19-5844; Rev 0; 5/11 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current General Description Features The MAX16977 is a 2A, 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. SWide 3.5V to 36V Input Voltage Range 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. SFrequency Synchronization Input 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. SOvervoltage, Undervoltage, Overtemperature, and Short-Circuit Protections Applications S42V Input Transients Tolerance SHigh Duty Cycle During Undervoltage Transients S5V Fixed or 1V to 10V Adjustable Output Voltage SIntegrated 2A Internal High-Side (70mI typ) Switch SFast Load-Transient Response and Current-Mode Architecture SAdjustable Switching Frequency (1MHz to 2.2MHz) S30µA Standby Mode Operating Current S5µA Typical Shutdown Current SSpread Spectrum (Optional) Ordering Information appears at end of data sheet. Automotive For related parts and recommended products to use with this part, refer to: www.maxim-ic.com/MAX16977.related Industrial/Military High-Voltage Input DC-DC Converters Point-of-Load Applications Typical Application Circuit VBAT CIN1 47µF CIN2 4.7µF SUP SUPSW BST EN LX FSYNC CCOMP1 2.2nF RCOMP 20kI COMP CCOMP2 12pF MAX16977 L1 2.2µH VOUT VOUT 5V AT 2A COUT 22µF D1 OUT VBIAS RFOSC 12kI VBIAS FOSC CBIAS 1µF CBST 0.1µF FB BIAS GND PGOOD RPGOOD 10kI POWER GOOD ����������������������������������������������������������������� 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. MAX16977 36V, 2A, 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....................................................3A Output Short-Circuit Duration.....................................Continuous Continuous Power Dissipation (TA = +70NC) TSSOP (derate 26.1mW/oC above +70oC)........... 2088.8mW* TQFN (derate 28.6mW/oC above +70oC)............. 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 *As per the JEDEC 51 standard (multilayer board). 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. 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 (VSUP = VSUPSW = 14V, VEN = 14V, CBIAS = 1FF, RFOSC = 12kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at TA = +25NC.) PARAMETER SYMBOL Supply Voltage Range VSUP, VSUPSW Load-Dump Event Supply Voltage VSUP_LD ISUP Supply Current CONDITIONS tLD < 1s MAX UNITS 36 V 42 V ILOAD = 1.5A 3.5 Standby mode, no load, VOUT = 5V 30 60 ISUP_STANDBY Standby mode, no load, VOUT = 5V, TA = +25°C 30 45 ISHDN VEN = 0V BIAS Regulator Voltage VBIAS VSUP = VSUPSW = 6V to 36V VBIAS rising BIAS Undervoltage-Lockout Hysteresis TYP 3.5 Shutdown Supply Current BIAS Undervoltage Lockout MIN VUVBIAS mA FA 5 12 FA 4.7 5 5.3 V 2.9 3.1 3.3 V 400 mV Thermal Shutdown Threshold +175 NC Thermal-Shutdown Threshold Hysteresis 15 NC ����������������������������������������������������������������� Maxim Integrated Products 2 MAX16977 36V, 2A, 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 SYMBOL CONDITIONS MIN TYP MAX UNITS VFB = VBIAS, normal operation 4.925 5 5.075 V 4.925 5 5.15 V 10 V OUTPUT VOLTAGE (OUT) Output Voltage VOUT Skip-Mode Output Voltage VOUT_SKIP No load, VFB = VBIAS Adjustable Output Voltage Range VOUT_ADJ FB connected to external resistive divider 1 Load Regulation VFB = VBIAS, 30mA < ILOAD < 2A 0.5 % Line Regulation VFB = VBIAS, 6V < VSUPSW < 36V High-side on, VBST - VLX = 5V 