP2 to cancel fZO, or to provide additional attenuation of the high-frequency output ripple: 1 fP3 = C × C CF 2π × R F × F C F + C CF fP3 attenuates the high-frequency output ripple. Place the zeros and poles such that the phase margin peaks around fO. Ensure that RF >> 2/gM and the parallel resistance of R1, R2, and RI is greater than 1/gM. Otherwise, a 180N phase shift is introduced to the response making the loop unstable. VOUT R1 R2 VREF gM RF CF COMP Use the following compensation procedures: 1) With RF >> 10kI, place the first zero (fZ1) at 0.8 x fPO: CCF fZ1 = 1 = 0.8 × fPO 2π × R F × C F Figure 3. Type II Compensation Network ______________________________________________________________________________________ 17 40V, High-Performance, Synchronous Buck Controller MAX15046 So: CF = 1 2π × R F × 0.8 × fPO Once fP2 is known, calculate RI: RI = 1 2π × fP2 × CI MAX15046 2) The gain of the modulator (GAINMOD), comprised of the pulse-width modulator, LC filter, feedback divider, and associated circuitry at crossover frequency is: 4) Place the second zero (fZ2) at 0.2 x fO or at fPO, whichever is lower and calculate R1 using the following equation: R1 = 1 - RI 2π × fZ2 × CI GAINMOD = VIN 1 × VRAMP (2π × f ) 2 × L O OUT × C OUT The gain of the error amplifier (GAINEA) in midband frequencies is: GAINEA = 2π × fO × CI × R F The total loop gain as the product of the modulator gain and the error amplifier gain at fO is 1. GAINMOD × GAINEA = 1 So : VIN 1 × × 2π × fO × CI × R F = 1 VRAMP (2π × f ) 2 × C O OUT × L OUT Solving for CI : V × (2π × fO × L OUT × C OUT ) CI = RAMP VIN × R F 5) Place the third pole (fP3) at one half the switching frequency and calculate CCF: C CF = (2π × 0.5 × fSW × R F × C F ) - 1 CF 6) Calculate R2 as: R2 = VFB × R1 VOUT − VFB The MAX15046 step-down controller drives two external logic-level n-channel MOSFETs. The key selection parameters to choose these MOSFETs include: U On-resistance (RDS(ON)) U Maximum Drain-to-Source Voltage (VDS(MAX)) U Minimum Threshold Voltage (VTH(MIN) U Total Gate Charge (QG) U Reverse Transfer Capacitance (CRSS) U Power Dissipation MOSFET Selection 3) Use the second pole (fP2) to cancel fZO when fPO < fO < fZO < fSW/2. The frequency response of the loop gain does not flatten out soon after the 0dB crossover, and maintains -20dB/decade slope up to 1/2 of the switching frequency. This is likely to occur if the output capacitor is low-ESR tantalum. Set fP2 = fZO. When using a ceramic capacitor, the capacitor ESR zero (fZO) is likely to be located even above one half of the switching frequency, fPO < fO < fSW/2 < fZO. In this case, place the frequency of the second pole (fP2) high enough in order not to significantly erode the phase margin at the crossover frequency. For example, set fP2 at 5 x fO so that the contribution to phase loss at the crossover frequency fO is only about 11N: fP2 = 5 x fO The two n-channel MOSFETs must be a logic-level type with guaranteed on-resistance specifications at VGS = 4.5V. For maximum efficiency, choose a highside MOSFET that has conduction losses equal to the switching losses at the typical input voltage. Ensure that the conduction losses at minimum input voltage do not exceed the MOSFET package thermal limits, or violate the overall thermal budget. Also ensure that the conduction losses plus switching losses at the maximum input voltage do not exceed package ratings or violate the 18 _____________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller VOUT CCF Use a low-ESR ceramic capacitor as the boost capacitor with a minimum value of 100nF. CF MAX15046 RF RI CI R1 A small-signal diode can be used for the bootstrap circuit and must have a minimum voltage rating of VIN + 3V to withstand the maximum BST voltage. The average forward current of the diode should meet the following requirement: IF > QGATE x fSW COMP R2 VREF gM where QGATE is the gate charges of the low-side MOSFET. The maximum power dissipation of the device depends on the thermal resistance from the die to the ambient environment and the ambient temperature. The thermal resistance depends on the device package, PCB copper area, other thermal mass, and airflow. The power dissipated into the package (PT) depends on the supply configuration (see the Typical Application Circuits). Use the following equation to calculate power dissipation: PT = VIN x [QG_TOTAL x fSW + IQ] where IQ is the quiescent supply current at the switching frequency. See the IIN vs. Switching Frequency graph in the Typical Operating Characteristics for the IQ. Use the following equation to estimate the temperature rise of the die: TJ = TA + (PT x BJA) where BJA is the junction-to-ambient thermal impedance of the package, PT is power dissipated in the device, and TA is the ambient temperature. The BJA is 103.7NC/W for the 16-pin QSOP and 53NC/W for the 16-pin QSOPEP package on multilayer boards, with the conditions specified by the respective JEDEC standards (JESD51-5, JESD51-7). An accurate estimation of the junction temperature requires a direct measurement of the case temperature (TC) when actual operating conditions significantly deviate from those described in the JEDEC standards. The junction temperature is then: TJ = TC + (PT x BJC) Use 37NC/W as BJC thermal impedance for the 16-pin