19-1462; Rev 0; 6/99 NUAL KIT MA ATION U EET L H A S V A E T WS DA O L L O F 28V, PWM, Step-Up DC-DC Converter The MAX618 CMOS, PWM, step-up DC-DC converter generates output voltages up to 28V and accepts inputs from +3V to +28V. An internal 2A, 0.3Ω switch eliminates the need for external power MOSFETs while supplying output currents up to 500mA or more. A PWM control scheme combined with Idle Mode™ operation at light loads minimizes noise and ripple while maximizing efficiency over a wide load range. No-load operating current is 500µA, which allows efficiency up to 93%. A fast 250kHz switching frequency allows the use of small surface-mount inductors and capacitors. A shutdown mode extends battery life when the device is not in use. Adaptive slope compensation allows the MAX618 to accommodate a wide range of input and output voltages with a simple, single compensation capacitor. The MAX618 is available in a thermally enhanced 16pin QSOP package that is the same size as an industrystandard 8-pin SO but dissipates up to 1W. An evaluation kit (MAX618EVKIT) is available to help speed designs. Features ♦ Adjustable Output Voltage Up to +28V ♦ Up to 93% Efficiency ♦ Wide Input Voltage Range (+3V to +28V) ♦ Up to 500mA Output Current at +12V ♦ 500µA Quiescent Supply Current ♦ 3µA Shutdown Current ♦ 250kHz Switching Frequency ♦ Small 1W 16-Pin QSOP Package Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX618EEE -40°C to +85°C 16 QSOP Applications Automotive-Powered DC-DC Converters Industrial +24V and +28V Systems LCD Displays Typical Operating Circuit Palmtop Computers Pin Configuration VIN 3V TO 28V TOP VIEW GND 1 15 PGND LX 3 LX 4 VOUT UP TO 28V MAX618 16 GND LX 2 LX IN SHDN PGND 14 PGND MAX618 13 PGND SHDN 5 12 GND COMP 6 11 VL FB 7 10 IN GND 8 9 VL COMP FB GND GND QSOP Idle Mode is a trademark of Maxim Integrated Products. ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. MAX618 General Description MAX618 28V, PWM, Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10sec) .............................+300°C IN to GND ...............................................................-0.3V to +30V LX to GND ..............................................................-0.3V to +30V VL to GND ................................................................-0.3V to +6V SHDN, COMP, FB to GND ............................-0.3V to (VL + 0.3V) PGND to GND.....................................................................±0.3V Continuous Power Dissipation (TA = +70°C) (Note 1) 16-Pin QSOP (derate 15mW/°C above +70°C)...................1W Note 1: With part mounted on 0.9 in.2 of copper. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VIN = +6V, PGND = GND, CVL = 4.7µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS VIN Supply Current, No Load IIN VIN = 3V to 28V, VFB = 1.6V, SHDN = VL Supply Current, Full Load, VL Connected to IN IIN VIN = 3V to 5.5V, VFB = 1.4V, SHDN = VL = IN Supply Current, Full Load IIN VIN = 3.4V to 28V, VFB = 1.4V, SHDN = VL, VVL < VIN Shutdown Supply Current IIN VIN = 28V, VFB = 1.6V, SHDN = GND VVL VIN = 3.5V or 28V, no load VL Output Voltage VL Load Regulation ∆VVL VL Undervoltage Lockout VFB FB Input Bias Current IFB V µA 5 6.5 mA 2.5 3.5 mA 3 8 µA 3.05 3.2 V 25 40 mV 2.58 2.7 2.8 V 1.47 1.5 1.53 V 1 50 nA 0.08 %/V 2.9 VFB = 1.6V VIN = 3V to 6V, VOUT = 12V 0.01 Load Regulation ∆VOUT VOUT = 12V, ILOAD = 10mA to 500mA 0.2 VLX ILXON PWM mode Idle