0.02 %/V BST Input Current IBST_ON LX Current Limit ILX Skip-Mode Threshold (Note 2) 2.4 ISKIP_TH 1.5 2.5 mA 3 4 A 300 mA % Spread Spectrum Spread spectrum enabled 6 Power-Switch On-Resistance RON measured between SUPSW and LX, ILX = 1A, VBIAS = 5V 70 RON High-Side Switch Leakage Current VSUP = 36V, VLX = 0V, TA = +25°C 150 mI 1 FA TRANSCONDUCTANCE AMPLIFIER (COMP) FB Input Current IFB FB Regulation Voltage VFB FB Line Regulation DVLINE Transconductance (from FB to COMP) gm Minimum On-Time tON_MIN Maximum Duty Cycle DCMAX 10 nA FB connected to an external resistive divider; 0°C < TA < +125°C 0.99 1.0 1.01 FB connected to an external resistive divider; -40°C < TA < +125°C 0.985 1.0 1.015 V 6V < VSUP < 36V 0.02 %/V VFB = 1V, VBIAS = 5V (Note 2) 900 FS 80 ns fSW = 2.2MHz 98 fSW = 1MHz 99 % OSCILLATOR FREQUENCY Oscillator Frequency RFOSC = 12kI 2.05 2.20 2.35 MHz 1 FA EXTERNAL CLOCK INPUT (FSYNC) FSYNC Input Current External Input Clock Acquisition Time TA at +25°C tFSYNC External Input Clock Frequency External Input Clock High Threshold 1 (Note 2) VFSYNC_HI VFSYNC rising Cycles fOSC + 10% Hz 1.4 V ����������������������������������������������������������������� Maxim Integrated Products 3 MAX16977 36V, 2A, 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 Low Threshold Soft-Start Time SYMBOL VFSYNC_LO CONDITIONS MIN TYP VFSYNC falling tSS MAX UNITS 0.4 V 8.5 ms ENABLE INPUT (EN) Enable Input-High Threshold VEN_HI Enable Input-Low Threshold VEN_LO Enable Threshold Voltage Hysteresis VEN,HYS Enable Input Current IEN 2 V 0.9 0.2 TA = +25NC V V 1 FA %VFB RESET Output Overvoltage Trip Threshold PGOOD Switching Level VOUT_OV VTH_RISING VTH_FALLING VFB rising, VPGOOD = high VFB falling, VPGOOD = low PGOOD Debounce PGOOD Output Low Voltage ISINK = 5mA PGOOD Leakage Current VOUT in regulation, TA = +25NC 105 110 115 93 95 97 90 92.5 95 10 35 60 %VFB Fs 0.4 V 1 FA Note 2: Guaranteed by design; not production tested. ����������������������������������������������������������������� Maxim Integrated Products 4 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current Typical Operating Characteristics (VSUP = VSUPSW = 14V, VEN = 14V, VOUT = 5V, VFSYNC = 0V, RFOSC = 12.1kHz, TA = +25NC, unless otherwise noted.) NO-LOAD STARTUP BEHAVIOR (5V/2.2MHz) FULL-LOAD STARTUP BEHAVIOR MAX16977 toc01 MAX16977 toc02 5V/2.2MHz RESISTIVE LOAD = 2.5Ω 5V/div SUP SHORTED TO SUPSW 5V/div VIN VIN 0V 0V 2V/div VOUT 2V/div VOUT 0V 1A/div 0A 5V/div 0V ILOAD VPGOOD 0V 10V/div 0V VPGOOD 2ms/div 2ms/div 60 50 40 70 60 3.3V 50 8V 40 5V 30 ISUP + ISUPSW 75 70 55 fSW = 2.2MHz L1 = 2.2µH (WURTH 744311220) D1: D360B-13-F FROM DIODES, INC. 50 0.0001 0.001 0.01 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.1 ILOAD (A) LOAD CURRENT (A) 2.5 2.0 1.5 1.0 VIN = 14V ILOAD = 1.5A 0 18 RFOSC (kI) 21 24 3.0 SWITCHING FREQUENCY (MHz) MAX16977 toc06 3.0 MAX16977 toc07 SWITCHING FREQUENCY vs. LOAD CURRENT (5V/2.2MHz) SWITCHING FREQUENCY vs. RFOSC 15 80 10 SUPPLY VOLTAGE (V) 12 85 60 0 5V 65 0 0.5 3.3V 90 20 5.5 9.0 12.5 16.0 19.5 23.0 26.5 30.0 33.5 8V 95 MAX16977 toc05 MAX16977 toc04 80 100 EFFICIENCY (%) 70 D1: B360B-13-F FROM DIODES, INC. L1: WURTH 744311220 90 EFFICIENCY (%) 80 30 20 10 0 100 MAX16977 toc03 90 SWITCHING FREQUENCY (MHz) SUPPLY CURRENT (µA) 120 110 100 EFFICIENCY vs. LOAD CURRENT (VIN = 14V) EFFICIENCY vs. LOAD