MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter General Description Benefits and Features The MAX17681/MAX17681A uses peak-current-mode control. The low-resistance, on-chip MOSFETs ensure high efficiency at full load while simplifying the PCB layout. Reduces Number of DC-DC Regulators to Stock * Wide 4.5V to 42V Input * 0.9V to 0.96 x VIN Primary Output Voltage * Delivers Up to 5W Output Power The MAX17681/MAX17681A is a high-voltage, highefficiency, iso-buck DC-DC converter designed to provide isolated power up to 5W. The device operates over a wide 4.5V to 42V input and uses primary-side feedback to regulate the output voltage. The MAX17681/MAX17681A devices generate a well regulated primary side voltage which is then scaled by a suitable transformer turns ratio to derive isolated secondary output rails. While both MAX17681 and MAX17681A support primary side overcurrent protection, the MAX17681A is an enhanced design that supports robust secondary-side overcurrent protection as well. The MAX17681/MAX17681A is available in a compact 10-pin (3mm x 2mm) TDFN package. Simulation models are available. Applications Reduces External Components and Total Cost * No Optocoupler * Synchronous Primary Operation * All-Ceramic Capacitors, Compact Layout Reduces Power Dissipation * Peak Efficiency > 90% * 0.9A (typ) Shutdown Current Operates Reliably in Adverse Industrial Environments * Peak and Sink Current-Limit Protection * 1.7% Feedback Accuracy * Programmable EN/UVLO Threshold * Adjustable Soft-Start * Overtemperature Protection * -40C to +125C Operation Short-Circuit Protection * MAX17681A Supports Robust Secondary-Side Short-Circuit Protection * MAX17681A Is Recommended for All New Designs Isolated Fieldbus Interfaces PLC I/O Modules Smart Meters Isolated Power Supplies in Medical Equipment Floating Power Supply Generation Ordering Information appears at end of data sheet. Application Circuit VIN 17V TO 32V LX VIN C1 1F EN/UVLO C3 1F C4 33nF 19-7053; Rev 4; 3/18 R3 4.75k C5 33nF PGND NPRI VOUT 24V, 100mA D1 NSEC C7 2.2F R4 49.9 Z1 VCC GND MAX17681 MAX17681A SS R1 105k C2 10uF FB R2 10k COMP C6 680pF T1 1:2.4 RESET C8 1nF MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Absolute Maximum Ratings VIN to GND.............................................................-0.3V to +48V EN/UVLO to GND...................................... -0.3V to (VIN + 0.3V) LX to PGND............................................... -0.3V to (VIN + 0.3V) VCC, FB, RESET, COMP, SS to GND.....................-0.3V to +6V PGND to GND.......................................................-0.3V to +0.3V LX Total RMS Current.........................................................1.6A Output Short-Circuit Duration.....................................Continuous Operating Temperature Range.......................... -40C to +125C Junction Temperature.......................................................+150C Storage Temperature Range............................. -65C to +160C Lead Temperature (soldering, 10s).................................. +300C Soldering Temperature (reflow)........................................+260C 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. Package Information PACKAGE TYPE: 10 TDFN Package Code T1032N+1 Outline Number 21-0429 Land Pattern Number 90-0082 THERMAL RESISTANCE, FOUR-LAYER BOARD (Note 1) Junction to Ambient (JA) 67.3C/W Junction to Case (JC) 18.2C/W For the latest package outline information and land patterns (footprints), go to www.maximintegrated.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 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 www.maximintegrated.com/thermal-tutorial. Note 1: Continuous Power Dissipation (TA = +70C) (derate 14.9mW/C above +70C) (multilayer board) 1188.7mW www.maximintegrated.com Maxim Integrated 2 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Electrical Characteristics (VIN = 24V, VGND = VPGND = 0V, CVIN = 2.2F, CVCC = 1F, VEN = 1.5V, CSS = 3300pF, VFB = 0.98 x VOUT, COMP = unconnected, LX = unconnected, RESET = unconnected. TA = TJ = -40C to +125C, unless otherwise noted. Typical values are at TA = +25C. All voltages are referenced to GND, unless otherwise noted.