MAX16907SATE

19-5779; Rev 0; 3/11 EVALUATION KIT AVAILABLE MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current General Description The MAX16907 is a 3A, current-mode, step-down converter with an integrated high-side switch. The device is designed to operate with input voltages from 3.5V to 36V while using only 30FA quiescent current at no load. The switching frequency is adjustable from 1MHz to 2.2MHz by an external resistor and can be synchronized to an external clock. The output voltage is pin selectable to be 5V fixed or adjustable from 1V to 10V. The wide input voltage range along with its ability to operate at high duty cycle during undervoltage transients make the device ideal for automotive and industrial applications. The device operates in skip mode for reduced current consumption in light-load applications. Protection features include overcurrent limit, overvoltage, and thermal shutdown with automatic recovery. The device also features a power-good monitor to ease power-supply sequencing. The device operates over the -40NC to +125NC automotive temperature range, and is available in 16-pin TSSOP and TQFN (5mm x 5mm) packages with exposed pads. S 42V Input Transients Tolerance S High Duty Cycle During Undervoltage Transients S 5V Fixed or 1V to 10V Adjustable Output Voltage S Integrated 3A Internal High-Side (70mI typ) Switch S Fast Load-Transient Response and Current-Mode Architecture S Adjustable Switching Frequency (1MHz to 2.2MHz) S Frequency Synchronization Input S 30µA Standby Mode Operating Current S 5µA Typical Shutdown Current S Spread Spectrum (Optional) S Overvoltage, Undervoltage, Overtemperature, and Short-Circuit Protections Features S Wide 3.5V to 36V Input Voltage Range Applications Automotive Industrial/Military High-Voltage Input DC-DC Converter Point-of-Load Applications Ordering Information appears at end of data sheet. For related parts and recommended products to use with this part, refer to: www.maxim-ic.com/MAX16907.related Typical Application Circuit VBAT CIN1 47µF CIN2 4.7µF SUP EN FSYNC COMP CCOMP2 12pF RFOSC 12kI FOSC FB SUPSW BST LX CBST 0.1µF L1 2.2µH D1 VBIAS VBIAS RPGOOD 10kI POWER GOOD VOUT 5V AT 3A COUT 22µF VOUT MAX16907 CCOMP1 1000pF RCOMP 20kI OUT CBIAS 1µF BIAS GND PGOOD ����������������������������������������������������������������� 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. MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current ABSOLUTE MAXIMUM RATINGS SUP, SUPSW, LX, EN to GND ............................... -0.3V to +42V SUP to SUPSW ..................................................... -0.3V to +0.3V BST to GND ........................................................... -0.3V to +47V BST to LX ............................................................... -0.3V to +6V OUT to GND .......................................................... -0.3V to +12V FOSC, COMP, BIAS, FSYNC, I.C., PGOOD, FB to GND............................................................ -0.3V to +6V LX Continuous RMS Current ...................................................4A Output Short-Circuit Duration .................................... Continuous *As per the JEDEC 51 standard (multilayer board). 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. Continuous Power Dissipation (TA = +70NC) TSSOP (derate 26.1mW/oC above +70NC) .......... 2088.8mW* TQFN (derate 28.6mW/oC above +70NC) ............ 2285.7mW* Operating Temperature Range ........................ -40NC to +125NC Junction Temperature .....................................................+150NC Storage Temperature Range............................ -65NC to +150NC Lead Temperature (soldering, 10s) ................................+300NC Soldering Temperature (reflow) ..................................... +260oC PACKAGE THERMAL CHARACTERISTICS (Note 1) TSSOP Junction-to-Ambient Thermal Resistance (BJA) ....... 38.3NC/W Junction-to-Case Thermal Resistance (BJC) ................. 3NC/W TQFN Junction-to-Ambient Thermal Resistance (BJA) .......... 35NC/W Junction-to-Case Thermal Resistance(BJC) ............... 