CURRENT (VIN = 14V) SUPPLY CURRENT vs. SUPPLY VOLTAGE (5V/2.2MHz) VIN = 14V 2.5 2.0 1.5 1.0 0.5 0 0 0.5 1.0 1.5 2.0 ILOAD (A) ����������������������������������������������������������������� Maxim Integrated Products 5 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current Typical Operating Characteristics (continued) (VSUP = VSUPSW = 14V, VEN = 14V, VOUT = 5V, VFSYNC = 0V, RFOSC = 12.1kHz, TA = +25NC, unless otherwise noted.) LOAD-TRANSIENT RESPONSE (SKIP MODE) LINE-TRANSIENT RESPONSE (5V/2.2MHz) MAX16977 toc08 MAX16977 toc09 5V/2.2MHz VOUT (AC-COUPLED) 100mV/div ILOAD VOUT AC-COUPLED 50mV/div 100mA/div 1A/div 500mA 0A ILOAD 0 100µs/div 100µs/div FSYNC TRANSITION FROM INTERNAL TO EXTERNAL FREQUENCY (3.3V/2.2MHz CONFIGURATION) UNDERVOLTAGE PULSE (5V/2.2MHz) MAX16977 toc11 MAX16977 toc10 fFSYNC = 2.475MHz VIN 5V/div 5V/div RESISTIVE LOAD = 2.5Ω VLX 0V 2V/div VFSYNC 0V 5V/div VLX 0V 20V/div 0V VBIAS 5V/div 0V 10ms/div OUTPUT RESPONSE TO SLOW INPUT RAMP (ILOAD = 2A) MAX16977 toc13 MAX16977 toc12 5V/2.2MHz 42V VIN 10V/div 10V/div 14V 0V VIN 5V/div 0V VOUT 0V VOUT 10V/div VLX 0V 5V/div 0V 5V/2.2MHz 2A/div 0A ILOAD 100ms/div 0V VOUT 200ns/div LOAD DUMP TEST 3.5V 4s/div ����������������������������������������������������������������� Maxim Integrated Products 6 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current Typical Operating Characteristics (continued) (VSUP = VSUPSW = 14V, VEN = 14V, VOUT = 5V, VFSYNC = 0V, RFOSC = 12.1kHz, TA = +25NC, unless otherwise noted.) SHORT CIRCUIT TO GROUND TEST (5V/2.2MHz) 5.10 VIN = 14V 5.08 2V/div VOUT 5.06 5.04 5V/div 0V VOUT (V) 0V VPGOOD MAX16977 toc15 VOUT LOAD REGULATION (5V/2.2MHz) MAX16977 toc14 5.02 5.00 4.98 4.96 ILX 10A/div 4.94 0A 4.92 4.90 10ms/div 0 0.4 0.8 1.2 1.6 2.0 ILOAD (A) VOUT vs. TEMPERATURE (5V/2.2MHz) 5.06 5.04 5.04 5.02 5.02 5.00 4.98 ILOAD = 0A ILOAD = 3A 4.96 5.00 4.98 4.96 4.94 4.94 4.92 4.92 4.90 4.90 14 VOUT LINE REGULATION (5V/2.2MHz) BIAS LOAD REGULATION (5V/2.2MHz) 5.08 5.06 5.04 5.02 5.02 VBIAS (V) 5.04 5.00 4.98 4.94 4.94 4.92 4.92 4.90 4.90 18 24 30 36 TA = -40°C 4.98 4.96 12 18 5.00 4.96 SUPPLY VOLTAGE (V) 16 MAX16977 toc19 5.10 MAX16977 toc18 5.06 6 12 SUPPLY VOLTAGE (V) ILOAD = 0A 0 10 TEMPERATURE (°C) 5.10 5.08 8 6 -40 -25 -10 5 20 35 50 65 80 95 110 125 VOUT (V) ILOAD = 2A 5.08 VOUT (V) VOUT (V) 5.06 MAX16977 toc17 VIN = 14V 5.08 5.10 MAX16977 toc16 5.10 VOUT LINE REGULATION (5V/2.2MHz) TA = +125°C 0 2 4 TA = +25°C 6 8 10 12 14 16 18 20 IBIAS (mA) ����������������������������������������������������������������� Maxim Integrated Products 7 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current Typical Operating Characteristics (continued) (VSUP = VSUPSW = 14V, VEN = 14V, VOUT = 5V, VFSYNC = 0V, RFOSC = 12.1kHz, TA = +25NC, unless otherwise noted.) ISHDN vs. SUPPLY VOLTAGE 16 TA = +125°C 12 8 6 5.6 5.4 TA = +25°C 10 5.2 5.0 4.8 4.6 TA = -40°C 4 4.4 2 4.2 0 VEN = 0V VIN = 14V 5.8 ISHDN (µA) 14 MAX16977 toc21 VEN = 0V 18 ISHDN (µA) ISHDN vs. TEMPERATURE 6.0 MAX16977 toc20 20 4.0 3 10 17 24 31 38 -40 -25 -10 5 20 35 50 65 80 95 110 125 45 SUPPLY VOLTAGE (V) TEMPERATURE (°C) DIPS AND DROP TEST LINE-TRANSIENT RESPONSE (ILOAD = 2A) MAX16977 toc22 MAX16977 toc23 5V/2.2MHz 5V/2.2MHz VIN 5V/div 0V 20V/div 0V VLX 5V/div 0V VPGOOD 10ms/div 5V 5V/div 0V VOUT 14V VIN 10V/div 0V VOUT 5V/div 0V 10V/div