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS INPUT SUPPLY (VIN) Input Voltage Range Input Supply Current 42 V IIN-SH VIN VEN = 0V, shutdown mode 4.5 0.9 3.5 A IIN-SW Normal switching mode, no load 1.95 2.8 mA VENR VEN rising 1.183 1.218 1.253 VENF VEN falling 1.1 1.135 1.17 V 8 200 nA 4.65 5 5.35 V 40 80 mA ENABLE/UVLO (EN/UVLO) EN Threshold VEN-TRUESD EN Input Leakage Current VEN falling, true shutdown IEN VEN = VIN = 42V, TA = +25C VCC 6V < VIN < 12V, 0mA < IVCC < 10mA, 12V < VIN < 42V, 0mA < IVCC < 2mA 0.7 LDO VCC Output Voltage Range VCC Current Limit IVCC-MAX VCC = 4.3V, VIN = 12V 15 VCC-DO VIN = 4.5V, IVCC = 5mA 4.1 VCC-UVR VCC rising 3.85 4 4.15 VCC-UVF VCC falling 3.55 3.7 3.85 0.55 0.85 High-Side pMOS On-Resistance RDS-ONH ILX = 0.5A (sourcing) Low-Side nMOS On-Resistance RDS-ONL ILX = 0.5A (sinking) LX Leakage Current ILX_LKG VEN = 0V, TA = +25C, VLX = (VPGND + 1V) to (VIN - 1V) VCC Dropout VCC UVLO V V POWER MOSFETs TA = +25C TA = TJ = +125C (Note 3) 1.2 TA = +25C 0.2 TA = TJ = +125C (Note 3) 0.35 0.47 1 A A SOFT-START (SS) Charging Current ISS VSS = 0.5V 4.7 5 5.3 0.884 0.9 0.916 V 100 nA FEEDBACK (FB) FB Regulation Voltage VFB_REG FB Input Bias Current IFB TA = +25C TRANSCONDUCTANCE AMPLIFIER (COMP) Transconductance GM ICOMP = 2.5A 510 590 650 S 19 32 55 A COMP Source Current ICOMP_SRC COMP Sink Current ICOMP_SINK 19 32 55 A RCS 0.45 0.5 0.55 V/A Current Sense Transresistance CURRENT LIMIT www.maximintegrated.com Maxim Integrated 3 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Electrical Characteristics (continued) (VIN = 24V, VGND = VPGND = 0V, CVIN = 2.2F, CVCC = 1F, VEN = 1.5V, CSS = 3300pF, VFB = 0.98 x VOUT, COMP = unconnected, LX = unconnected, RESET = unconnected. TA = TJ = -40C to +125C, unless otherwise noted. Typical values are at TA = +25C. All voltages are referenced to GND, unless otherwise noted.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Peak Current-Limit Threshold IPEAK-LIMIT 1.4 1.65 1.9 A Runaway Current-Limit Threshold IRUNAWAY- 1.45 1.7 2 A Sink Current-Limit Threshold ISINK-LIMIT 1.05 1.25 1.45 A fSW 186 200 213 kHz 73.14 % LIMIT TIMINGS Switching Frequency Events to Hiccup After Crossing Runaway Current Limit VOUT Undervoltage Trip Level to Cause Hiccup 1 VOUT-HICF VSS > 0.95V (soft-start is done) 67.86 Hiccup Timeout Minimum On-Time Maximum Duty Cycle 70.5 32768 tON_MIN DMAX VFB = 0.98 x VFB-REG Cycles 200 300 415 ns 96.5 97.5 98.5 % LX Dead Time 12 RESET ns RESET Output Level Low IRESET = 1mA 0.02 V RESET Output Leakage Current High VFB = 1.01 x VFB-REG, TA= 25C 0.45 A FB Threshold for RESET Falling VFB-OKF VFB falling 90.5 92.5 94.5 % FB Threshold for RESET Rising VFB-OKR VFB rising 93.5 95.5 97.5 % RESET Delay After FB Reaches 95% Regulation VFB rising 1024 Cycles Temperature rising 165 C 10 C THERMAL SHUTDOWN Thermal Shutdown Threshold Thermal Shutdown Hysteresis Note 2: All limits are 100% tested at +25C. Limits over temperature are guaranteed by design. Note 3: Guaranteed by design, not production tested. www.maximintegrated.com Maxim Integrated 4 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Typical Operating Characteristics (VIN = 24V, VGND = VPGND = 0V, CVIN = 1F, CVCC = 1F, VEN = 1.5V, CSS = 33nF, VFB = 0.98 x VPRI, TA = +25C, unless otherwise noted.) OUTPUT VOLTAGE REGULATION toc2 8 6 REGULATION (%) 4 VIN = 32V 2 VIN = 24V 0 VIN = 17V -2 VIN = 19V -4 -6 FIGURE 9 APPLICATION CIRCUIT 0 10 20 30 40 50 60 70 80 90 100 LOAD CURRENT (mA) EN/UVLO THRESHOLD VOLTAGE VS. TEMPERATURE EN/UVLO THRESHOLD VOLTAGE (V) 1.26 toc4 RISING 1.22 1.18 1.14 FALLING 1.1 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C) www.maximintegrated.com Maxim Integrated 5 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Typical Operating Characteristics (continued) (VIN = 24V, VGND = VPGND = 0V, CVIN = 1F, CVCC = 1F, VEN = 1.5V, CSS = 33nF, VFB = 0.98 x VPRI, TA = +25C, unless otherwise noted.) LOAD TRANSIENT RESPONSE, (LOAD CURRENT STEPPED FROM 50mA to 100mA) toc9 VOUT (AC) FIGURE6 FIGURE 9 APPLICATION APPLICATION CIRCUIT CIRCUIT VOUT=5V 500mV/ div 50mA/ div IOUT 400s/div SOFT-START BODE PLOT toc12 toc15 5V/div 10V/div VOUT 5V/div VPRI 100mA/div I OUT FIGURE 9 APPLICATION CIRCUIT PHASE GAIN (dB) UVLO IOUT GAIN PHASE () VEN/ fCR = 6.7kHz, PHASE MARGIN = 80 FIGURE 9 APPLICATION CIRCUIT 1ms/div www.maximintegrated.com Maxim Integrated 6 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Pin Configuration TOP VIEW LX 10 + 1 GND RESET COMP SS 9 8 6 7 MAX17681 MAX17681A 2 PGND VIN 3 4 EP 5 EN/ VCC UVLO FB TDFN 3mm x 2mm *EP = EXPOSED PAD, CONNECTED TO GND Pin Description PIN NAME FUNCTION 1 PGND Power Ground. Connect PGND externally to the power ground plane. Connect GND and PGND pins together at the ground return path of the VCC bypass capacitor. 