Mode Current-Limit Threshold UNITS 700 ∆VOUT LX Switch Current Limit MAX 500 Line Regulation LX Voltage % 28 V 1.7 2.2 2.7 A 0.25 0.35 0.45 A 0.3 0.6 Ω 0.02 10 µA LX On-Resistance RLXON LX Leakage Current ILXOFF VLX = 28V COMP Maximum Output Current ICOMP FB = GND 100 200 µA ∆FB = 0.1V 0.8 1 mmho COMP Current vs. FB Voltage Transconductance SHDN Input Logic Low VIL SHDN Input Logic High VIH 0.8 V 1 µA 300 kHz 2.0 V SHDN = GND or VL Shutdown Input Current 2 TYP 28 ILOAD = 0 to 2mA, VFB = 1.6V Rising edge, 1% hysteresis FB Set Voltage MIN 3 Input Voltage Switching Frequency f 200 250 Maximum Duty Cycle DC 90 95 _______________________________________________________________________________________ % 28V, PWM, Step-Up DC-DC Converter MAX618 ELECTRICAL CHARACTERISTICS (VIN = +6V, PGND = GND, CVL = 4.7µF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX V VIN = 3V to 28V, VFB = 1.6V, SHDN = VL 800 µA IIN VIN = 3V to 5.5, VFB = 1.4V, SHDN = VL = IN 7.5 mA IIN VIN = 3.4V to 28V, VFB = 1.4V, SHDN = VL, VL < VIN 4 mA Supply Current Shutdown IIN VIN = 28V, VFB = 1.6V, SHDN = GND VL Output Voltage VVL VIN = 3.5V or 28V, no load VL Undervoltage Lockout VVL Rising edge, 1% hysteresis FB Set Voltage VFB VIN Supply Current, No Load IIN Supply Current, Full Load, VL Connected to IN Supply Current, Full Load LX Voltage Range VLXON LX Switch Current Limit ILXON LX On-Resistance RLXON Switching Frequency 3 UNITS 28 Input Voltage 10 µA 3.3 V 2.55 2.85 V 1.455 1.545 V 28 V 2.85 1.4 PWM mode 188 f 3 A 0.6 Ω 312 kHz Note 2: Specifications to -40°C are guaranteed by design, not production tested. Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C.) VIN = 8V 90 VIN = 3V 60 50 40 VIN = 3V 70 60 50 40 30 30 20 20 10 10 0 VIN = 5V 80 VIN = 5V 70 VIN = 12V 90 EFFICIENCY (%) EFFICIENCY (%) 80 100 MAX618 toc01 100 EFFICIENCY vs. OUTPUT CURRENT (VOUT = 28V) MAX618 toc02 EFFICIENCY vs. OUTPUT CURRENT (VOUT = 12V) 0 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 _______________________________________________________________________________________ 3 Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C.) NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE SHUTDOWN CURRENT vs. SUPPLY VOLTAGE SUPPLY CURRENT vs. TEMPERATURE 0.55 0.50 600 550 VIN = 5V 500 VIN = 8V 450 400 0.45 3.5 SHUTDOWN CURRENT (µA) 650 MAX618 toc06 VIN = 3V SUPPLY CURRENT (µA) 0.60 4.0 MAX618 toc05 700 MAX618 toc04 0.65 SUPPLY CIRRENT (mA) 3.0 2.5 2.0 1.5 1.0 0.5 350 INCLUDES CAPACITOR LEAKAGE CURRENT 0 300 0 5 10 15 20 INPUT VOLTAGE (V) 25 30 -50 -30 -10 10 30 50 70 TEMPERATURE (°C) 90 MAX618 toc09 0 VLX (10V/div) VOUT (100mV/div) VOUT (100mV/ div) 3V 2ms/div VIN = 5V, VOUT = 12V, IOUT = 500mA VIN = 5V, VOUT = 12V, IOUT = 200mA IOUT = 200mA, VOUT = 12V MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE SHUTDOWN RESPONSE LOAD-TRANSIENT RESPONSE MAX618 toc11 MAX618 toc10 1.6 SHDN (2V/div) 0 12V IOUT (100mA/div) VOUT (2V/div) 5V 4 6V VIN (5V/div) 2µs/div 2µs/div 0 32 VOUT (50mV/div) 0 VLX (10V/div) VOUT (200mV/div) 27 LINE-TRANSIENT RESPONSE IL (1A/div) VIN = 5V, VOUT = 12V 12 17 22 SUPPLY VOLTAGE (V) MAX618 toc08 MAX618 toc07 5ms/div 7 HEAVY-LOAD SWITCHING WAVEFORMS MEDIUM-LOAD SWITCHING WAVEFORMS IL (1A/div) 2 110 500µs/div VIN = 5V, VOUT = 12V, ILOAD = 500mA VOUT = 12V 1.4 MAX618 toc12 0.40 MAXIMUM OUTPUT CURRENT (A) MAX618 28V, PWM, Step-Up DC-DC Converter 1.2 1.0 0.8 0.6 0.4 0.2 0 2 3 4 5 6 7 8 9 INPUT VOLTAGE (V) _______________________________________________________________________________________ 10 11 12 28V, PWM, Step-Up DC-DC Converter PIN NAME FUNCTION 1, 8, 9, 12, 16 GND 2, 3, 4 LX 5 SHDN Shutdown Input. A logic low puts the MAX618 in shutdown mode and reduces supply current to 3µA. SHDN must not exceed VL. In shutdown, the output falls to VIN less one diode drop. 6 COMP Compensation Input. Bypass to GND with the capacitance value shown in Table 2. 