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19-1462; Rev 0; 6/99
UAL IT MAN TION K T VALUA A SHEE E S DAT OLLOW F
28V, PWM, Step-Up DC-DC Converter
Features
o Adjustable Output Voltage Up to +28V o Up to 93% Efficiency o Wide Input Voltage Range (+3V to +28V) o Up to 500mA Output Current at +12V o 500A Quiescent Supply Current o 3A Shutdown Current o 250kHz Switching Frequency o Small 1W 16-Pin QSOP Package
General Description
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, 0.3 switch eliminates the need for external power MOSFETs while supplying output currents up to 500mA or more. A PWM control scheme combined with Idle ModeTM operation at light loads minimizes noise and ripple while maximizing efficiency over a wide load range. No-load operating current is 500A, 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.
MAX618
Ordering Information
PART MAX618EEE TEMP. RANGE -40C to +85C PIN-PACKAGE 16 QSOP
Applications
Automotive-Powered DC-DC Converters Industrial +24V and +28V Systems LCD Displays Palmtop Computers
Typical Operating Circuit Pin Configuration
TOP VIEW
GND 1 LX 2 LX 3 LX 4 SHDN 5 COMP 6 FB 7 GND 8 16 GND 15 PGND 14 PGND
VIN 3V TO 28V
IN
LX
VOUT UP TO 28V
MAX618
SHDN PGND
MAX618
13 PGND 12 GND 11 VL 10 IN 9 GND
VL
FB
COMP
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.
28V, PWM, Step-Up DC-DC Converter MAX618
ABSOLUTE MAXIMUM RATINGS
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 = +70C) (Note 1) 16-Pin QSOP (derate 15mW/C above +70C)...................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.
Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) .............................+300C
ELECTRICAL CHARACTERISTICS
(VIN = +6V, PGND = GND, CVL = 4.7F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Input Voltage Supply Current, No Load Supply Current, Full Load, VL Connected to IN Supply Current, Full Load Shutdown Supply Current VL Output Voltage VL Load Regulation VL Undervoltage Lockout FB Set Voltage FB Input Bias Current Line Regulation Load Regulation LX Voltage LX Switch Current Limit Idle Mode Current-Limit Threshold LX On-Resistance LX Leakage Current COMP Maximum Output Current COMP Current vs. FB Voltage Transconductance SHDN Input Logic Low SHDN Input Logic High Shutdown Input Current Switching Frequency Maximum Duty Cycle f DC VIL VIH SHDN = GND or VL 200 90 250 95 2.0 1 300 RLXON ILXOFF ICOMP VLX = 28V FB = GND FB = 0.1V 100 0.8 VFB IFB VOUT VOUT VLX ILXON PWM mode 1.7 0.25 2.2 0.35 0.3 0.02 200 1 0.8 VFB = 1.6V VIN = 3V to 6V, VOUT = 12V VOUT = 12V, ILOAD = 10mA to 500mA SYMBOL VIN IIN IIN IIN IIN VVL VVL VIN = 3V to 28V, VFB = 1.6V, SHDN = VL VIN = 3V to 5.5V, VFB = 1.4V, SHDN = VL = IN VIN = 3.4V to 28V, VFB = 1.4V, SHDN = VL, VVL < VIN VIN = 28V, VFB = 1.6V, SHDN = GND VIN = 3.5V or 28V, no load ILOAD = 0 to 2mA, VFB = 1.6V Rising edge, 1% hysteresis 2.58 1.47 2.9 CONDITIONS MIN 3 500 5 2.5 3 3.05 25 2.7 1.5 1 0.01 0.2 28 2.7 0.45 0.6 10 TYP MAX 28 700 6.5 3.5 8 3.2 40 2.8 1.53 50 0.08 UNITS V A mA mA A V mV V V nA %/V % V A A A A mmho V V A kHz %
2
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28V, PWM, Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS
(VIN = +6V, PGND = GND, CVL = 4.7F, TA = -40C to +85C, unless otherwise noted.) (Note 2) PARAMETER Input Voltage Supply Current, No Load Supply Current, Full Load, VL Connected to IN Supply Current, Full Load Supply Current Shutdown VL Output Voltage VL Undervoltage Lockout FB Set Voltage LX Voltage Range LX Switch Current Limit LX On-Resistance Switching Frequency SYMBOL VIN IIN IIN IIN IIN VVL VVL VFB VLXON ILXON RLXON f 188 PWM mode 1.4 VIN = 3V to 28V, VFB = 1.6V, SHDN = VL VIN = 3V to 5.5, VFB = 1.4V, SHDN = VL = IN VIN = 3.4V to 28V, VFB = 1.4V, SHDN = VL, VL < VIN VIN = 28V, VFB = 1.6V, SHDN = GND VIN = 3.5V or 28V, no load Rising edge, 1% hysteresis 2.85 2.55 1.455 CONDITIONS MIN 3 TYP MAX 28 800 7.5 4 10 3.3 2.85 1.545 28 3 0.6 312 UNITS V A mA mA A V V V V A kHz
MAX618
Note 2: Specifications to -40C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C.)