2 VIN 3 EN/UVLO 4 VCC 5 FB Output Feedback Connection. Connect FB to a resistor-divider between VPRI and GND to set the output voltage. See the Adjusting the Primary Output Voltage section for details. 6 SS Soft-Start Input. Connect a ceramic capacitor from SS to GND to set the soft-start time. 7 COMP Compensation Input. Connect an RC network from COMP to GND. See the External Loop Compensation section. 8 RESET Open-Drain Reset Output. Pull up RESET to an external power supply with an external resistor. RESET pulls low if FB voltage drops below 92.5% of its set value. RESET goes high impedance 1024 clock cycles after FB voltage rises above 95.5% of its set value. 9 GND 10 LX Switching Node. Connect LX to the switching side of the transformer. LX is high impedance when the device is in shutdown mode. -- EP Exposed Pad. Connect to the GND pin of the IC. Connect to a large copper plane below the IC to improve heat dissipation capability. www.maximintegrated.com Switching Regulator Input. Connect a X7R ceramic capacitor from VIN to PGND for bypassing. Active-High, Enable/Undervoltage-Detection Input. Pull EN/UVLO to GND to disable the regulator output. Connect EN/UVLO to VIN for always-on operation. Connect a resistor-divider between VIN, EN/UVLO, and GND to program the input voltage at which the device is enabled and turns on. Internal LDO Output. Bypass VCC to GND with a minimum 1F capacitor. Signal Ground. Maxim Integrated 7 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Block Diagram MAX17681 MAX17681A LDO REGULATOR VCC HIGH-SIDE CURRENT SENSE PEAK CURRENT LIMIT POK RUNAWAY EN/UVLO CHIPEN 1.218 HIGH-SIDE DRIVER PWM CONTROL LOGIC THERMAL SHUTDOWN * CLX CHIPEN OSCILLATOR LX HICCUP SECONDARY OVERCURRENT PROTECTION LOGIC SLOPE LOW-SIDE DRIVER SINK LIMIT PGND LIMIT SLOPE COMP CS FB VIN CS + + LOW-SIDE CURRENT SENSE PWM GND REF VCC RESET 0.8595 5A SS HICCUP www.maximintegrated.com FB REFERENCE SWITCHOVER CIRCUIT 5.12ms DELAY REF * SECONDARY OVER-CURRENT PROTECTION LOGIC ONLY FOR MAX17681A Maxim Integrated 8 MAX17681 Detailed Description The MAX17681/MAX17681A is a high-voltage, highefficiency, iso-buck DC-DC converter designed to provide isolated power up to 5W. The device operates over a wide 4.5V to 42V input and uses primary side feedback to regulate the output voltage. The MAX17681/MAX17681A uses peak-current-mode control. The low-resistance, on-chip MOSFETs ensure high efficiency at full load while simplifying the PCB layout. The programmable soft-start feature allows users to reduce input inrush current. The device also incorporates an output enable/undervoltage lockout pin (EN/UVLO) that allows the user to turn on the part at the desired input-voltage level. An open-drain RESET pin provides a delayed power-good signal to the system upon achieving successful regulation of the primary output voltage. The device operates over the -40C to +125C industrial temperature range and is available in a compact 10-pin (3mm x 2mm) TDFN package. Linear Regulator (VCC) An internal linear regulator (VCC) provides a 5V nominal supply to power the internal blocks and the low-side MOSFET driver. The output of the VCC linear regulator should be bypassed with a 1F ceramic capacitor to GND. The device employs an undervoltage-lockout circuit that disables the internal linear regulator when VCC falls below 3.7V (typ). The internal VCC linear regulator can source up to 40mA (typ) to supply the device and to power the low-side gate driver. Enable Input (EN/UVLO) and Soft-Start (SS) When the EN/UVLO voltage increases above 1.218V (typ), the device initiates a soft-start sequence with the duration of the soft-start being dependent on the value of the capacitor connected from SS to GND. A 5A current source charges the capacitor and ramps up the SS pin voltage. The SS pin voltage is used as reference for the internal error amplifier. The reference ramp-up allows the output voltage to increase monotonically from zero to the target value. www.maximintegrated.com 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter The EN/UVLO can be used as an input-voltage UVLOadjustment input. An external voltage-divider between VIN and EN/UVLO to GND adjusts the input voltage at which the device turns on or turns off. See the Setting the Input Undervoltage Lockout Level section for details. If input UVLO programming is not desired, connect the EN/UVLO to VIN (see the Electrical Characteristics table for the EN/ UVLO rising and falling-threshold voltages). Driving the EN/UVLO low disables both power MOSFETs as well as other internal circuitry and reduces VIN quiescent current to 0.9A (typ). The SS capacitor is discharged with an internal pulldown resistor when the EN/UVLO is low. If the EN/UVLO pin is driven from an external signal source, a series resistance of minimum 1k is recommended to be placed between the signal source output and the EN/ UVLO pin to reduce voltage ringing on the line. Overcurrent Protection/HICCUP Mode The MAX17681/MAX17681A are provided with an overcurrent-protection scheme that protects the device under overload and output short-circuit conditions. A cycleby-cycle peak current limit turns off the high-side MOSFET whenever the switch current exceeds the internal limit of 1.65A (typ). Additionally, the sink current limit turns off the low-side switch when the low side MOSFET negative current exceeds 1.25A (typ). A runaway current limit on the highside MOSFET current at 1.7A (typ) protects the devices under high input voltage, short-circuit conditions. The MAX17681 enters hiccup mode, either on one occurrence of the runaway current limit or when the primary output voltage (VPRI) drops to 70.5% (typ) of its nominal value after the soft-start is completed. In the MAX17681, when hiccup is triggered, the converter is protected by suspending switching for a hiccup timeout period of 32,768 clock cycles. Once the hiccup timeout period expires, soft-start is attempted again. This behaviour works well for primary output over-current events. However, when secondaryside overcurrent events occur, additional measures are required for the Iso-Buck topology to enter into hiccup mode reliably, and also support robust output voltage recovery after overcurrent removal. These measures are not supported by MAX17681. Maxim Integrated 9 MAX17681 The MAX17681A provides the robust secondary overcurrent protection, and smooth output voltage recovery after removal of overcurrent, by entering into hiccup mode after detecting 16 consecutive negative current limit events. This is supported by implementing a scheme where the primary capacitor voltage is actively discharged during the hiccup timeout period, and soft-starting both primary and secondary-side outputs. The MAX17681A enters hiccup mode, either on one occurrence of the runaway current limit, when the primary output voltage drops to 71.14% (typ) of its nominal value after the soft-start is completed, or when 16 consecutive negative current limit events occur. When hiccup is triggered, the converter enters a hiccup timeout period of 32,768 clock cycles. During this period, the high side switch is kept off and the low side switch is turned on each cycle until the low side MOSFET negative current reaches 0.6A limit. This mode of operation effectively produces a negative current in the primary capacitor and discharges it towards zero. Once the hiccup timeout period expires, the MAX17681A smoothly soft starts both primary and secondary output voltages. If the output capacitance is such that it is discharged to zero within one hiccup timeout period, the MAX17681A executes a normal soft-start operation upon exit from the hiccup timeout period. For cases, where the capacitor is sized such that it does not discharge to zero in one hiccup timeout period, during the next soft-start attempt the converter may re-enter the hiccup time period due to one of the event which triggers hiccup mode. Eventually the primary capacitor is completely discharged and the smooth output voltage recovery is ensured. www.maximintegrated.com 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter In summary the MAX17681 provides primary side overcurrent protection, whereas the MAX17681A provides both primary and secondary side over-current protection. RESET Output The device includes a RESET comparator to monitor the primary output voltage. The open-drain RESET