7 FB Feedback Input. Connect a resistor-divider network to set VOUT. FB threshold is 1.5V. 10 IN LDO Regulator Supply Input. IN accepts inputs up to +28V. Bypass to GND with a 1µF ceramic capacitor as close to pins 10 and 12 as possible. 11 VL Internal 3.1V LDO Regulator Output. Bypass to GND with a 4.7µF capacitor. 13, 14, 15 PGND Ground Drain of internal N-channel switch. Connect the inductor between IN and LX. Power Ground, source of internal N-channel switch _______________ Detailed Description L 3V TO 28V VIN CIND ECB1Q503L LX IN COUT 1µF VOUT UP TO 28V MAX618 SHDN R1 PGND VL FB 4.7µF R2 CP GND COMP CCOMP VOUT R1 R2 CIND L COUT CP CCOMP 8V 12V 28V 402kΩ 715kΩ 574kΩ 93.1kΩ 100kΩ 32.4kΩ 150µF 100µF 86µF 12µH 15µH 39µH 150µF 100µH 33µF 220pF 56pF 47pF 0.082µF 0.1µF 0.47µF Figure 1. Single-Supply Operation The MAX618 pulse-width modulation (PWM) DC-DC converter with an internal 28V switch operates in a wide range of DC-DC conversion applications including boost, SEPIC, and flyback configurations. The MAX618 uses fixed-frequency PWM operation and Maxim’s proprietary Idle Mode control to optimize efficiency over a wide range of loads. It also features a shutdown mode to minimize quiescent current when not in operation. PWM Control Scheme and Idle Mode Operation The MAX618 combines continuous-conduction PWM operation at medium to high loads and Idle Mode operation at light loads to provide high efficiency over a wide range of load conditions. The MAX618 control scheme actively monitors the output current and automatically switches between PWM and Idle Mode to optimize efficiency and load regulation. Figure 2 shows a functional diagram of the MAX618’s control scheme. The MAX618 normally operates in low-noise, continuous-conduction PWM mode, switching at 250kHz. In PWM mode, the internal MOSFET switch turns on with each clock pulse. It remains on until either the error comparator trips or the inductor current reaches the 2A switch-current limit. The error comparator compares the feedback-error signal, current-sense signal, and slopecompensation signal in one circuit block. When the switch turns off, energy transfers from the inductor to _______________________________________________________________________________________________________ 5 MAX618 Pin Description MAX618 28V, PWM, Step-Up DC-DC Converter IDLE MODE CURRENT LIMIT MAX618 PWM CURRENT LIMIT CURRENTSENSE CIRCUIT PGND IN VL ERROR COMPARATOR PWM LOGIC NMOS R 250kHz OSCILLATOR GND SLOPE COMPENSATION LX FB 14R REFERENCE INTEGRATOR SHDN THERMAL SHUTDOWN SHUTDOWN OUT COMP LINEAR REGULATOR IN VL Figure 2. Functional Diagram the output capacitor. Output current is limited by the 2A MOSFET current limit and the MAX618’s package power-dissipation limit. See the Maximum Output Current section for details. In Idle Mode, the MAX618 improves light-load efficiency by reducing inductor current and skipping cycles to reduce the losses in the internal switch, diode, and inductor. In this mode, a