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19-0171; Rev 1; 9/93
High-Efficiency PWM, Step-Down P-Channel DC-DC Controller
_______________General Description
The MAX747 high-efficiency, high-current, step-down controller drives external P-channel FETs. It provides 90% to 95% efficiency from a 6V supply with load currents ranging from 50mA up to 2.5A. It uses a pulse-width-modulating (PWM) current-mode control scheme to provide precise output regulation and low output noise. The MAX747's 4V to 15V input voltage range, a fixed 5V/adjustable (Dual-ModeTM) output, and a current limit set with an external resistor make this device ideal for a wide range of applications. High efficiency is maintained with light loads due to a proprietary dual-control (Idle-ModeTM) scheme that minimizes switching losses by reducing the switching frequency at light loads. The low 800A quiescent current and ultra-low 0.6A shutdown current further extend battery life. External components are protected by the MAX747's cycle-by-cycle current limit. The MAX747 also features a 2V 1.5% reference, a comparator for low-battery detection or level translating, as well as soft-start and shutdown capability. The MAX746, discussed in a separate data sheet, functions similarly to the MAX747, but it drives N-channel logic level FETs on the high side.
____________________________Features
o 90% to 95% Efficiency for 50mA to 2.5A Output Currents o 4V to 15V Input Voltage Range o Low 800A Supply Current o 0.6A Shutdown Current o Drives External P-Channel FETs o Cycle-by-Cycle Current Limiting o 2V 1.5% Accurate Reference Output o Adjustable Soft-Start o Precision Comparator for Power-Fail or Low-Battery Warning
MAX747
______________Ordering Information
PART MAX747CPD MAX747CSD MAX747C/D MAX747EPD MAX747ESD MAX747MJD TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 14 Plastic DIP 14 Narrow SO Dice* 14 Plastic DIP 14 Narrow SO 14 CERIDIP
* Contact factory for dice specifications.
________________________Applications
Notebook Power Supplies Personal Digital Assistants Battery-Operated Equipment Cellular Phones 5V to 3.3V Green PC Applications
__________Typical Operating Circuit
INPUT 6V TO 15V V+ AV+ 5Om
100F
MAX747
CS EXT P 5OH OUTPUT 5V 2.3A
__________________Pin Configuration
TOP VIEW
ON/OFF SHDN
430F
LBI SS REF SHDN FB CC
1 2 3 4 5 6
14 LBO 13 GND
OUT LOW-BATTERY DETECTOR INPUT LBI 0.1F
MAX747
12 V+ 11 EXT 10 AGND 9 8 CS OUT
REF SS
LBO CC FB AGND GND
LOW-BATTERY DETECTOR OUTPUT
AV+ 7
DIP/SO
TM Dual-Mode and Idle-Mode are trademarks of Maxim Integrated Products.
________________________________________________________________ Maxim Integrated Products 1
Call toll free 1-800-998-8800 for free samples or literature.
High-Efficiency PWM, Step-Down P-Channel DC-DC Controller MAX747
ABSOLUTE MAXIMUM RATINGS
Supply Voltage V+, AV+ to GND ..............................-0.3V to 17V AGND to GND..........................................................-0.3V to 0.3V All Other Pins................................................-0.3V to (V+ + 0.3V) Reference Current (IREF) ....................................................2mA Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 10.00mW/C above +70C) ..........800mW SO (derate 8.33mW/C above +70C) .........................667mW CERDIP (derate 9.09mW/C above +70C) .................727mW Operating Temperature Ranges: MAX747C_D .......................................................0C to +70C MAX747E_D.....................................................-40C to +85C MAX747MJD ..................................................-55C to +125C Junction Temperature MAX747C_D/E_D .........................................................+150C MAX747MJD ...............................................................+175C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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