output requires an external pullup resistor. RESET can sink 2mA of current while low. RESET goes high (high-impedance) 1024 switching cycles after the primary output increases above 95.5% of the nominal regulated voltage. RESET goes low when the primary output voltage drops to below 92.5% of the nominal regulated voltage. In MAX17681A, when the secondary output is shorted, the primary output voltage is discharged as well during the hiccup period. So, in this case, even for a fault on the isolated output, the RESET can be used as an indicator. RESET also goes low during thermal shutdown. RESET is valid when the device is enabled and VIN is above 4.5V. Thermal-Overload Protection Thermal-overload protection limits total power dissipation in the device. When the junction temperature of the device exceeds +165C, an on-chip thermal sensor shuts down the device, allowing the device to cool. The thermal sensor turns the device on again after the junction temperature cools by 10C. Carefully evaluate the total power dissipation (see the Power Dissipation section) to avoid unwanted triggering of the thermal-overload protection in normal operation. Maxim Integrated 10 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Applications Information secondary side diode is reverse-biased and the load current is supplied by the secondary-side filter capacitor COUT. Operation of the Iso-Buck Converter The iso-buck is a synchronous-buck-converter-based topology, useful for generating isolated outputs at low power level without using an optocoupler. Figure 1 shows the basic circuit of an iso-buck converter, consists of a half-bridge transformer driver and secondary side filter. Figure 2 shows the equivalent circuit when the high-side switch (QHS) is ON. During this time, the primary current ramps up and stores energy in the transformer magnetizing inductance LPRI and the primary capacitor CPRI. The Figure 3 shows the equivalent circuit when the low-side switch (QLS) is on. During this time, the secondary diode gets forward-biased. The primary current ramps down and releases stored energy in the transformer magnetizing inductance and the primary capacitor to the load. Operating waveforms of the converter are shown in Figure 4. Neglecting diode drop VD, transformer resistances, and leakage inductance, the output voltage VOUT is proportional to the primary output voltage VPRI and is regulated by the MAX17681/MAX17681A control loop. QHS QHS D1 T1 VIN + NPRI NSEC - COUT RLOAD - + QLS + VPRI NSEC VPRI CPRI - COUT RLOAD + - Figure 1. Iso-Buck Topology VOUT NPRI VIN QLS D1 T1 VOUT CPRI Figure 3. Off-Period Equivalent Circuit QHS QLS QHS D1 T1 VOUT IPRI I IPK_PRI INEGPK_ VIN QLS PRI + NPRI NSEC - COUT RLOAD IPK_SE ISEC VPRI DxTS + - CPRI Figure 2. On-Period Equivalent Circuit www.maximintegrated.com C (1-D)xTS IOUT Figure 4. Iso-Buck Operating Waveforms Maxim Integrated 11 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Primary Output Voltage Selection Primary output voltage is regulated by the MAX17681/ MAX17681A control loop. The primary output voltage can be calculated by using the equation: Primary Inductance Selection Primary inductance value determines the ripple current in the transformer. The required primary inductance is given by the equation: = VPRI D MAX x VIN_MIN L PRI= 7 x VPRI where DMAX is the maximum duty cycle of the converter and VIN_MIN is the minimum input voltage. Maximum duty cycle should be in the range of 0.4 to 0.6 for ideal iso-buck operation. where LPRI is the primary inductance in H and VPRI is the primary output voltage. Adjusting the Primary Output Voltage The primary output voltage is set with a resistor-divider from primary output to FB to GND (see Figure 5). Choose R2 in the range of 10k to 49.9k and calculate R1 using the equation: V R1 =x R2 PRI - 1 0.9 Neglecting diode drop VD, transformer resistances, and leakage inductance, the iso-buck output voltage VOUT is proportional to the primary output voltage VPRI. The turns ratio (K) is given by the equation: N SEC VOUT + VD = NPRI VPRI N SEC NPRI Turns ratio can be adjusted to match with the readily available off-the-shelf transformer turns ratio by adjusting the primary output voltage. D1 LX + NPRI NSEC MAX17681 MAX17681A R1 + - FB V VPRI x 1 - PRI V IN I = f SW x L PRI where LPRI is the primary inductance in H, fSW