EFFICIENCY vs. OUTPUT CURRENT (VOUT = 12V)
MAX618 toc01
EFFICIENCY vs. OUTPUT CURRENT (VOUT = 28V)
VIN = 12V VIN = 5V VIN = 3V 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
MAX618 toc02
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 10 100 OUTPUT CURRENT (mA) VIN = 3V
VIN = 8V VIN = 5V
100
1000
0.1
1 10 100 OUTPUT CURRENT (mA)
1000
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28V, PWM, Step-Up DC-DC Converter MAX618
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C.)
NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE
MAX618 toc04
SUPPLY CURRENT vs. TEMPERATURE
VIN = 3V 650 SUPPLY CURRENT (A) 600 550 500 450 400 350 INCLUDES CAPACITOR LEAKAGE CURRENT VIN = 5V VIN = 8V
MAX618 toc05
SHUTDOWN CURRENT vs. SUPPLY VOLTAGE
3.5 SHUTDOWN CURRENT (A) 3.0 2.5 2.0 1.5 1.0 0.5 0 2 7 12 17 22 SUPPLY VOLTAGE (V) 27 32
MAX618 toc06
0.65
700
4.0
0.60 SUPPLY CIRRENT (mA)
0.55
0.50
0.45
0.40 0 5 10 15 20 INPUT VOLTAGE (V) 25 30
300 -50 -30 -10 10 30 50 70 TEMPERATURE (C) 90 110
MEDIUM-LOAD SWITCHING WAVEFORMS
MAX618 toc07
HEAVY-LOAD SWITCHING WAVEFORMS
MAX618 toc08
LINE-TRANSIENT RESPONSE
MAX618 toc09
IL (1A/div) VLX (10V/div)
IL (1A/div) 0 VLX (10V/div) 0
VOUT (50mV/div)
VOUT (100mV/div) 2s/div VIN = 5V, VOUT = 12V, IOUT = 200mA
VOUT (100mV/ div) 2s/div VIN = 5V, VOUT = 12V, IOUT = 500mA
VIN (5V/div) 2ms/div IOUT = 200mA, VOUT = 12V
6V 3V
LOAD-TRANSIENT RESPONSE
MAX618 toc10
SHUTDOWN RESPONSE
MAX618 toc11
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 2 3 4 56789 INPUT VOLTAGE (V) 10 11 12 VOUT = 12V
MAX618 toc12
1.6 MAXIMUM OUTPUT CURRENT (A)
VOUT (200mV/div)
SHDN (2V/div) 0
12V IOUT (100mA/div) 0 VOUT (2V/div) 5V 5ms/div VIN = 5V, VOUT = 12V 500s/div VIN = 5V, VOUT = 12V, ILOAD = 500mA
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28V, PWM, Step-Up DC-DC Converter
Pin Description
PIN 1, 8, 9, 12, 16 2, 3, 4 5 6 7 10 11 13, 14, 15 NAME GND LX SHDN COMP FB IN VL PGND Ground Drain of internal N-channel switch. Connect the inductor between IN and LX. Shutdown Input. A logic low puts the MAX618 in shutdown mode and reduces supply current to 3A. SHDN must not exceed VL. In shutdown, the output falls to VIN less one diode drop. Compensation Input. Bypass to GND with the capacitance value shown in Table 2. Feedback Input. Connect a resistor-divider network to set VOUT. FB threshold is 1.5V. LDO Regulator Supply Input. IN accepts inputs up to +28V. Bypass to GND with a 1F ceramic capacitor as close to pins 10 and 12 as possible. Internal 3.1V LDO Regulator Output. Bypass to GND with a 4.7F capacitor. Power Ground, source of internal N-channel switch FUNCTION
MAX618
_______________ Detailed Description
3V TO 28V VIN CIND ECB1Q503L IN 1F LX COUT VOUT UP TO 28V L
MAX618
SHDN PGND R1
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
VL 4.7F
FB
CP COMP CCOMP GND
R2
VOUT 8V 12V 28V
R1 402k 715k 574k
R2 93.1k 100k 32.4k
CIND 150F 100F 86F
L 12H 15H 39H
COUT 150F 100H 33F
CP 220pF 56pF 47pF
CCOMP 0.082F 0.1F 0.47F
Figure 1. Single-Supply Operation
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28V, PWM, Step-Up DC-DC Converter MAX618
IDLE MODE CURRENT LIMIT
MAX618
PWM CURRENT LIMIT CURRENTSENSE CIRCUIT VL ERROR COMPARATOR PWM LOGIC R 250kHz OSCILLATOR GND SLOPE COMPENSATION REFERENCE INTEGRATOR COMP PGND IN
NMOS LX FB OUT
14R
SHDN
SHUTDOWN
THERMAL SHUTDOWN
LINEAR REGULATOR VL
IN
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.