2.7NC/W Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial. ELECTRICAL CHARACTERISTICS (VSUP = VSUPSW = 14V, VEN = 14V, CBIAS = 1FF, RFOSC = 12kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = +25NC.) PARAMETER Supply Voltage Range Load-Dump Event Supply Voltage SYMBOL VSUP, VSUPSW VSUP_LD ISUP Supply Current tLD < 1s ILOAD = 1.5A Standby mode, no load, VOUT = 5V ISUP_STANDBY Standby mode, no load, VOUT = 5V, TA = +25°C ISHDN VBIAS VUVBIAS VEN = 0V VSUP = VSUPSW = 6V to 36V VBIAS rising 4.7 2.9 3.5 30 30 5 5 3.1 400 +175 15 60 45 12 5.3 3.3 FA FA V V mV NC NC CONDITIONS MIN 3.5 TYP MAX 36 42 UNITS V V mA Shutdown Supply Current BIAS Regulator Voltage BIAS Undervoltage Lockout BIAS Undervoltage-Lockout Hysteresis Thermal-Shutdown Threshold Thermal-Shutdown Threshold Hysteresis ����������������������������������������������������������������� Maxim Integrated Products 2 *The parametric values (min, typ, max limits) shown in the Electrical Characteristics table supersede values quoted elsewhere in this data sheet. MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current ELECTRICAL CHARACTERISTICS* (continued) (VSUP = VSUPSW = 14V, VEN = 14V, CBIAS = 1FF, RFOSC = 12kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = +25NC.) PARAMETER OUTPUT VOLTAGE (OUT) Output Voltage Skip-Mode Output Voltage Adjustable Output Voltage Range Load Regulation Line Regulation BST Input Current LX Current Limit Skip-Mode Threshold Spread Spectrum Power-Switch On-Resistance High-Side Switch Leakage Current TRANSCONDUCTANCE AMPLIFIER (COMP) FB Input Current IFB FB connected to an external resistive divider, 0°C < TA < +125°C -40°C < TA < +125°C 6V < VSUP < 36V VFB = 1V, VBIAS = 5V (Note 2) 0.99 0.985 10 1.0 1.0 0.02 900 80 fSW = 2.2MHz fSW = 1MHz RFOSC = 12kI TA = +25°C tFSYNC (Note 2) fOSC + 10% 1 2.05 98 99 2.20 2.35 1.01 1.015 %/V FS ns % nA RON IBST_ON ILX ISKIP_TH Spread spectrum enabled RON measured between SUPSW and LX, ILX = 1A, VBIAS = 5V VSUP = 36V, VLX = 0V, TA = +25°C VOUT VOUT_SKIP VOUT_ADJ VFB = VBIAS, normal operation No load, VFB = VBIAS FB connected to external resistive divider VFB = VBIAS, 30mA < ILOAD < 3A VFB = VBIAS, 6V < VSUPSW < 36V High-side on, VBST - VLX = 5V (Note 2) 3.4 4.925 4.925 1 0.5 0.02 1.5 4.1 300 6 70 150 1 2.5 6 5 5 5.075 5.15 10 V V V % %/V mA A mA % mI FA SYMBOL CONDITIONS MIN TYP MAX UNITS FB Regulation Voltage VFB V FB Line Regulation Transconductance (from FB to COMP) Minimum On-Time Maximum Duty Cycle OSCILLATOR FREQUENCY Oscillator Frequency EXTERNAL CLOCK INPUT (FSYNC) FSYNC Input Current External Input Clock Acquisition Time External Input Clock Frequency DVLINE gm tON_MIN DCMAX MHz 1 FA Cycles Hz ����������������������������������������������������������������� Maxim Integrated Products 3 *The parametric values (min, typ, max limits) shown in the Electrical Characteristics table supersede values quoted elsewhere in this data sheet. MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current ELECTRICAL CHARACTERISTICS* (continued) (VSUP = VSUPSW = 14V, VEN = 14V, CBIAS = 1FF, RFOSC = 12kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = +25NC.) PARAMETER External Input Clock High Threshold External Input Clock Low Threshold Soft-Start Time ENABLE INPUT (EN) Enable Input-High Threshold Enable Input-Low Threshold Enable Threshold Voltage Hysteresis Enable Input Current RESET Output Overvoltage Trip Threshold PGOOD Switching Level PGOOD Debounce PGOOD Output Low Voltage PGOOD Leakage Current ISINK = 5mA VOUT in regulation, TA = +25NC VOUT_OV VTH_RISING SYMBOL VFSYNC_HI VFSYNC_LO tSS VEN_HI VEN_LO VEN,HYS IEN TA = +25°C VFSYNC rising CONDITIONS MIN 1.4 TYP MAX UNITS V VFSYNC falling 8.5 2 0.4 V ms V 0.9 0.2 1 V V FA 105 VFB rising, VPGOOD = high VFB falling, VPGOOD = low 93 90 10 110 95 92.5 35 115 97 95 60 0.4 1 %VFB %VFB Fs V FA VTH_FALLING Note 2: Guaranteed by design; not production tested. ����������������������������������������������������������������� Maxim Integrated Products 4 *The parametric values (min, typ, max limits) shown in the Electrical Characteristics table supersede values quoted elsewhere in this data sheet. MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current