is the switching frequency in Hz, VPRI is the primary output voltage, VIN is the input voltage. Winding Peak and RMS Currents Turns Ratio Selection K= The primary ripple current can be calculated using the equation: CPRI R2 - COUT Windings peak and RMS current ratings should be specified for selecting the iso-buck transformer. Primary and secondary winding peak currents are given by the equations: n IHS_AVG = IPRI + I OUT x K i i i=1 I IPK_PRI IHS_AVG + = 2 I OUT i IPK_SEC i (1 - D) V D = PRI VIN where n is the total number of isolated outputs, i is the individual isolated output, IPRI is the primary load current, IOUTi is the individual secondary load current, Ki is the individual secondary turns ratio, D is the duty cycle, and I is the primary ripple current. Primary RMS current is the sum of the high-side and lowside switch RMS currents. High-side switch RMS current: I 2 IHS_RMS = Dx IHS_AVG 2 + 12 Figure 5. Adjusting the Primary Output Voltage www.maximintegrated.com Maxim Integrated 12 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Primary Output Capacitor Selection Low-side switch RMS current: ILS_RMS = n 1 x (I OUT x K i ) (IPK_PRI - i (1 - D) i=1 (1 - D) x I 2 I 2 ) - + 2 12 X7R ceramic output capacitors are preferred due to their stability over temperature in industrial applications. The minimum required output capacitance is given by the equation: C PRI = IHS_RMS 2 + ILS_RMS 2 Secondary winding RMS current is given by the equation: I SEC_RMS = i I OUT i (1 - D) Leakage Inductance Transformer leakage inductance (LLEAK) plays a key role in determining the output voltage regulation. For better output voltage regulation, leakage inductance should be reduced to less than 1% of the primary inductance value. Higher leakage inductance also limits the amount of power delivered to the output. Primary Negative Peak Current The primary current can go negative when the low side switch is turned on. Steady-state primary negative peak current should be verified not to exceed -1A. The primary negative peak current can be calculated using the equation: n 1 INEGPK = x (I OUT x K i ) - I _PRI IPK _PRI - i (1 - D) i=1 Specifying the Iso-Buck Transformer An off-the-shelf transformer or coupled inductor can be used as an Iso-buck transformer. If readily not available, use the table below to specify the Iso-buck transformer parameters to transformer vendors. www.maximintegrated.com f SW x 0.01x VPRI D MAX = Primary winding RMS current: = IPRI_RMS IHS_AVG x D MAX VPRI VIN_MIN Where IOUT is the load current, K is the turns ratio, fSW is the switching frequency, VPRI is the primary output voltage, VIN_MIN is the minimum input voltage. Secondary Output Capacitor Selection A secondary side capacitor supplies load current when the high-side switch is on. The required output capacitance to support 1% steady state ripple is given by the equation: C OUT = I OUT x D MAX f SW x 0.01x VOUT It should be noted that dielectric materials used in ceramic capacitors exhibit capacitance loss due to DC bias levels and should be appropriately derated to ensure the required output capacitance is obtained in the application. Table 1. Specifying Iso-Buck Transformer PARAMETER SYMBOL Primary Inductance LPRI Leakage Inductance LLEAK Primary Ripple Current I Primary Peak Current IPK_PRI Primary RMS Current IPRI_RMS Secondary Peak Current IPK_SEC Secondary RMS Current ISEC_RMS Working Voltage VAC, VDC Insulation Level VAC, VDC Maxim Integrated 13 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Input Capacitor Selection Ceramic input capacitors are recommended for the IC. The input capacitor reduces peak current drawn from the power source and reduces noise and voltage ripple on the input caused by the switching circuitry. In applications where the source is located distant from the device input, an electrolytic capacitor should be added in parallel to the input ceramic capacitor to provide necessary damping for potential oscillations caused by the longer input power path and input ceramic capacitor. The required input capacitance can be calculated using the equation: C IN = IHS_AVG x D MAX x (1 - D MAX ) Soft-Start Capacitor Selection The MAX17681/MAX17681A implements adjustable softstart operation to reduce inrush current. A capacitor connected from the SS pin to GND programs the soft-start period. The