(V+ = 10V, ILOAD = 0mA, IREF = 0mA, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER Input Voltage Range Output Voltage Feedback Voltage Line Regulation Load Regulation Efficiency OUT Leakage Current FB Input Logic Low FB Input Leakage Current Reference Voltage Reference Load Regulation Soft-Start Source Current Soft-Start Fault Current Supply Current Oscillator Frequency Maximum Duty Cycle CS Amp ILIM Threshold EXT Output High EXT Output Low EXT Sink Current EXT Source Current CC Impedance LBI Threshold Voltage LBO Output Voltage Low LBI Input Leakage Current LBO Output Leakage Current SHDN Input Voltage Low SHDN Input Voltage High SHDN Input Leakage Current 2 SYMBOL CONDITIONS V+ For regulated outputs V+ = 6V to 15V, 0V < V+ - CS < 0.125V, FB = 0V VOUT (includes line and load regulation) V+ - CS = 0V, external MAX747C feedback mode MAX747E/M V+ = 6V to 15V, FB = 0V V+ = 4V to 15V, external feedback mode 0V < V+ - CS < 0.125V Circuit of Figure 1, ILOAD = 0.5A to 2.5A VOUT = 5V For dual-mode switchover FB = 2V MAX747C VREF IREF = 0A MAX747E/M IREF = 0A to 100A SS = 0V SS = 2V Operating, V+ = 15V Operating, V+ = 10V Shutdown mode MAX747C fOSC MAX747E/M V+ = 6V VLIMIT V+ - CS IEXT = -1mA (sourcing) IEXT = 1mA (sinking) VEXT = 7.5V VEXT = 2.5V VTH LBI falling MAX747C MAX747EM MIN 4 4.85 1.96 1.95 TYP 5.08 2.00 2.00 0.05 1.3 91 50 0.1 2.00 2.00 9 1 500 0.95 0.8 0.6 100 100 96 150 MAX 15 5.25 2.04 2.05 0.1 2.5 80 40 100 2.03 2.04 20 UNITS V V V %V % % A mV nA V mV A A mA A kHz % mV V V mA mA k V V nA A V V nA
1.97 1.96
100
1.3 20 115 120 175 0.25
85 80 91 125 V+ - 0.1
1.97 1.96
110 170 24 2.00 2.00
ISINK = 0.5mA LBI = 2.5V V+ = 15V, LBO = 15V, LBI = 2.5V VIL VIH SHDN = 10V 2.0 0.1
2.03 2.04 0.4 100 1 0.4 100
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High-Efficiency PWM, Step-Down P-Channel DC-DC Controller
__________________________________________Typical Operating Characteristics
(Circuit of Figure 1, V+ = 9V, TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE
MAX747-TOC1
MAX747
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX747-TOC2
PEAK INDUCTOR CURRENT vs. OUTPUT CURRENT (VOUT = 5V)
MAX747-TOC5
4 VIN = 9V VOUT = 5V SUPPLY CURRENT (mA) 3
1.0
3 PEAK INDUCTOR CURRENT (A)
SUPPLY CURRENT (mA)
0.9
2
2 ENTIRE CIRCUIT 1 SCHOTTKY DIODE LEAKAGE EXCLUDED 0 -75 -50 -25 0 25 50 75 100 125 TEMPERATURE (C)
0.8
1
VIN = 12V VIN = 9V VIN = 6V
0.7
0.6 5 7 9 11 13 15 SUPPLY VOLTAGE (V)
0 0.01
0.1
1
10
OUTPUT CURRENT (A)
PEAK INDUCTOR CURRENT vs. OUTPUT CURRENT (VOUT = 3.3V)
MAX747-TOC6
EFFICIENCY vs. OUTPUT CURRENT (VOUT = 5V)
VIN = 6V
MAX1747-TOC7
EFFICIENCY vs. OUTPUT CURRENT (VOUT = 3.3V)
MAX747-TOC8
3 PEAK INDUCTOR CURRENT (A)
100
100
VIN = 6V EFFICIENCY (%) EFFICIENCY (%) 2 90 VIN = 9V 90 VIN = 9V VIN = 12V 80
VIN = 9V 1 VIN = 6V VIN = 5V 0 0.01
80
VIN = 12V
0.1
1
10
70 0.01
0.1
1
10
70 0.01
0.1
1
10
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
CONTINUOUS-CONDUCTION MODE BOUNDARY AND CORRESPONDING PEAK INDUCTOR CURRENT (VOUT = 3.3V)
MAX747-TOC3
CONTINUOUS-CONDUCTION MODE BOUNDARY AND CORRESPONDING PEAK INDUCTOR CURRENT (VOUT = 5V)
DISCONTINUOUS CONDUCTION REGION PEAK INDUCTOR CURRENT VOUT = 5V L = 50H RSENSE = 50m
MAX747-TOC4
18 VOUT = 3.3V L = 33H RSENSE = 50m SUPPLY VOLTAGE (V) 14 DISCONTINUOUS CONDUCTION REGION PEAK INDUCTOR CURRENT CONTINUOUS CONDUCTION REGION
2.0 PEAK INDUCTOR CURRENT (A)
15
2.0 PEAK INDUCTOR CURRENT (A)
13 SUPPLY VOLTAGE (V)
1.6
1.6
11 9
10
1.2
1.2
6
0.8
7
CONTINUOUS CONDUCTION REGION
0.8
2 0.4 0.6 0.8 1.0 1.2 1.4 OUTPUT CURRENT (A)
0.4
5 0.5 0.7 0.9 1.1 1.3 OUTPUT CURRENT (A)
0.4
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High-Efficiency PWM, Step-Down P-Channel DC-DC Controller MAX747