Typical Operating Characteristics (VSUP = VSUPSW = VEN = 14V, VOUT = 5V, FB connected to VOUT, L1 = 2.2µH (Wurth 744311220), D1 = D360B-13-F (Diodes, Inc.), TA = +25NC, unless otherwise noted.) PWM MODE STARTUP BEHAVIOR (5V/2.2MHz) MAX16907 toc01 SKIP MODE STARTUP BEHAVIOR (5V/2.2MHz) MAX16907 toc02 SUP SHORTED TO SUPSW VIN 5V/div VIN 0V 2V/div VOUT SUP SHORTED TO SUPSW 5V/div 0V 2V/div 0V 10V/div 0V VOUT ILOAD VPGOOD 0V 2A/div 0A 10V/div 0V 2ms/div VPGOOD 2ms/div SUPPLY CURRENT vs. SUPPLY VOLTAGE (5V/2.2MHz) MAX16907 toc03 EFFICIENCY vs. LOAD CURRENT VIN = 14V 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 0.0001 0.001 0.01 0.1 EFFICIENCY vs. LOAD CURRENT VIN = 14V MAX16907 toc04 D1: B360B-13-F FROM DIODES L1: WURTH 744311220 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 SUPPLY CURRENT (µA) 90 80 70 60 50 40 30 20 10 0 3.3V 5V 8V 3.3V 8V 5V ISUP + ISUPSW 5.5 9.0 12.5 16.0 19.5 23.0 26.5 30.0 33.5 SUPPLY VOLTAGE (V) D1: B360B-13-F FROM DIODES L1: WURTH 744311220 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 LOAD CURRENT (A) LOAD CURRENT (A) SWITCHING FREQUENCY vs. RFOSC MAX16907 toc06 SWITCHING FREQUENCY vs. LOAD CURRENT(5V/2.2MHz) MAX16907 toc07 3.0 SWITCHING FREQUENCY (MHz) 2.5 2.0 1.5 1.0 0.5 0 12 15 18 RFOSC (kI) 21 3.0 SWITCHING FREQUENCY (MHz) 2.5 2.0 1.5 1.0 0.5 0 VIN = 14V ILOAD = 1.5A 24 VIN = 14V ILOAD = 1.5A 0 0.5 1.0 1.5 2.0 2.5 3.0 LOAD CURRENT (A) ����������������������������������������������������������������� Maxim Integrated Products 5 MAX16907 toc05 120 110 100 100 100 MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current Typical Operating Characteristics (continued) (VSUP = VSUPSW = VEN = 14V, VOUT = 5V, FB connected to VOUT, L1 = 2.2µH (Wurth 744311220), D1 = D360B-13-F (Diodes, Inc.), TA = +25NC, unless otherwise noted.) LOAD-TRANSIENT RESPONSE (5V/2.2MHz) MAX16907 toc08 LOAD-TRANSIENT RESPONSE (SKIP MODE) MAX16907 toc09 5V/2.2MHz VOUT AC-COUPLED 200mV/div VOUT AC-COUPLED 50mV/div 1A/div ILOAD 0A 100µs/div 100µs/div ILOAD 100mA/div 0 FSYNC TRANSITION FROM INTERNAL TO EXTERNAL FREQUENCY (3.3V/2.2MHz CONFIGURATION) MAX16907 toc10 UNDERVOLTAGE PULSE (5V/2.2MHz) MAX16907 toc11 fFSYNC = 2.475MHz 5V/div VLX 0V VOUT 2V/div VFSYNC 0V VBIAS 200ns/div 10ms/div VLX VIN RESISTIVE LOAD = 1.6I 5V/div 0V 5V/div 0V 20V/div 0V 5V/div 0V LOAD DUMP TEST (5V/2.2MHz) MAX16907 toc12 OUTPUT RESPONSE TO SLOW INPUT RAMP (ILOAD = 3A) MAX16907 toc13 VIN VIN 10V/div VOUT 5V/2.2MHz SUP SHORTED TO SUPSW 10V/div 0V 5V/div 0V 10V/div 0V 5A/div 0A 0V VLX VOUT SUP SHORTED TO SUPSW 100ms/div 5V/div 0V ILOAD 4s/div ����������������������������������������������������������������� Maxim Integrated Products 6 MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current Typical Operating Characteristics (continued) (VSUP = VSUPSW = VEN = 14V, VOUT = 5V, FB connected to VOUT, L1 = 2.2µH (Wurth 744311220), D1 = D360B-13-F (Diodes, Inc.), TA = +25NC, unless otherwise noted.) SHORT CIRCUIT TO GROUND TEST (5V/2.2MHz) MAX16907 toc14 VOUT LOAD REGULATION (5V/2.2MHz) 2V/div 5.08 5.06 5.04 VOUT (V) 0V 5.02 5.00 4.98 4.96 10A/div 4.94 4.92 4.90 0 0.2 0.4 0.6 ILOAD (A) 0.8 1.0 1.2 VIN = 14V VOUT VPGOOD 5V/div 0V ILX 10ms/div 0A VOUT vs. TEMPERATURE (5V/2.2MHz) MAX16907 toc16 VOUT LINE REGULATION (5V/2.2MHz) 5.08 5.06 5.04 VOUT (V) 5.02 5.00 4.98 4.96 4.94 4.92 5.08 5.06 5.04 VOUT (V) 5.02 5.00 4.98 4.96 4.94 4.92 4.90 VIN = 14V ILOAD = 3A ILOAD = 3A ILOAD = 0A -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) 4.90 6 8 10 12 14 16 18 SUPPLY VOLTAGE (V) VOUT LINE REGULATION (5V/2.2MHz) MAX16907 toc18 BIAS LOAD REGULATION (5V/2.2MHz) 5.08 5.06 5.04 VBIAS (V) 5.02 5.00 4.98 4.96 4.94 4.92 MAX16907 toc19 5.10 5.08 5.06 5.04 VOUT (V) 5.02 5.00 4.98 4.96 4.94 4.92 4.90 0 6 12 18 24 30 5.10 ILOAD = 0A TA = -40°C TA = +125°C TA = +25°C 36 4.90 0 2 4 6 8 10 12 14 16 18 20 SUPPLY VOLTAGE (V) IBIAS (mA) ����������������������������������������������������������������� Maxim Integrated Products 7 MAX16907 toc17 5.10 5.10 MAX16907 toc15 5.10 MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current Typical Operating Characteristics (continued) (VSUP = VSUPSW = VEN = 14V, VOUT = 5V, FB connected to VOUT, L1 = 2.2µH (Wurth 744311220), D1 = D360B-13-F (Diodes, Inc.), TA = +25NC, unless otherwise noted.) ISHDN vs. SUPPLY VOLTAGE 18 16 14 ISHDN (µA) 12 10 8 6 4 2 0 3 10 17 24 31 38 45 ISHDN vs. TEMPERATURE MAX16907 toc20 VEN = 0V TA = +125°C 5.8 5.6 5.4 ISHDN (µA) 5.2 5.0 4.8 4.6 4.4 4.2 4.0 VEN = 0V VIN = 14V TA = +25°C TA = -40°C -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) SUPPLY VOLTAGE (V) DIPS AND DROP TEST (5V/2.2MHz) MAX16907 toc22 LINE TRANSIENT TEST (5V/2.2MHz) MAX16907 toc23 VIN RESISTIVE LOAD = 1.6I VOUT 10V/div 0V 5V/div 0V 5V/div 0V VIN SUP SHORTED TO SUPSW RESISTIVE LOAD = 1.6I VOUT VLX VLX VPGOOD 10ms/div 5V/div 0V VPGOOD 10ms/div ����������������������������������������������������������������� Maxim Integrated Products 8 MAX16907 toc21 20 6.0 10V/div 0V 5V/div 0V 10V/div 0V 5V/div 0V MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current Pin Configurations SUPSW SUPSW SUPSW SUPSW SUP TOP VIEW I.C. EN TOP VIEW BST LX LX 16 15 14 13 12 11 10 9 12 11 LX 10 LX 9 EN 13 I.C. 14 8 7 SUP BST GND BIAS MAX16907 FSYNC 15 EP FOSC 16 MAX16907 6 + 1 FSYNC 2 FOSC 3 PGOOD 4 OUT 5 FB 6 COMP 7 BIAS 8 GND + 1 PGOOD 2 OUT 3 FB EP 5 4 COMP TSSOP TQFN (5mm × 5mm) Pin Descriptions PIN TSSOP 1 TQFN 15 NAME FUNCTION Synchronization Input. The device synchronizes to an external signal applied to FSYNC. The external clock frequency must be 10% greater than the internal clock frequency for proper operation. Connect FSYNC to GND if the internal clock is used. Resistor-Programmable Switching-Frequency Setting Control Input. Connect a resistor from FOSC to GND to set the switching frequency. Open-Drain, Active-Low Output. PGOOD asserts when VOUT is below the 92.5% regulation point. PGOOD deasserts when VOUT is above the 95% regulation point. Switch Regulator Output. OUT also provides power to the internal circuitry when the output voltage of the converter is set between 3V and 5V during standby mode. Feedback Input. Connect an external resistive divider from OUT to FB and GND to set the output voltage. Connect to BIAS to set the output voltage to 5V. Error-Amplifier Output. Connect an RC network from COMP to GND for stable operation. See the Compensation Network section for more details. Linear Regulator Output. BIAS powers up the internal circuitry. Bypass with a 1FF capacitor to ground. Ground High-Side Driver Supply. Connect a 0.1FF capacitor between LX and BST for proper operation. FSYNC 2 3 4 5 6 7 8 9 16 1 2 3 4 5 6 7 FOSC PGOOD OUT FB COMP BIAS GND BST ����������������������������������������������������������������� Maxim Integrated Products 9 MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current Pin Descriptions (continued) PIN TSSOP 10 11, 12 13, 14 15 16 — TQFN 8 9, 10 11, 12 13 14 — NAME SUP LX SUPSW EN I.C. EP FUNCTION Voltage Supply Input. SUP powers up the internal linear regulator. Connect a minimum 4.7FF capacitor to ground. Inductor Switching Node. Connect a Schottky diode between LX and GND. Internal High-Side Switch-Supply Input. SUPSW provides power to the internal switch. Connect a 0.1FF decoupling capacitor and a 4.7FF ceramic capacitor to ground. SUP Voltage-Compatible Enable Input. Drive EN low to disable the device. Drive EN high to enable the device. Internally Connected. Connect to ground for proper operation. Exposed Pad. Connect EP to a large-area contiguous copper ground plane for effective power dissipation. Do not use as the only IC ground connection. EP must be connected to GND. Internal Block Diagram OUT COMP PGOOD EN SUP BIAS FB FBSW FBOK AON HVLDO SWITCHOVER BST SUPSW PWM REF SOFTSTART SLOPE COMP CS LX LOGIC HSD EAMP OSC MAX16907 FSYNC FOSC ���������������������������������������������������������������� Maxim Integrated Products 10 MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current Detailed Description The MAX16907 is a constant-frequency, current-mode, automotive buck converter with an integrated high-side switch. The device operates with input voltages from 3.5V to 36V and tolerates input transients up to 42V. During undervoltage events, such as