QSOP package and 6NC/W for the 16-pin QSOP-EP package. The case-to-ambient thermal impedance (BCA) is dependent on how well the heat is transferred from the PCB to the ambient. Use large copper areas to keep the PCB temperature low. Power Dissipation Figure 4. Type III Compensation Network overall thermal budget. Ensure that the DL gate driver can drive the low-side MOSFET. In particular, check that the dv/dt caused by the high-side MOSFET turning on does not pull up the low-side MOSFET gate through the drain-to-gate capacitance of the low-side MOSFET, which is the most frequent cause of crossconduction problems. Check power dissipation when using the internal linear regulator to power the gate drivers. Select MOSFETs with low gate charge so that VCC can power both drivers without overheating the device: PDRIVE = VCC x QG_TOTAL x fSW where QG_TOTAL is the sum of the gate charges of the two external MOSFETs. The MAX15046 uses a bootstrap circuit to generate the necessary gate-to-source voltage to turn on the high-side MOSFET. The selected n-channel high-side MOSFET determines the appropriate boost capacitance value (CBST in the Typical Application Circuits) according to the following equation: C BST = QG ∆VBST Boost Capacitor and Diode Selection where QG is the total gate charge of the high-side MOSFET and DVBST is the voltage variation allowed on the high-side MOSFET driver after turn-on. Choose DVBST such that the available gate-drive voltage is not significantly degraded (e.g. DVBST = 100mV to 300mV) when determining CBST. ______________________________________________________________________________________ 19 40V, High-Performance, Synchronous Buck Controller MAX15046 Careful PCB layout is critical to achieve clean and stable operation. The switching power stage requires particular attention. Follow these guidelines for good PCB layout: PCB Layout Guidelines 1) Place decoupling capacitors as close as possible to the IC. Connect the power ground plane (connected to PGND) and signal ground plane (connected to GND) at one point near the device. 2) Connect input and output capacitors to the power ground plane; connect all other capacitors to the signal ground plane. 3) Keep the high-current paths as short and wide as possible. Keep the path of switching current (C2 to IN and C2 to PGND) short. Avoid vias in the switching paths. 4) Connect CSP to the drain of the low-side FET using a Kelvin connection for accurate current-limit sensing. 5) Ensure all feedback connections are short and direct. Place the feedback resistors as close as possible to the IC. 6) Route high-speed switching nodes (BST, LX, DH, and DL) away from sensitive analog areas (RT, FB, COMP, and LIM). Typical Application Circuit 1 in the Typical Application Circuits section shows an application circuit that operates out of 24V and outputs up to 10A at 3.3V. R5 sets the switching frequency to 350kHz. Typical Application Circuit 2 in the Typical Application Circuits section shows an application circuit for a single +4.5V to +5.5V power-supply operation. Typical Application Circuit 3 in the Typical Application Circuits section shows an application circuit for a +24V supply to drive the external MOSFETs and an auxiliary +5V supply to power the device 24V Supply, 3.3V Output Operation Single 4.5V to 5.5V Supply Operation Auxiliary 5V Supply Operation 20 _____________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller Typical Application Circuits TYPICAL APPLICATION CIRCUIT 1 VIN +24V C1 330FF C2 10FF C3 10FF C4 0.1FF MAX15046 R7 51kI ON OFF C16 1FF IN VCC PGOOD EN LIM MAX15046 CSP DH LX BST DL D1 C5 0.47FF Q1 L1 1.4mH C8 470FF C9 22FF C10 22FF VOUT +3.3V Q2 R1 1kI C7 2700pF R9 32.4kI C13 68pF R8 22.6kI C14 1500pF C15 15pF COMP DRV C6 2.2FF R2 2.2I FB RT R5 32.4kI PGND GND C12 1200pF R3 10.7kI C11 4.7FF R6 10kI R4 45.3kI Q1: VISHAY SILICONIX Si7850DP Q2: VISHAY SILICONIX Si7460DP D1: DIODES INC. ZHCS506 L1: COILCRAFT MSS1278-142ML C1: PANASONIC EEVFK1H331Q C8: SANYO 6CE470EX ______________________________________________________________________________________ 21 40V, High-Performance, Synchronous Buck Controller MAX15046 Typical Application Circuits (continued) TYPICAL APPLICATION CIRCUIT 2 VIN +4.5V TO +5.5V C1 C2 IN VCC PGOOD ENABLE PGOOD EN LIM R4 C7 R3 C8 C9 R1 R3 COMP FB RT MAX15046 CSP DH L1 LX C3 BST DL D1 DRV C6 PGND GND Q2 C4 C5 VOUT Q1 R2 TYPICAL APPLICATION CIRCUIT 3 VAUX +4.5V TO +5.5V C1 C2 IN VCC PGOOD ENABLE PGOOD EN LIM R4 C7 R3 C8 C9 R1 R3 COMP FB RT MAX15046 CSP DH L1 LX C3 BST DL D1 DRV C6 PGND GND Q2 C4 C5 VOUT VIN +24V Q1 R2 22 _____________________________________________________________________________________ 40V, High-Performance, Synchronous Buck Controller Pin Configurations TOP VIEW IN 1 VCC 2 PGOOD 3 EN 4 LIM 5 COMP 6 FB 7 RT 8 MAX15046 + 16 CSP 15 DH 14 LX IN 1 VCC 2 PGOOD 3 EN 4 LIM 5 COMP 6 FB 7 RT 8 + 16 CSP 15 DH 14 LX MAX15046A 13 BST 12 DL 11 DRV 10 PGND 9 GND MAX15046B 13 BST 12 DL 11 DRV EXPOSED PAD 10 PGND 9 GND QSOP QSOP-EP Chip Information PROCESS: BiCMOS PACKAGE TYPE Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE CODE DOCUMENT NO. 16 QSOP 16 QSOP-EP E16+4 E16E+9 21-0055 21-0112 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 23 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.