____________________________Typical Operating Characteristics (continued)
CONTINUOUS-CONDUCTION MODE WAVEFORMS
MAX747-SCOPE1
DISCONTINUOUS-CONDUCTION IDLE-MODE WAVEFORMS
MAX747-SCOPE2
a
a
b
b
c 20s/div V+ = 9V, IOUT = 125mA a) EXT VOLTAGE, 10V/div b) INDUCTOR CURRENT, 200mA/div c) VOUT RIPPLE, 50mV/div
c
5s/div V+ = 9V, IOUT = 2.5A a) EXT VOLTAGE, 10V/div b) INDUCTOR CURRENT, 1A/div c) VOUT RIPPLE, 50mV/div
LINE-TRANSIENT RESPONSE
MAX747-SCOPE4 MAX747-SCOPE5
LOAD-TRANSIENT RESPONSE
a
a
b
b
5ms/div IOUT = 2.0A a) V+ = 6V to 12V, 5V/div b) VOUT RIPPLE, 100mV/div
MAX747-SCOPE3
100s/div V+ = 9V, COUT = 430F a) LOAD CURRENT, 0.1A TO 2.5A, 1A/div b) VOUT RIPPLE, 100mV/div
MODERATE LOAD, IDLE-MODE WAVEFORMS
a
b
c
5s/div V+ = 9V, IOUT = 560mA a) EXT VOLTAGE, 5V/div b) INDUCTOR CURRENT, 0.5A/div c) VOUT RIPPLE 100mV/div
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High-Efficiency PWM, Step-Down P-Channel DC-DC Controller MAX747
______________________________________________________________Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 NAME LBI SS REF SHDN FB CC AV+ OUT CS AGND EXT V+ GND LBO FUNCTION Input to the internal low-battery comparator. Tie to V+ or GND if not used. Soft-start limits start-up surge currents. On power-up, it charges the soft-start capacitor, slowly raising the peak current limit to the level set by the sense resistor. 2V reference output that can source 100A for external loads. Bypass with 0.22F. The reference is disabled in shutdown mode. Active-high TTL/CMOS logic-level input. In shutdown mode, VOUT = 0V and the supply current is reduced to 20A. Feedback input for adjustable-output operation. Connect to GND for fixed +5V output. Use a resistor divider network to adjust the output voltage. See the section Setting the Output Voltage. Compensation capacitor. AC compensation input for the error amplifier. Connect a capacitor between CC and GND for fixed +5V output operation. See Compensation Capacitor section. Quiet supply voltage for sensitive analog circuitry. A bypass capacitor is not required for AV+. Output voltage sense input. Connects to internal resistor divider. Leave unconnected for adjustable output. Bypass to AGND with a 0.1F capacitor close to the IC. Negative input to the current-sense amplifier. Connect the current-sense resistor (RSENSE) from V+ to CS. Quiet analog ground Power MOSFET gate drive output that swings between V+ and GND. EXT is not protected against short circuits to V+ or AGND. High-current supply voltage for the output driver High-current ground return for the output driver Low-battery output is an open-drain output that goes low when LBI is less than 2V. Connect to V+ through a pull-up resistor. Leave floating if not used. LBO is disabled in shutdown mode.
____________________Getting Starting
Figure 1a shows the 5V output 11.4W standard application circuit and Figure 1b shows the 3.3V output 7.5W standard application circuit. Most applications will be served by these circuits. To learn more about component selection for particular applications, refer to the Design Procedure section. To learn more about the operation of the MAX747, refer to the Detailed Description.
_______________Detailed Description
The MAX747 monolithic, CMOS, step-down switchmode power-supply controller drives external P-channel FETs. It uses a unique current-mode pulsewidth-modulating (PWM) control scheme that results in high efficiency over a wide range of load currents, tight output voltage regulation, excellent load- and linetransient response, and low noise. Efficiency at light loads is further enhanced by a proprietary Idle-Mode switching control scheme that skips oscillator cycles in order to reduce switching losses.