soft-start time (tSS) is related to the capacitor connected at SS (CSS) by the following equation: f SW x VIN D MAX = A resistor connected in series with a Zener diode (See R4, Z1 in Figure 9) can be used as an overvoltage protection circuit to limit the overvoltage under absolute no load conditions. The Zener diode threshold can be selected as 15% higher than the nominal regulated output voltage VOUT. The series resistor, R1, value can be in the range of 30 to 60. VPRI C= SS 5.55 x t SS VIN_MIN VIN is the input voltage ripple, normally 2% of the minimum input voltage, DMAX is the maximum duty cycle, and fSW is the switching frequency of operation. Secondary Diode Selection A secondary rectifier diode should be rated to carry peak secondary current and to withstand reverse voltage when the high-side switch is on. A Schottky diode with less forward-voltage drop should be selected for better output voltage regulation. where tSS is in milliseconds and CSS is in nanofarads. Setting the Input Undervoltage Lockout Level The device offers an adjustable input undervoltagelockout level. Set the voltage at which the device turns on with a resistive voltage-divider connected from VIN to GND (see Figure 6). Connect the center node of the divider to EN/UVLO. Choose R1 to be 3.3M max and then calculate R2 as follows: The peak current rating of the diode is given by: IPK_DIODE = i R2 = I OUT i (1 - D) The peak reverse voltage rating of the diode is given by: (( ) VDIODE = 2 x VIN_MAX - VPRI x K + VOUT ) Power dissipated in the diode can be calculated using the equation: PDIODE = VD x I OUT Minimum Load Requirements Under light-load conditions, the iso-buck converter output voltage increases excessively due to the transformer leakage inductance and parasitic capacitance. Normally, a minimum load of 10% to 20% of the full load is sufficient to keep the converter output voltage regulation within 5%. The output voltage regulation should be verified after testing prototype. www.maximintegrated.com R1x 1.218 V ( INU - 1.218) where VINU is the voltage at which the device is required to turn on. VIN R1 VIN MAX17681 MAX17681A EN/UVLO R2 Figure 6. Adjustable EN/UVLO Network Maxim Integrated 14 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter where POUT is the output power, is the efficiency of power conversion, RPRI is the primary resistance of the transformer, RSEC is the secondary resistance of the transformer and VD is the diode drop. MAX17681 MAX17681A The junction temperature (TJ) of the device can be estimated at any ambient temperature (TA) from the following equation: COMP RCOMP TJ= T A + ( JA x PLOSS ) CP CCOMP where JA is the junction-to-ambient thermal impedance of the package. PCB Layout Guidelines Figure 7. External Compensation Network External Loop Compensation The MAX17681/MAX17681A uses peak current-mode control scheme and needs only a simple RC network to have a stable control loop. The compensation network is shown in Figure 7. The following equations can be used for calculating the compensation components: C OUT x (1 - D) x VPRI = 6000 x f C x R COMP x K2 + C PRI where RCOMP is in , and the maximum limit for RCOMP is 12k. fC is bandwidth of the converter in Hz. Choose fC in the range of 2kHz to 10kHz. C COMP = 5 x f C x R COMP 1 CP = 2 x 50000 x R COMP Power Dissipation Ensure that the junction temperature of the device does not exceed +125C under the operating conditions specified for the power supply. At a particular operating condition, the power losses that lead to temperature rise of the device can be estimated as follows: ( 1 PLOSS = POUT x - 1 - IPRI_RMS 2 x R PRI Careful PCB layout is critical to achieve clean and stable operation. For a sample layout that ensures first-pass success, refer to the MAX17681/MAX17681A evaluation kit layouts available at www.maximintegrated.com. Follow these guidelines for good PCB layout: 1) All connections carrying pulsed currents must be very short and as wide as possible. The loop area of these connections must be made very small to reduce stray inductance and radiated EMI. 2) A ceramic input filter capacitor should be placed close to the VIN pin of the device. The bypass capacitor for the VCC pin should also be placed close to the VCC pin. External compensation components should be placed close to the IC and far from the LX node. The feedback trace should be routed as far as possible from the LX node. 3) Signal and power grounds must be kept separate. They should be connected together at a point where switching noise is minimum, typically the return terminal of the VCC bypass capacitor. The ground plane should be kept continuous as much as possible. 