switching cycle initiates only when the error comparator senses that the output voltage is about to drop out of regulation. When this occurs, the NMOS switch turns on and remains on until the inductor current exceeds the nominal 350mA Idle Mode current limit. Refer to Table 1 for an estimate of load currents at which the MAX618 transitions between PWM and Idle Mode. Compensation Scheme Although the higher loop gain of voltage-controlled architectures tends to provide tighter load regulation, current-controlled architectures are generally easier to compensate over wide input and output voltage 6 ranges. The MAX618 uses both control schemes in parallel: the dominant, low-frequency components of the error signal are tightly regulated with a voltage-control loop, while a current-control loop improves stability at higher frequencies. Compensation is achieved through the selection of the output capacitor (COUT), the integrator capacitor (CCOMP), and the pole capacitor (CP) from FB to GND. CP cancels the zero formed by COUT and its ESR. Refer to the Capacitor Selection section for guidance on selecting these capacitors. VL Low-Dropout Regulator The MAX618 contains a 3.1V low-dropout linear regulator to power internal circuitry. The regulator’s input is IN and its output is VL. The IN to VL dropout voltage is 100mV, so that when IN is less than 3.2V, VL is typically 100mV below IN. The MAX618 still operates when the LDO is in dropout, as long as VL remains above the 2.7V undervoltage lockout. Bypass VL with a 4.7µF ceramic capacitor placed as close to the VL and GND pins as possible. _______________________________________________________________________________________ _______________________________________________________________________________________ 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 4 5 6 7 0.20 0.20 0.18 0.15 0.18 0.21 0.20 0.16 0.20 0.15 8 0.12 0.17 0.21 0.20 0.17 9 0.10 0.15 0.19 0.21 0.19 0.19 10 0.09 0.13 0.17 0.20 0.21 0.18 0.20 11 0.08 0.12 0.16 0.19 0.21 0.20 0.17 0.21 12 0.07 0.10 0.14 0.18 0.20 0.21 0.20 0.16 0.22 13 0.06 0.09 0.13 0.16 0.19 0.20 0.21 0.19 0.15 0.23 14 0.05 0.08 0.11 0.15 0.17 0.20 0.21 0.20 0.19 0.15 0.24 15 0.04 0.07 0.10 0.13 0.16 0.19 0.20 0.21 0.20 0.18 0.16 0.25 16 0.04 0.07 0.09 0.12 0.15 0.17 0.19 0.21 0.21 0.20 0.17 0.17 0.25 VOUT 17 0.04 0.06 0.09 0.11 0.14 0.16 0.18 0.20 0.21 0.21 0.19 0.17 0.18 0.26 18 0.03 0.05 0.08 0.10 0.13 0.15 0.18 0.19 0.20 0.21 0.20 0.19 0.16 0.19 0.26 19 0.03 0.05 0.07 0.10 0.12 0.14 0.17 0.18 0.20 0.21 0.21 0.20 0.18 0.16 0.20 0.27 20 0.03 0.04 0.07 0.09 0.11 0.13 0.16 0.17 0.19 0.20 0.21 0.21 0.20 0.18 0.15 0.20 0.27 21 0.03 0.04 0.06 0.08 0.10 0.13 0.15 0.17 0.18 0.20 0.20 0.21 0.20 0.19 0.17 0.15 0.21 0.27 22 0.03 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.17 0.19 0.20 0.21 0.21 0.20 0.19 0.17 0.16 0.21 0.28 23 0.02 0.03 0.05 0.07 0.09 0.11 0.13 0.15 0.17 0.18 0.19 0.20 0.21 0.21 0.20 0.19 0.17 0.17 0.22 0.28 24 0.02 0.03 0.05 0.07 0.08 0.10 0.12 0.14 0.16 0.18 0.19 0.20 0.21 0.21 0.20 0.20 0.18 0.16 0.17 0.22 0.28 25 0.02 0.03 0.04 0.06 0.08 0.10 0.12 0.13 0.15 0.17 0.18 0.19 0.20 0.21 0.21 0.20 0.19 0.18 0.16 0.18 0.23 0.28 26 0.02 0.03 0.04 0.06 0.07 0.09 0.11 0.13 0.14 0.16 0.17 0.19 0.20 0.20 0.21 0.21 0.20 0.19 0.18 0.15 0.18 0.23 0.29 27 0.02 0.03 0.04 0.05 0.07 0.09 0.10 0.12 0.14 0.15 0.17 0.18 0.19 0.20 0.21 0.21 0.21 0.20 0.19 0.17 0.15 0.19 0.24 0.29 28 0.02 0.03 0.04 0.05 