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5
High-Efficiency PWM, Step-Down P-Channel DC-DC Controller MAX747
VIN (7.5V TO15V) C2 100F C3 0.1F VIN (4.5V TO 15V) C2 100F C3 0.1F
R2 R2 R3 100k 14 1 R1 C6 470pF 6 2 C4 0.1F 3 C5 0.22F 5 4 10 LBO LBI RSENSE 50m R1 12 V+ AV+ 7
R3 100k 14 1 N.C. 8 LBO LBI OUT
12 V+ AV+ 7
RSENSE 50m
MAX747
CC SS REF FB SHDN AGND
CS
9 11 Q1 SI9405DY P L1 50H 5V C1 @ 2.3A D1 430F NSQ03A03
6 N.C. C4 0.1F 2 3 C5 0.22F 4 10
MAX747
CC SS REF
CS
9 11 Q1 SI9405DY P L1 33H D1 NSQ03A03 3.3V @ 2.3A R5 13k R4 20k
EXT
EXT
SHDN AGND GND 13 FB 5 C6 2.7nF
OUT 8 GND 13 C7 0.1F C1 430F
C1 880F
Figure 1a. +5V Standard Application Circuit
Figure 1b. +3.3V Standard Application Circuit
Operating Principle
Figure 2 is the MAX747 block diagram. The MAX747 regulates using an inner current-feedback loop and an outer voltage-feedback loop. The current loop is stabilized by a slope compensation scheme and the voltage loop is stabilized by the dominant pole formed by the filter output capacitor and the load.
that to transfer equal amounts of energy to the load in one cycle, the peak current level for the discontinuous waveform must be much larger than the continuous waveform peak current.
Slope Compensation
Stability of the inner current-feedback loop is provided by a slope-compensation scheme that adds a ramp signal to the current-sense amplifier output. Ideal slope compensation can be achieved by adding a linear ramp with the same slope as the declining inductor current to the rising inductor current-sense voltage. Therefore, the inductor must be scaled to the currentsense resistor value. Overcompensation adds a pole to the outer voltagefeedback loop response that degrades loop stability. This may cause voltage-mode pulse-frequencymodulation instead of PWM operation. Undercompensation results in inner current-feedback loop instability, and may cause the inductor current to staircase. Ideal matching between the sense resistor and inductor is not required. The matching can be 30% or more.
Discontinuous-/ContinuousConduction Modes
The MAX747 operates in continuous-conduction mode (CCM) under heavy loads, but operates in discontinuous-conduction mode (DCM) at light loads, making it ideal for variable load applications. In DCM, the inductor current starts and ends at zero on each cycle. In CCM, the inductor current never returns to zero. It is composed of a small AC component superimposed on a DC level, which results in higher load-current capability and lower output noise. Output noise is reduced because the inductor does not exhibit the ringing that occurs when the inductor current reaches zero, and because there is a smaller AC component in the inductor-current waveform (see inductor waveforms in the Typical Operating Characteristics section). Note
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High-Efficiency PWM, Step-Down P-Channel DC-DC Controller MAX747
LBO LBI LOW-BATTERY COMPARATOR N +2V REFERENCE 100kHz OSCILLATOR EXT V+
REF OUT 60k ERROR AMPLIFIER EXT CONTROL
CC 40k DUAL-MODE COMPARATOR PWM COMPARATOR SHDN
FB
100mV
AV+
CURRENT-SENSE AMPLIFIER
CS SLOPE COMPENSATION RAMP VRAMP
50mV
IDLE-MODE COMPARATOR
CURRENT-LIMIT COMPARATOR
SS
SOFT-START CIRCUITRY
AGND
GND
Figure 2. Block Diagram
The Oscillator and EXT Control
The switching frequency is nominally 100kHz and the duty cycle varies from 5% to 96%, depending on the input/output voltage ratio. EXT, which provides the gate drive for the external P-FET, is switched between V+ and GND at the switching frequency. EXT is controlled by a unique two-comparator control scheme composed of a PWM comparator and an idle-mode comparator
(Figure 2). The PWM comparator determines the cycleby-cycle peak current with heavy loads, and the light-load comparator sets the light-load peak current. As VOUT begins to drop, EXT goes low and remains low until both comparators trip. With heavy loads, the idlemode comparator trips quickly, and the PWM control comparator determines the EXT on-time; with light loads, the idle-mode comparator sets the EXT on-time.
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High-Efficiency PWM, Step-Down P-Channel DC-DC Controller MAX747
MAX747-FIG3
3
Figure 3 shows how the peak current limit increases as the voltage on SS rises for two RSENSE values.