VLX 0V VPGOOD 5V/div 0V 10ms/div ����������������������������������������������������������������� Maxim Integrated Products 8 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current SUPSW SUPSW LX LX 16 15 14 13 12 11 10 TOP VIEW BST SUP LX LX SUPSW EN I.C. TOP VIEW SUPSW Pin Configurations 12 11 10 9 9 EN 13 I.C. 14 MAX16977 7 BST 6 GND 5 BIAS 8 1 GND PGOOD TSSOP 2 3 4 TQFN (5mm × 5mm) COMP 7 BIAS PGOOD 6 COMP FOSC 5 FB FSYNC 4 OUT 3 EP + FB FOSC 16 OUT EP + 2 SUP MAX16977 FSYNC 15 1 8 Pin Descriptions PIN NAME FUNCTION 15 FSYNC 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. 2 16 FOSC Resistor-Programmable Switching-Frequency Setting Control Input. Connect a resistor from FOSC to GND to set the switching frequency. 3 1 PGOOD 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. 4 2 OUT 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. 5 3 FB 6 4 COMP 7 5 BIAS Linear-Regulator Output. BIAS powers up the internal circuitry. Bypass with a 1FF capacitor to ground. 8 6 GND Ground 9 7 BST High-Side Driver Supply. Connect a 0.1FF capacitor between LX and BST for proper operation. TSSOP TQFN 1 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. ����������������������������������������������������������������� Maxim Integrated Products 9 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current Pin Descriptions (continued) PIN NAME FUNCTION TSSOP TQFN 10 8 SUP 11, 12 9, 10 LX 13, 14 11, 12 SUPSW 15 13 EN SUP Voltage-Compatible Enable Input. Drive EN low to disable the device. Drive EN high to enable the device. 16 14 I.C. Internally Connected. Connect to ground for proper operation. — — EP 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. Voltage Supply Input. SUP powers up the internal linear regulator. Connect a 1FF 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 1FF and 4.7FF capacitor to ground. Internal Block Diagram OUT FB COMP FBSW PGOOD FBOK EN SUP AON HVLDO BIAS SWITCHOVER BST SUPSW EAMP LOGIC PWM HSD REF LX CS SOFTSTART SLOPE COMP OSC FSYNC MAX16977 FOSC ���������������������������������������������������������������� Maxim Integrated Products 10 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current Detailed Description The MAX16977 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. Wide Input Voltage Range (3.5V to 36V) 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. Linear-Regulator Output (BIAS) The device includes a 5V linear regulator, BIAS, that provides power to the internal circuitry. Connect a 1FF ceramic capacitor from BIAS to GND. External Clock Input (FSYNC) 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. Soft-Start The device includes an 8.5ms fixed soft-start time for up to 500FF capacitive load with a 2A resistive load. Minimum On-Time 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 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. System Enable (EN) 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 converter 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. Overvoltage Protection 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. Fast Load-Transient Response 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 design 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. Overload Protection 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 off the high-side switch for 16ms