0.07 0.08 0.10 0.11 0.13 0.15 0.16 0.17 0.19 0.20 0.20 0.21 0.21 0.20 0.20 0.19 0.17 0.15 0.19 0.24 0.29 MAX618 VIN Table 1. PWM/Idle-Mode Transition Load Current (IOUT in Amps) vs. Input and Output Voltage 28V, PWM, Step-Up DC-DC Converter 7 MAX618 28V, PWM, Step-Up DC-DC Converter VL can be overdriven by an external supply between 2.7V and 5.5V. In systems with +3.3V or +5V logic power supplies available, improve efficiency by powering VL and VIN directly from the logic supply as shown in Figure 3. Operating Configurations The MAX618 can be connected in one of three configurations described in Table 2 and shown in Figures 1, 3, and 4. The VL linear regulator allows operation from a single supply between +3V and +28V as shown in Figure 1. The circuit in Figure 3 allows a logic supply to power the MAX618 while using a separate source for DC-DC conversion power (inductor voltage). The logic supply (between 2.7V and 5.5V) connects to VL and IN. VL = IN; voltages of 3.3V or more improve efficiency by providing greater gate drive for the internal MOSFET. The circuit in Figure 4 allows separate supplies to power IN and the inductor voltage. It differs from the connection in Figure 3 in that the MAX618 chip supply is not limited to 5.5V. Table 2. Input Configurations CIRCUIT CONNECTION VIN RANGE Figure 1 Input voltage connects to IN and inductor. 3V to VOUT (up to 28V) IN and VL connect together. Inductor voltage supplied by a separate source. Figure 3 INDUCTOR VOLTAGE 2.7V to 5.5V BENEFITS/COMMENTS VIN • Single-supply operation. • SHDN must be connected to or pulled up to VL. On/off control requires an open-drain or open-collector connection to SHDN. 0 to VOUT (up to 28V) • Increased efficiency. • SHDN can be driven by logic powered from the supply connected to IN and VL, or can be connected to or pulled up to VL. • Input power source (inductor voltage) is separate from the MAX618’s bias (VIN = VL) and can be less than or greater than VIN. • Input power source (inductor voltage) is separate from the IN and inductor voltage supplied by separate sources. Figure 4 VIND UP TO 28V 0 to VOUT (up to 28V) 3V to 28V MAX618’s bias (VIN) and can be less than or greater than VIN. • SHDN must be connected to or pulled up to VL. On/off control requires an open-drain or open-collector connection to SHDN. VIND UP TO 28V L CIND IN 2.7V TO 5.5V L CIND OUT UP TO 28V IN IN 3V TO 28V 1µF COUT MAX618 R1 SHDN LX 1µF COUT MAX618 OUT UP TO 28V IN LX SHDN PGND R1 PGND VL VL 4.7µF COMP CCOMP 4.7µF FB CP R2 GND Figure 3. Dual-Supply Operation (VIN = 2.7V to 5.5V) 8 FB COMP CP CCOMP R2 GND Figure 4. Dual-Supply Operation (VIN = 3V to 28V) _______________________________________________________________________________________ 28V, PWM, Step-Up DC-DC Converter OPEN-DRAIN LOGIC MAX618 MAX618 MAX618 VL IN SYSTEM LOGIC SUPPLY 100k VL SHDN ON/OFF CONTROL SHDN SYSTEM LOGIC Figure 5. Adding On/Off Control to Circuit of Figure 1 or 4 ON/OFF CONTROL Figure 6. Adding On/Off Control to Circuit of Figure 3 Shutdown Mode Determining the Inductor Value In shutdown mode (SHDN = 0), the MAX618’s feedback and control circuit, reference, and internal biasing circuitry turn off and reduce the IN supply current to 3µA (10µA max). When in shutdown, a current path remains from the input to the output through the external inductor and diode. Consequently, the output falls to VIN less one diode drop in shutdown. SHDN may not exceed