PEAK CURRENT LIMIT (A)
RSENSE = 50m 2 V+ -VCS = 150mV 1
Shutdown Mode
When SHDN is high, the MAX747 enters shutdown mode. In this mode, the internal biasing circuitry (including EXT) is turned off, VOUT drops to 0V, and the supply current drops to 0.6A (20A max). This excludes external component leakage, which may add several microamps to the shutdown supply current for the entire circuit. SHDN is a TTL/CMOS logic-level input. Connect SHDN to GND for normal operation.
RSENSE = 100m 0 0 1 2 3 4 SOFT-START VOLTAGE (V)
Low-Battery Detector
The MAX747 provides a low-battery comparator that compares the voltage on LBI to the reference voltage. LBO, an open-drain output, goes low when the LBI voltage is below VREF. Use a resistor-divider network as shown in Figure 4 to set the trip voltage (V TRIP) to the desired level. In this circuit, LBO goes low when V+ VTRIP. LBO is high impedance in shutdown mode.
Figure 3. Peak Current Limit vs. Soft-Start Voltage
With decreasing loads, as the inductor current becomes discontinuous, traditional PWM converters continue to switch at a fixed frequency, decreasing light-load efficiency. However, the MAX747's idle-mode comparator increases the peak inductor current, allowing more energy to be transferred per cycle. Since fewer cycles are required, the switching frequency is reduced. This keeps the external PFET off for longer periods, minimizing switching losses and increasing efficiency. The light-load output noise spectrum widens due to variable switching frequency in idle-mode, but output ripple remains low. Using the Typical Operating Circuit, with a 9V input and a 125mA load current, output ripple is less than 40mV.
__________________Design Procedure
Setting the Output Voltage
The MAX747's output voltage can be set to 5V by grounding FB, or adjusted from 2V to 14V using external resistors R4 and R5, configured as shown in Figure 5. Select feedback resistor R4 from the 10k to 1M range. R5 is given by: V R5 = (R4) OUT - 1 2V
Soft-Start and Current Limiting
The MAX747 draws its highest current at power-up. If The power source to the MAX747 cannot provide this initial elevated current, the circuit may not function correctly. For example, after prolonged use, a battery's increased series resistance may prevent it from providing adequate initial surge currents when the MAX747 is brought out of shutdown. Using Soft-Start (SS) minimizes the possibility of overloading the incoming supply at power-up by gradually increasing the peak current limit. Connect an external capacitor from SS to ground to reduce the initial peak currents drawn from the supply. The steady-state SS pin voltage is typically 3.8V. On power-up, SS sources 1A until the SS voltage reaches 3.8V. The current-limit comparator inhibits EXT switching until the SS voltage reaches 1.8V. The maximum current limit is set by: IPK = VLIMIT 150mV (typ) = RSENSE RSENSE
Selecting RSENSE First, approximate the peak current assuming IPK is (1.1)(ILOAD), where ILOAD is the maximum load current. Once all component values have been determined, the actual peak current is given by:
VOUT VOUT IPK = ILOAD + 1- (2L) (fOSC ) VIN Next, determine the value of RSENSE such that: RSENSE = VLIMIT (MIN) IPK = 125mV IPK
For example, to obtain 5V at 3A, IPK = 3.3A and RSENSE = 125mV/3.3A = 38m. The sense resistor should have a power rating greater than (IPK2)(RSENSE) (with an adequate safety margin). With a 3A load current, IPK = 3.3A and RSENSE = 38m. The power dissipated by the resistor (assuming an 80%
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High-Efficiency PWM, Step-Down P-Channel DC-DC Controller MAX747
duty cycle) is 331mW. Metal film resistors are recommended. Do not use wire-wound resistors because their inductance will adversely affect circuit operation. Determine the duty cycle for CCM from the following equation: VOUT + VDIODE Duty cycle (%) = (100%) V + - VSW + VDIODE where VSW is the voltage drop across the external PFET and sense resistor, and can be approximated as (ILOAD)[RDS(ON) + RSENSE]. For highest efficiency, use a coil with low DC resistance. To minimize radiated noise, use a toroid, pot core, or shielded coil.