and re-enters soft-start. If the overload condition is still present, the device repeats the cycle. ���������������������������������������������������������������� Maxim Integrated Products 11 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current Skip Mode/Standby Mode 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. Overtemperature Protection 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 VOUT RFB1 MAX16977 FB RFB2 Figure 1. Adjustable Output-Voltage Setting SWITCHING FREQUENCY vs. RFOSC MAX16977 toc06 SWITCHING FREQUENCY (MHz) 3.0 2.5 2.0 internal bias regulator and the step-down converter, allowing the IC to cool. The thermal sensor turns on the IC again after the junction temperature cools by 15NC. Applications Information Setting the Output Voltage 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 V FB where VFB = 1V (see the Electrical Characteristics table). Internal Oscillator 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 1.5 1.0 VIN = 14V ILOAD = 1.5A 0.5 0 12 15 18 RFOSC (kI) Figure 2. Switching Frequency vs. RFOSC 21 24 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 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current Table 1. Inductor Size Comparison INDUCTOR SIZE SMALLER LARGER Lower price Smaller ripple Smaller form factor Higher efficiency Faster load response Larger fixed-frequency range in skip mode 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 Input Capacitor 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 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 (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. ���������������������������������������������������������������� Maxim Integrated Products 13 MAX16977 36V, 2A, 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. Compensation Network 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 × 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 = VOUT R1 R2 VREF RC CC Figure 3. Compensation Network 1 R ×f ×L 2π × C OUT × LOAD SW + ESR R + (f × L) SW LOAD The output capacitor and its ESR also introduce a zero at: COMP gm R LOAD × fSW × L R LOAD + (fSW × L) fzMOD = CF 1 2π × ESR × C OUT 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. ���������������������������������������������������������������� Maxim Integrated Products 14 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current The feedback voltage-divider has a gain of GAINFB = VFB/VOUT, where VFB is 1V (typ). The transconductance error amplifier has a DC gain of GAINEA(DC) = gm,EA x ROUT,EA, where gm,EA is the error-amplifier 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 fpdEA = 2π × C C × (R OUT,EA + R C ) 1 fzEA = 2π × C C × R C fpEA = 1 2π × C F × R C f fpMOD << fC ≤ SW 5 The total loop gain as the product of the modulator gain, the feedback voltage-divider gain, and the error-amplifier gain at fC should be equal to 1. So: V GAINMOD(fC) × FB × GAINEA(fC) = 1 VOUT For the case where fzMOD is greater than fC: GAINEA(fC) = gm,EA × RC Therefore: GAINMOD(fC) × RC = VOUT g m,EA × VFB × GAINMOD(fC) 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: 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): GAINMOD(fC) = GAINMOD(DC) × Solving for RC: fpMOD fC VFB × g m,EA × R C = 1 VOUT GAINMOD(fC) = GAINMOD(DC) × fpMOD fzMOD The error-amplifier gain at fC is: f GAINEA(fC) = g