4) Multiple thermal vias that connect to a large ground plane should be provided under the exposed pad of the device, for efficient heat dissipation. Figure 8 show the recommended component placement for the MAX17681/MAX17681A iso-buck converter. ) ) ( - I SEC_RMS 2 x R SEC - (VD x I OUT ) P= OUT VOUT x I OUT www.maximintegrated.com Maxim Integrated 15 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter U1 VIN CIN R1 VCC CVCC COUT IGND GND MAX17681 MAX17681A CPRI R3 SS FB CSS R4 COMP CCF VOUT PGND EN/UVLO R2 D1 T1 LX VIN CY RESET RF CF CY D1 VOUT VPRI COUT CPRI U1 PGND PLANE PGND CIN VIN PLANE GND VIN R1 EN/UVLO R2 RESET COMP VCC CVCC FB R3 IGND T1 LX SS CSS RF CCF CF R4 SGND PLANE VIAS TO BOTTOM SIDE GROUND PLANE VIAS TO BOTTOM SIDE PGND TRACK VIAS TO BOTTOM SIDE VPRI TRACK Figure 8. Recommended Component Placement www.maximintegrated.com Maxim Integrated 16 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Typical Application Circuits VIN 17V TO 32V LX VIN C1 1F EN/UVLO PGND VOUT 24V, 100mA C7 2.2F NSEC R4 49.9 GND SS C2 10uF R1 105k FB R2 10k COMP C6 680pF NPRI MAX17681 MAX17681A C4 33nF D1 Z1 VCC C3 1F T1 1:2.4 C8 1nF RESET R3 4.75k C1 : MURATA 1F/X7R/50V/1206 (GRM31MR71H105K) C2 : MURATA 10F/X7R/16V/1206 (GRM31CR71C106K) C7 : MURATA 2.2F/X7R/50V/1206 (GRM31CR71H225K) C8 : AVX 1nF/X7R/1.5KV/1206 (1206SC102KAT3A) D1 : DIODES INC, DFLS1200-7 Z1 : DIODES INC, MMSZ5254B-7-F T1 : ETAL, 303993 C5 33nF Figure 9. Low-Profile 24V to 24V, 100mA Isolated Output Application Circuit VIN 17V TO 32V C1 1F EN/UVLO C3 1F C4 33nF NSEC1 PGND SS C6 680pF Z1 NSEC2 MAX17681 MAX17681A R4 49.9 NPRI VCC C8 2.2F GND R5 49.9 Z2 R1 86.6k FB R2 10k COMP R3 4.75k C7 2.2F LX VIN VOUT1 +15V, 75mA D1 T1 1:1.8:1.8 RESET C5 33nF C2 10uF D2 C9 1nF VOUT2 -15V, 75mA C1 : MURATA 1F/X7R/50V/1206 (GRM31MR71H105K) C2 : MURATA 10F/X7R/16V/1206 (GRM31CR71C106K) C7, C8 : MURATA 2.2F/X7R/25V/1206 (GRM31MR71E225K) C9 : AVX 1nF/X7R/1.5KV/1206 (1206SC102KAT3A) D1, D2 : DIODES INC, DFLS1200-7 Z1, Z2 : DIODES INC, DDZ17-7 T1 : WURTH, 750342557 Figure 10. 24V to 15V, 75mA Isolated Output Application Circuit www.maximintegrated.com Maxim Integrated 17 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Typical Application Circuits (continued) VIN 18V TO 30V LX VIN C1 4.7F EN/UVLO C3 1F C4 33nF C6 470pF C7 1F R4 1.5k C8 1F R5 1.5k NSEC1 PGND NPRI VCC MAX17681 MAX17681A GND NSEC2 VOUT2 +3.3V/80mA R1 26.7k SS FB C2 22uF R2 10k COMP VOUT1 +16V, 65mA D1 T1 1:5.3:5.3 RESET R3 6.8k C5 47nF C9 1nF D2 VOUT2 -16V, 65mA C1 : MURATA 4.7F/X7R/50V/1210 (GRM32ER71H475KA88) C2 : MURATA 22F/X7R/10V/1210 (GRM32ER71A226ME20) C7, C8 : MURATA 1F/X7R/50V/1206 (GRM31MR71H105KA88) C9 : AVX 1nF/X7R/1.5KV/1206 (1206SC102KAT3A) D1, D2 : DIODES INC, BAS521 T1 : SUMIDA,CEI-120-06340-T294 Figure 11. 24V to 3.3V/80mA Non-Isolated and 16V,65mA Isolated Output Application circuit Ordering Information PART TEMP RANGE PIN-PACKAGE MAX17681ATB+ -40C to +125C 10L TDFN-EP* MAX17681AATB+ -40C to +125C 10L TDFN-EP* +Denotes a lead (Pb)-free/RoHS-compliant package. *EP = Exposed pad. Chip Information PROCESS: BiCMOS www.maximintegrated.com Maxim Integrated 18 MAX17681 4.5V to 42V Input, High-Efficiency, Iso-Buck DC-DC Converter Revision History REVISION NUMBER REVISION DATE PAGES CHANGED 0 9/14 Initial release -- 1 10/15 Equation updated 12 2 1/17 Added MAX17681A to Ordering Information table 3 5/17 Updated Ordering Information footnote 1 4 3/18 Reversed Ordering Information footnote and updated Benefits and Features section. 1 DESCRIPTION 1-17 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated's website at www.maximintegrated.com. Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses are implied. Maxim Integrated 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. Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc. (c) 2018 Maxim Integrated Products, Inc. 19 Mouser Electronics Authorized Distributor Click to View Pricing, Inventory, Delivery & Lifecycle Information: Maxim Integrated: MAX17681ATB+ MAX17681ATB+T