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 4.7F ceramic capacitor placed as close to the VL and GND pins as possible.
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
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Table 1. PWM/Idle-Mode Transition Load Current (IOUT in Amps) vs. Input and Output Voltage
VOUT
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 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
VIN
MAX618
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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
4 5 6 7 0.20 0.20 0.18 0.15 0.18 0.21 0.20 0.16 0.20 0.15
7
28V, PWM, Step-Up DC-DC Converter MAX618
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. 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.
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.
Table 2. Input Configurations
CIRCUIT CONNECTION Input voltage connects to IN and inductor. VIN RANGE 3V to VOUT (up to 28V) INDUCTOR VOLTAGE VIN BENEFITS/COMMENTS * 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. * 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.
Figure 1
Figure 3
IN and VL connect together. Inductor voltage supplied by a separate source.
2.7V to 5.5V
0 to VOUT (up to 28V)
* Input power source (inductor voltage) is separate from the
Figure 4 IN and inductor voltage supplied by separate sources. 3V to 28V 0 to VOUT (up 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 CIND OUT UP TO 28V IN LX IN 3V TO 28V IN COUT R1 PGND VL 4.7F FB VL 4.7F FB COMP CCOMP GND 1F LX COUT OUT UP TO 28V
VIND UP TO 28V CIND IN 2.7V TO 5.5V 1F
L
L
MAX618
SHDN
MAX618
SHDN PGND R1
COMP CCOMP GND
CP
R2
CP
R2
Figure 3. Dual-Supply Operation (VIN = 2.7V to 5.5V)
8
Figure 4. Dual-Supply Operation (VIN = 3V to 28V)
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28V, PWM, Step-Up DC-DC Converter MAX618
MAX618
VL SYSTEM LOGIC SUPPLY VL SHDN ON/OFF CONTROL SYSTEM LOGIC ON/OFF CONTROL SHDN IN
MAX618
OPEN-DRAIN LOGIC
100k
Figure 5. Adding On/Off Control to Circuit of Figure 1 or 4
Figure 6. Adding On/Off Control to Circuit of Figure 3
Shutdown Mode
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 3A (10A 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 (10k to 100k) 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.
Determining the Inductor Value
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: L= 7 10 5 VOUT
__________________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 10k and 200k. Calculate R1 with: V R1 = R2 OUT - 1 VFB where VFB is 1.5V.
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 200m. 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 + 2s 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.
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9
28V, PWM, Step-Up DC-DC Converter MAX618
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 +150C. When the device cools by 10C, 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 +85C ambient, whichever is lower. The values in Table 3 assume a 40m inductor resistance. 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)
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, 68F 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 0.3 are generally appropriate. In addition to the input bypass capacitor, bypass IN with a 1F ceramic capacitor placed as close to the IN and GND pins as possible. Bypass VL with a 4.7F 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 50m 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
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 1F input bypass capacitor as close as possible to IN and GND. Refer to the MAX618 evaluation kit for an example of proper board layout.
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Table 3. Typical Output Current vs. Input and Output Voltages
VOUT
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 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
VIN
MAX618
______________________________________________________________________________________
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 0.77 0.59 0.49 0.41 0.96 0.76 0.64 1.09 0.89 1.18
28V, PWM, Step-Up DC-DC Converter
11
MAX618
28V, PWM, Step-Up DC-DC Converter
12
Table 4. Minimum COUT for Stability (F)
VOUT
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 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 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
VIN
______________________________________________________________________________________
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
4 173
5 128 151
6 100 118 132
Table 5. Minimum CCOMP for Stability (nF)
VOUT
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 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
VIN
MAX618
______________________________________________________________________________________
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 40
5 46 42
28V, PWM, Step-Up DC-DC Converter
13
28V, PWM, Step-Up DC-DC Converter MAX618
Package Information
QSOP.EPS
___________________Chip Information
TRANSISTOR COUNT: 1794
14
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