m,EA × R C × zMOD fC Therefore: GAINMOD(fC) × f VFB × g m,EA × R C × zMOD = 1 VOUT fC Solving for RC: RC = VOUT × fC g m,EA × VFB × GAINMOD(fC) × fzMOD Set the error-amplifier compensation zero formed by RC and CC at the fpMOD (fzEA = fpMOD) CC = 1 2π × fpMOD × R C ���������������������������������������������������������������� Maxim Integrated Products 15 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current If fzMOD is less than 5 x fC, add a second capacitor CF from COMP to GND. Set fpEA = fzMOD and calculate CF as follows: 1 CF = 2π × fzMOD × R C PCB Layout Guidelines Careful PCB layout is critical to achieve low switching losses and clean, stable operation. Use a multilayer board whenever possible for better noise immunity and power dissipation. Follow these guidelines for good PCB layout: 1) Use a large contiguous copper plane under the IC package. Ensure that all heat-dissipating components have adequate cooling. The bottom pad of the device must be soldered down to this copper plane for effective heat dissipation and for getting the full power out of the IC. Use multiple vias or a single large via in this plane for heat dissipation. 2) Isolate the power components and high-current path from the sensitive analog circuitry. This is essential to prevent any noise coupling into the analog signals. 3) Keep the high-current paths short, especially at the ground terminals. This practice is essential for stable, jitter-free operation. The high-current path composed of input capacitor, high-side FET, inductor, and the output capacitor should be as short as possible. 4) Keep the power traces and load connections short. This practice is essential for high efficiency. Use thick copper PCBs (2oz vs. 1oz) to enhance full-load efficiency. 5) The analog signal lines should be routed away from the high-frequency planes. This ensures integrity of sensitive signals feeding back into the IC. 6) The ground connection for the analog and power section should be close to the IC. This keeps the ground current loops to a minimum. In cases where only one ground is used, enough isolation between analog return signals and high-power signals must be maintained. Chip Information PROCESS: BiCMOS Ordering Information SPREAD SPECTURM TEMP RANGE PIN-PACKAGE MAX16977RAUE/V+ MAX16977SAUE/V+† PART Disabled -40NC to +125NC 16 TSSOP-EP* Enabled -40NC to +125NC 16 TSSOP-EP* MAX16977RATE/V+† MAX16977SATE/V+† Disabled -40NC to +125NC 16 TQFN-EP* Enabled -40NC to +125NC 16 TQFN-EP* /V denotes an automotive qualified part. +Denotes a lead(Pb)-free/RoHS-compliant package. †Future product—contact factory for availability. *EP = Exposed pad. Package Information For the latest package outline information and land patterns (footprints), go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE 16 TSSOP-EP 16 TQFN-EP U16E+3 T1655+4 OUTLINE NO. 21-0108 21-0140 LAND PATTERN NO. 90-0120 90-0121 ���������������������������������������������������������������� Maxim Integrated Products 16 MAX16977 36V, 2A, 2.2MHz Step-Down Converter with Low Operating Current Revision History REVISION NUMBER REVISION DATE 0 5/11 DESCRIPTION Initial release PAGES CHANGED — 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. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance. 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