cold-crank conditions, the internal pass device maintains 98% duty cycle. The switching frequency is resistor programmable from 1MHz to 2.2MHz to allow optimization for efficiency, noise, and board space. A synchronization input FSYNC allows the device to synchronize to an external clock frequency. During light-load conditions, the device enters skip mode for high efficiency. The 5V fixed output voltage eliminates the need for external resistors and reduces the supply current to 30FA. See the Internal Block Diagram for more information. The device includes two separate supply inputs, SUP and SUPSW, specified for a wide 3.5V to 36V input voltage range. VSUP provides power to the device and VSUPSW provides power to the internal switch. When the device is operating with a 3.5V input supply, certain conditions such as cold crank can cause the voltage at SUPSW to drop below the programmed output voltage. As such, the device operates in a high duty-cycle mode to maintain output regulation. The device includes a 5V linear regulator, BIAS, that provides power to the internal circuitry. Connect a 1FF ceramic capacitor from BIAS to GND. The device synchronizes to an external clock signal applied at FSYNC. The signal at FSYNC must have a 10% higher frequency than the internal clock frequency for proper synchronization. The device includes an 8.5ms fixed soft-start time for up to 500FF capacitive load with a 3A resistive load. where the board space is limited and the converter needs to maintain a well-regulated output voltage using an input voltage that varies from 9V to 18V. Additionally, the device incorporates an innovative design for fast-loop response that further ensures good output-voltage regulation during transients. An enable-control input (EN) activates the device from its low-power shutdown mode. EN is compatible with inputs from automotive battery level down to 3.3V. The highvoltage compatibility allows EN to be connected to SUP, KEY/KL30, or the INH pin of a CAN transceiver. EN turns on the internal regulator. Once VBIAS is above the internal lockout threshold, VUVL = 3.1V (typ), the controller activates and the output voltage ramps up within 8.5ms. A logic-low at EN shuts down the device. During shutdown, the internal linear regulator and gate drivers turn off. Shutdown is the lowest power state and reduces the quiescent current to 5FA (typ). Drive EN high to bring the device out of shutdown. The device includes overvoltage protection circuitry that protects the device when there is an overvoltage condition at the output. If the output voltage increases by more than 110% of its set voltage, the device stops switching. The device resumes regulation once the overvoltage condition is removed. Current-mode buck converters include an integrator architecture and a load-line architecture. The integrator architecture has large loop gain but slow transient response. The load-line architecture has fast transient response but low loop gain. The device features an integrator architecture with innovative designs to improve transient response. Thus, the device delivers high outputvoltage accuracy, plus the output can recover quickly from a transient overshoot, which could damage other on-board components during load transients. The overload protection circuitry is triggered when the device is in current limit and VOUT is below the reset threshold. Under these conditions the device turns the high-side FET off for 16ms and re-enters soft-start. If the overload condition is still present, the device repeats the cycle. System Enable (EN) Wide Input Voltage Range (3.5V to 36V) Overvoltage Protection Linear Regulator Output (BIAS) Fast Load-Transient Response External Clock Input (FSYNC) Soft-Start Overload Protection The device features a 80ns minimum on-time that ensures proper operation at 2.2MHz switching frequency and high differential voltage between the input and the output. This feature is extremely beneficial in automotive applications Minimum On-Time ���������������������������������������������������������������� Maxim Integrated Products 11 MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current During light-load operation, IINDUCTOR P 185mA, the device enters skip mode operation. Skip mode turns off the majority of circuitry and allows the output to drop below regulation voltage before the switch is turned on again. The lower the load current, the longer it takes for the regulator to initiate a new cycle. Because the converter skips unnecessary cycles and turns off the majority of circuitry, the converter efficiency increases. When the high-side FET stops switching for more than 50Fs, most of the internal circuitry, including LDO, draws power from VOUT (for VOUT = 3V to 5.5V), allowing current consumption from the battery to drop to only 30FA. Thermal-overload protection limits the total power dissipation in the device. When the junction temperature exceeds +175NC (typ), an internal thermal sensor shuts down the internal bias regulator and the step-down converter, VOUT RFB1 Skip Mode/Standby Mode allowing the IC to cool. The thermal sensor turns on the IC again after the junction temperature cools by 15NC. Applications Information Connect FB to BIAS for a fixed 5V output voltage. To set the output to other voltages between 1V and 10V, connect a resistive divider from output (OUT) to FB to GND (Figure 1). Calculate RFB1 (OUT to FB resistor) with the following equation:  V  R FB1 = R FB2  OUT  − 1 VFB      where VFB = 1V (see the Electrical Characteristics table). The switching frequency, fSW, is set by a resistor (RFOSC) connected from FOSC to GND. See Figure 2 to select the correct RFOSC value for the desired switching frequency. For example, a 2.2MHz switching frequency is set with RFOSC = 12kI. Higher frequencies allow designs with lower inductor values and less output capacitance. Consequently, peak currents and I2R losses are lower at higher switching frequencies, but core losses, gate charge currents, and switching losses increase. Inductor Selection Three key inductor parameters must be specified for operation with the device: inductance value (L), inductor saturation current (ISAT), and DC resistance (RDCR). To select inductance value, the ratio of inductor peak-topeak AC current to DC average current (LIR) must be selected first. 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 then determine the inductor value as follows: V (V − VOUT ) L = OUT SUP VSUP fSWI OUTLIR Setting the Output Voltage Overtemperature Protection Internal Oscillator MAX16907 FB RFB2 Figure 1. Adjustable Output-Voltage Setting SWITCHING FREQUENCY vs. RFOSC MAX16907 toc06 3.0 SWITCHING FREQUENCY (MHz) 2.5 2.0 1.5 1.0 0.5 0 12 15 18 RFOSC (kI) 21 VIN = 14V ILOAD = 1.5A 24 Figure 2. Switching Frequency vs. RFOSC where VSUP, VOUT, and IOUT are typical values (so that efficiency is optimum for typical conditions). The switching frequency is set by RFOSC (see the Internal Oscillator section). The exact inductor value is not critical and can be adjusted to make trade-offs among size, cost, efficiency, and transient response requirements. Table 1 shows a comparison between small and large inductor sizes. ���������������������������������������������������������������� Maxim Integrated Products 12 MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current Table 1. Inductor Size Comparison INDUCTOR SIZE SMALLER Lower price Smaller form factor Faster load response LARGER Smaller ripple Higher efficiency Larger fixed-frequency range in skip mode IRMS has a maximum value when the input voltage equals twice the output voltage (VSUP = 2VOUT), so IRMS(MAX) = ILOAD(MAX)/2. Choose an input capacitor that exhibits less than +10NC self-heating temperature rise at the RMS input current for optimal long-term reliability. The input-voltage ripple is composed of DVQ (caused by the capacitor discharge) and DVESR (caused by the equivalent series resistance (ESR) of the capacitor). Use low-ESR ceramic capacitors with high ripple-current capability at the input. Assume the contribution from the ESR and capacitor discharge equal to 50%. Calculate the input capacitance and ESR required for a specified input-voltage ripple using the following equations: ESRIN = where ∆IL = and ∆VESR ∆I I OUT + L 2 The inductor value must be chosen so that the maximum inductor current does not reach the device’s minimum current limit. The optimum operating point is usually found between 25% and 35% ripple current. When pulse skipping (FSYNC low and light loads), the inductor value also determines the load-current value at which PFM/ PWM switchover occurs. Find a low-loss inductor having the lowest possible DC resistance that fits in the allotted dimensions. Most inductor manufacturers provide inductors in standard values, such as 1.0FH, 1.5FH, 2.2FH, 3.3FH, etc. Also look for nonstandard values, which can provide a better compromise in LIR across the input voltage range. If using a swinging inductor (where the no-load inductance decreases linearly with increasing current), evaluate the LIR with properly scaled inductance values. For the selected inductance value, the actual peak-to-peak inductor ripple current (DIINDUCTOR) is defined by: V (V − VOUT ) ∆IINDUCTOR = OUT SUP VSUP × fSW × L where DIINDUCTOR is in A, L is in H, and fSW is in Hz. Ferrite cores are often the best choices, although powdered iron is inexpensive and can work well at 200kHz. The core must be large enough not to saturate at the peak inductor current (IPEAK): ∆I IPEAK = ILOAD(MAX) + INDUCTOR 2 The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit’s switching. The input capacitor RMS current requirement (IRMS) is defined by the following equation: IRMS = ILOAD(MAX) VOUT (VSUP − VOUT ) VSUP (VSUP − VOUT ) × VOUT VSUP × fSW × L I × D(1 − D) VOUT CIN = OUT and D = ∆VQ × fSW VSUPSW where IOUT is the maximum output current, and D is the duty cycle. Output Capacitor The output filter capacitor must have low enough ESR to meet output ripple and load-transient requirements, yet have high enough ESR to satisfy stability requirements. The output capacitance must be high enough to absorb the inductor energy while transitioning from full-load to no-load conditions without tripping the overvoltage fault protection. When using high-capacitance, low-ESR capacitors, the filter capacitor’s ESR dominates the output-voltage ripple. So the size of the output capacitor depends on the maximum ESR required to meet the output-voltage ripple (VRIPPLE(P-P)) specifications: VRIPPLE(P-P) = ESR × ILOAD(MAX) × LIR The actual capacitance value required relates to the physical size needed to achieve low ESR, as well as to the chemistry of the capacitor technology. Thus, the capacitor is usually selected by ESR and voltage rating rather than by capacitance value. Input Capacitor ���������������������������������������������������������������� Maxim Integrated Products 13 MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current When using low-capacity filter capacitors, such as ceramic capacitors, size is usually determined by the capacity needed to prevent voltage droop and voltage rise from causing problems during load transients. Generally, once enough capacitance is added to meet the overshoot requirement, undershoot at the rising load edge is no longer a problem. However, low-capacity filter capacitors typically have high-ESR zeros that can affect the overall stability. Rectifier Selection The device requires an external Schottky diode rectifier as a freewheeling diode. Connect this rectifier close to the device using short leads and short PCB traces. Choose a rectifier with a voltage rating greater than the maximum expected input voltage, VSUPSW. Use a low forward-voltage-drop Schottky rectifier to limit the negative voltage at LX. Avoid higher than necessary reversevoltage Schottky rectifiers that have higher forwardvoltage drops. The device uses an internal transconductance error amplifier with its inverting input and its output available to the user for external frequency compensation. The output capacitor and compensation network determine the loop stability. The inductor and the output capacitor are chosen based on performance, size, and cost. Additionally, the compensation network optimizes the control-loop stability. The controller uses a current-mode control scheme that regulates the