VL. For always-on operation, connect SHDN to VL. To add on/off control to the circuit of Figure 1 or 4, pull SHDN to VL with a resistor (10kΩ to 100kΩ) and drive SHDN with an open-drain logic gate or switch as shown in Figure 5. Alternatively, the circuit of Figure 3 allows direct SHDN drive by any logic-level gate powered from the same supply that powers VL and IN, as shown in Figure 6. The MAX618’s high switching frequency allows the use of a small value inductor. The recommended inductor value is proportional to the output voltage and is given by the following: __________________Design Procedure The MAX618 operates in a number of DC-DC converter configurations including step-up, SEPIC, and flyback. The following design discussion is limited to step-up converters. Setting the Output Voltage Two external resistors (R1 and R2) set the output voltage. First, select a value for R2 between 10kΩ and 200kΩ. Calculate R1 with: V R1 = R2 OUT − 1 V FB where VFB is 1.5V. L= VOUT 7 ⋅10 5 After solving for the above equation, round down as necessary to select a standard inductor value. When selecting an inductor, choose one rated to 250kHz, with a saturation current exceeding the peak inductor current, and with a DC resistance under 200mΩ. Ferrite core or equivalent inductors are generally appropriate (see MAX618 EV kit data sheet). Calculate the peak inductor current with the following equation: V VOUT − VIN VOUT IN ILX(PEAK) = IOUT + 2µs VOUT VIN L Note that the peak inductor current is internally limited to 2A. ( ) Diode Selection The MAX618’s high switching frequency demands a high-speed rectifier. Schottky diodes are preferred for most applications because of their fast recovery time and low forward voltage. Make sure that the diode’s peak current rating exceeds the 2A peak switch current, and that its breakdown voltage exceeds the output voltage. _______________________________________________________________________________________ 9 MAX618 28V, PWM, Step-Up DC-DC Converter Maximum Output Current The MAX618’s 2.2A LX current limit determines the output power that can be supplied for most applications. In some cases, particularly when the input voltage is low, output power is sometimes restricted by package dissipation limits. The MAX618 is protected by a thermal shutdown circuit that turns off the switch when the die temperature exceeds +150°C. When the device cools by 10°C, the switch is enabled again. Table 3 details output current with a variety of input and output voltages. Each listing in Table 3 is either the limit set by an LX current limit or by package dissipation at +85°C ambient, whichever is lower. The values in Table 3 assume a 40mΩ inductor resistance. Capacitor Selection Input Capacitors The input bypass capacitor, CIND, reduces the input ripple created by the boost configuration. High-impedance sources require high CIND values. However, 68µF is generally adequate for input currents up to 2A. Low ESR capacitors are recommended because they will decrease the ripple created on the input and improve efficiency. Capacitors with ESR below 0.3Ω are generally appropriate. In addition to the input bypass capacitor, bypass IN with a 1µF ceramic capacitor placed as close to the IN and GND pins as possible. Bypass VL with a 4.7µF ceramic capacitor placed as close to the VL and GND pins as possible. Output Capacitor Use Table 4 to find the minimum output capacitance necessary to