output voltage by forcing the required current through the external inductor. The device uses the voltage drop across the high-side MOSFET to sense inductor current. Current-mode control eliminates the double pole in the feedback loop caused by the inductor and output capacitor, resulting in a smaller phase shift and requiring less elaborate error-amplifier compensation than voltagemode control. Only a simple single-series resistor (RC) and capacitor (CC) are required to have a stable, highbandwidth loop in applications where ceramic capacitors are used for output filtering (Figure 3). For other types of capacitors, due to the higher capacitance and ESR, the frequency of the zero created by the capacitance and ESR is lower than the desired closed-loop crossover frequency. To stabilize a nonceramic output capacitor loop, add another compensation capacitor (CF) from COMP to GND to cancel this ESR zero. The basic regulator loop is modeled as a power modulator, output feedback divider, and an error amplifier. The power modulator has a DC gain set by gmc x RLOAD, with a pole and zero pair set by RLOAD, the output capacitor (COUT), and its ESR. The following equations allow to approximate the value for the gain of the power modulator (GAINMOD(DC)), neglecting the effect of the ramp stabilization. Ramp stabilization is necessary when the duty cycle is above 50% and is internally done for the device. GAINMOD(DC) = g mc × R LOAD × fSW × L R LOAD + (fSW × L) Compensation Network where RLOAD = VOUT/ILOUT(MAX) in I, fSW is the switching frequency in MHz, L is the output inductance in H, and gmc = 3S. In a current-mode step-down converter, the output capacitor, its ESR, and the load resistance introduce a pole at the following frequency: fpMOD = 1  R LOAD × fSW × L  2π × C OUT ×  + ESR RLOAD + (fSW × L)   1 2π × ESR × C OUT VOUT R1 The output capacitor and its ESR also introduce a zero at: COMP RC CC fzMOD = R2 gm VREF CF When COUT is composed of “n” identical capacitors in parallel, the resulting COUT = n x COUT(EACH) and ESR = ESR(EACH)/n. Note that the capacitor zero for a parallel combination of alike capacitors is the same as for an individual capacitor. The feedback voltage-divider has a gain of GAINFB = VFB/VOUT, where VFB is 1V (typ). Figure 3. Compensation Network ���������������������������������������������������������������� Maxim Integrated Products 14 MAX16907 36V, 2.2MHz Step-Down Converter with Low Operating Current The transconductance error amplifier has a DC gain of GAINEA(DC) = gm,EA x ROUT,EA, where gm,EA is the erroramplifier transconductance, which is 900FS (typ), and ROUT,EA is the output resistance of the error amplifier. A dominant pole (fdpEA) is set by the compensation capacitor (CC) and the amplifier output resistance (ROUT,EA). A zero (fzEA) is set by the compensation resistor (RC) and the compensation capacitor (CC). There is an optional pole (fpEA) set by CF and RC to cancel the output capacitor ESR zero if it occurs near the crossover frequency (fC, where the loop gain equals 1 (0dB)). Thus: 1 fdpEA = 2π × C C × (R OUT,EA + R C ) fzEA = fpEA = 1 2π × C C × R C 1 2π × C F × R C Set the error-amplifier compensation zero formed by RC and CC (fzEA) at the fpMOD. Calculate the value of CC a follows: 1 CC = 2π × fpMOD × R C If fzMOD is less than 5 x fC, add a second capacitor, CF, from COMP to GND and set the compensation pole formed by RC and CF (fpEA) at the fzMOD. Calculate the value of CF as follows: 1 CF = 2π × fzMOD × R C As the load current decreases, the modulator pole also decreases; however, the modulator gain increases accordingly and the crossover frequency remains the same. For the case where fzMOD is less than fC: The power-modulator gain at fC is: GAINMOD(fC) = GAINMOD(DC) × The error-amplifier gain at fC is: f GAINEA(fC) = g m,EA × R C × zMOD fC Therefore: GAINMOD(fC) × Solving for RC: RC = VOUT × fC g m,EA × VFB × GAINMOD(fC) × fzMOD f VFB × g m,EA × R C × zMOD = 1 VOUT fC fpMOD fzMOD The loop-gain crossover frequency (fC) should be set below 1/5th of the switching frequency and much higher than the power-modulator pole (fpMOD): f fpMOD