ensure stable operation. In addition, choose an output capacitor with low ESR to reduce the output ripple. The dominant component of output ripple is the product of the peak-to-peak inductor ripple current and the ESR of the output capacitor. ESR below 50mΩ generates acceptable levels of output ripple for most applications. Integrator Capacitor The compensation capacitor (CCOMP) sets the dominant pole in the MAX618’s transfer function. The proper compensation capacitance depends upon output capacitance. Table 5 shows the capacitance value 10 needed for the output capacitances specified in Table 4. However, if a different output capacitor is used (e.g., a standard value), then recalculate the value of capacitance needed for the integrator capacitor with the following formula: CCOMP = CCOMP (Table 5) ⋅ COUT COUT (Table 4) Pole Compensation Capacitor The pole capacitor (CP) cancels the unwanted zero introduced by COUT’s ESR, and thereby ensures stability in PWM operation. The exact value of the pole capacitor is not critical, but it should be near the value calculated by the following equation: CP = RESR ⋅ COUT (R2 + R2) R1 ⋅ R2 where RESR is COUT’s ESR. Layout Considerations Proper PC board layout is essential due to high current levels and fast switching waveforms that radiate noise. Use the MAX618 evaluation kit or equivalent PC layout to perform initial prototyping. Breadboards, wire-wrap, and proto-boards are not recommended when prototyping switching regulators. It is important to connect the GND pin, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point to minimize ground noise and improve regulation. Also, minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise, with preference given to the feedback circuit, the ground circuit, and LX. Place the feedback resistors as close to the FB pin as possible. Place a 1µF input bypass capacitor as close as possible to IN and GND. Refer to the MAX618 evaluation kit for an example of proper board layout. ______________________________________________________________________________________ ______________________________________________________________________________________ 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 VIN 4 5 6 7 0.77 0.59 0.49 0.41 0.96 0.76 0.64 1.09 0.89 1.18 8 0.34 0.56 0.76 0.99 1.26 9 0.29 0.49 0.67 0.85 1.07 1.32 10 0.25 0.43 0.60 0.76 0.93 1.13 1.37 11 0.22 0.38 0.54 0.68 0.83 1.00 1.19 1.41 12 0.20 0.34 0.50 0.63 0.76 0.90 1.06 1.24 1.44 13 0.18 0.31 0.45 0.58 0.70 0.82 0.96 1.11 1.28 1.47 14 0.17 0.28 0.41 0.54 0.65 0.76 0.88 1.01 1.15 1.31 1.49 15 0.15 0.26 0.37 0.50 0.60 0.71 0.81 0.93 1.05 1.19 1.34 1.52 16 0.14 0.24 0.34 0.46 0.57 0.66 0.76 0.86 0.97 1.10 1.23 1.37 1.53 VOUT 17 0.13 0.22 0.32 0.42 0.53 0.62 0.71 0.81 0.91 1.02 1.13 1.26 1.40 1.55 18 0.12 0.21 0.30 0.39 0.50 0.59 0.67 0.76 0.85 0.95 1.05 1.16 1.29 1.42 1.57 19 0.12 0.19 0.28 0.37 0.46 0.56 0.64 0.72 0.80 0.89 0.99 1.09 1.19 1.31 1.44 1.58 20 0.11 0.18 0.26 0.34 0.43 0.53 0.61 0.68 0.76 0.84 0.93 1.02 1.12 1.22 1.33 1.46 1.59 21 0.10 0.17 0.25 0.32 0.41 0.50 0.58 0.65 0.72 0.80 0.88 0.96 1.05 1.14 1.25 1.36 1.47 1.60 22 0.10 0.16 0.23 0.31 0.38 0.47 0.55 0.62 0.69 0.76 0.83 0.91 0.99 1.08 1.17 1.27 1.37 1.49 1.61 23 0.09 0.16 0.22 0.29 0.36 0.44 0.53 0.59 0.66 0.73 0.80 0.87 0.94 1.02 1.11 1.20 1.29 1.39 1.50 1.62 24 0.09 0.15 0.21 0.28 0.35 0.42 0.50 0.57 0.63 0.70 0.76 0.83 0.90 0.97 1.05 1.13 1.22 1.31 1.41 1.51 1.63 25 0.08 0.14 0.20 0.26 0.33 0.40 0.47 0.55 0.61 0.67 0.73 0.79 0.86 0.93 1.00 1.07 1.15 1.24 1.33 1.42 1.53 1.64 26 0.08 0.14 0.19 0.25 0.31 0.38 0.45 0.52 0.58 0.64 0.70 0.76 0.82 0.89 0.95 1.02 1.10 1.18 1.26 1.35 1.44 1.54 1.64 27 0.08 0.13 0.18 0.24 0.30 0.36 0.43 0.50 0.56 0.62 0.67 0.73 0.79 0.85 0.91 0.98 1.05 1.12 1.20 1.28 1.36 1.45 1.55 1.65 28 0.07 0.12 0.18 0.23 0.29 0.35 0.41 0.47 0.54 0.60 0.65 0.71 0.76 0.82 0.88 0.94 1.00 1.07 1.14 1.22 1.29 1.38 1.46 1.56 1.66 MAX618 Table 3. Typical Output Current vs. Input and Output Voltages 28V, PWM, Step-Up DC-DC Converter 11 12 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 VIN 4 173 5 128 151 6 100 118 132 7 80 96 107 117 8 65 80 90 97 104 9 54 68 77 83 89 94 10 46 59 67 72 77 82 86 11 40 51 59 64 68 72 76 79 12 35 45 52 57 61 64 67 70 73 Table 4. Minimum COUT for Stability (µF) 13 31 39 46 51 55 58 61 63 66 68 14 28 35 41 46 50 52 55 57 59 62 64 15 25 32 37 42 45 48 50 52 54 56 58 60 16 23 29 34 38 42 44 46 48 50 51 53 55 56 VOUT 17 21 27 31 35 39 41 42 44 46 47 49 50 52 53 18 19 24 29 32 35 38 39 41 43 44 45 47 48 49 50 19 18 23 26 30 33 35 37 38 40 41 42 43 44 46 47 48 20 17 21 25 28 30 33 34 36 37 38 39 40 42 43 44 45 46 21 15 20 23 26 28 31 32 34 35 36 37 38 39 40 41 42 43 43 22 15 18 21 24 26 29 30 32 33 34 35 36 37 37 38 39 40 41 42 23 14 17 20 23 25 27 29 30 31 32 33 34 35 35 36 37 38 38 39 40 24 13 16 19 21 23 25 27 28 29 30 31 32 33 33 34 35 36 36 37 38 38 25 12 15 18 20 22 24 25 27 28 29 29 30 31 32 32 33 34 34 35 36 36 37 26 12 15 17 19 21 22 24 25 26 27 28 29 29 30 31 31 32 33 33 34 34 35 35 27 11 14 16 18 20 21 23 24 25 26 27 27 28 29 29 30 30 31 32 32 33 33 34 34 28 10 13 15 17 19 20 21 23 24 25 25 26 27 27 28 28 29 29 30 31 31 32 32 33 33 MAX618 28V, PWM, Step-Up DC-DC Converter ______________________________________________________________________________________ ______________________________________________________________________________________ 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 VIN 4 40 5 46 42 6 54 45 43 7 64 51 45 44 8 73 58 49 45 45 9 83 66 54 48 45 46 10 94 74 60 52 47 45 46 11 105 82 67 57 50 47 46 47 12 118 91 75 62 54 49 47 46 47 13 130 100 81 68 58 52 48 46 46 48 14 143 109 88 74 63 56 51 48 46 47 48 15 157 119 96 80 68 60 54 50 48 47 47 49 16 172 130 103 86 74 64 57 52 49 47 47 47 49 VOUT 17 187 141 111 92 79 68 61 55 51 49 47 47 47 49 18 203 152 120 99 85 73 64 58 54 50 48 47 47 48 49 19 219 164 128 105 90 78 68 61 56 52 50 48 47 47 48 50 20 236 176 137 112 95 83 73 65 59 55 52 49 48 47 47 48 50 21 253 188 147 119 101 88 77 69 62 57 54 51 49 48 47 47 48 50 22 271 201 156 127 107 93 82 72 65 60 56 53 50 49 48 47 47 48 50 23 290 214 166 134 113 98 86 77 69 63 58 55 52 50 48 48 47 48 49 50 24 309 228 176 142 119 103 91 81 72 66 61 57 53 51 49 48 48 47 48 49 50 25 329 242 187 150 125 108 95 85 76 69 63 59 55 53 51 49 48 48 47 48 49 51 26 349 257 197 159 132 113 99 89 80 72 66 61 57 54 52 50 49 48 48 48 48 49 51 27 370 272 209 167 139 119 104 93 84 75 69 64 59 56 53 51 50 49 48 48 48 48 49 51 28 391 287 220 176 146 124 109 97 88 79 72 66 62 58 55 53 51 49 48 48 48 48 48 49 51 MAX618 Table 5. Minimum CCOMP for Stability (nF) 28V, PWM, Step-Up DC-DC Converter 13 MAX618 28V, PWM, Step-Up DC-DC Converter QSOP.EPS Package Information ___________________Chip Information TRANSISTOR COUNT: 1794 14 ______________________________________________________________________________________