External P-FET Selection
To ensure the external P-FET is fully on, use logic-level, or low threshold P-FETs when the minimum input voltage is less than 8V. When selecting the P-FET, three important parameters to note are total gate charge (Q g ), on resistance (RDS(ON)), and reverse transfer capacitance (CRSS). Qg, the total gate charge, includes all capacitances associated with charging the gate. Use the typical Qg value for best results; the maximum value is usually overspecified since it is a guaranteed limit and not the measured value. The typical total gate charge should be 50nC. Larger numbers mean that EXT may not be able to adequately drive the gate. EXT sink/source capability (IEXT) is typically 140mA. There are two losses associated with the P-FET's power dissipation: I 2 R losses and switching losses. CCM power dissipation (PD) is approximated by:
2 PD = Duty Cycle IPK RDS(ON) +
Inductor Selection
Once the sense resistor value is determined, the inductor is determined from the following equation. The value of inductor L ensures proper slope compensation. Continuing with the above example, (RSENSE ) (VOUT(MAX) ) L= (VRAMP(MAX) ) (fOSC ) = (38m) (5V) = 38H (50mV) (100kHz)
Although 38H is the calculated value, the component used may have a tolerance of 30% or more. Make sure the inductor's saturation current rating (the current at which the core begins to saturate and the inductance starts to fall) exceeds the peak current set by RSENSE. Inductors with molypermalloy powder (MPP), Kool M, or ferrite are recommended. Inexpensive iron powder core inductors are not suitable due to their increased core losses. MPP and Kool M cores have low permeability, allowing larger currents.
(
)
[
]
2 V + CRSS IPK IEXT
(
) ( ) (fOSC )

where the duty cycle is approximated by VOUT/V+, fOSC = 100kHz, and RDS(ON) and CRSS are given in the data sheet of the chosen P-FET. In the equation, RDS(ON) is assumed to be constant, but is actually a function of temperature. Note that the equation does not account for losses incurred by charging and discharging the
VIN 12 V+ R2 1 ...TO VOUT OR VIN R3 100k LBO 14 LOW-BATTERY OUTPUT
VIN 12 V+ FB 5 C6* R4 R5 ...to VOUT
MAX747
LBI
MAX747
OUT GND 8 N.C. R4 = 10k TO 1M VOUT R5 = R4 -1 2V
R1 GND 13 R2 = R1
( VVTRIP -1) TH
13
(
)
VTH = 2.0V
* SEE COMPENSATION CAPACITOR SECTION
Figure 5. Adjustable Output Circuit
9
Figure 4. Input Voltage Monitor Circuit
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High-Efficiency PWM, Step-Down P-Channel DC-DC Controller MAX747
gate capacitance, because that energy is dissipated by the gate-drive circuitry, not the P-FET. The Standard Application Circuit (Figure 1a, 1b) uses an 8-pin Si9405DY surface-mount P-FET that has 0.1 on resistance with a 10V VGS. Optimum efficiency is obtained when the voltage at the drain swings between the supply rails (within a few hundred mV). required at the V+ input. As with the output filter capacitor, a low-ESR capacitor (Sanyo OS-CON, Sprague 595D, or equivalent) is recommended for input bypassing.
Diode Selection
The MAX747's high switching frequency demands a high-speed rectifier. Schottky diodes are recommended. Ensure that the Schottky diode average current rating exceeds the load current level.
Soft-Start and Reference Capacitors A typical value for the soft-start capacitor C4 is 0.1F, which provides a 380ms ramp to full current limit. Use values in the 0.001F and 1F range. The nominal time for C4 to reach its steady-state value is given by:
t SS (sec) = (C4) (3.8 x 106 ) Note that tSS does not equal the time it takes for the MAX747 to power up, although it does affect start-up time. Start-up time is also a function of the input voltage and load current. With a 2.5A load current, a 7V input voltage, and a 0.1F soft-start capacitor, power-up takes typically 360ms. Bypass REF with a 0.22F capacitor (C5).
Capacitor Selection
Output Filter Capacitor The output filter capacitor C1 should have a low effective series resistance (ESR), and its capacitance should remain fairly constant over temperature. This is especially true when in CCM, since the output filter capacitor and the load form the dominant pole that stabilizes the loop. 430F is adequate for load currents up to 2.3A in Figure 1a. At low input/output differentials, it may be necessary to use much larger output filter capacitors to maintain adequate loadtransient response. See the AC Stability with Low Input/Output Differentials section. Sprague 595D surface-mount solid tantalum capacitors and Sanyo OS-CON through-hole capacitors are recommended due to their extremely low ESR. OS-CON capacitors are particularly useful at low temperatures. For best results when using other capacitors, increase the output filter capacitor's size or use capacitors in parallel to reduce ESR. Input Bypass Capacitor The input bypass capacitor C2 reduces peak currents drawn from the voltage source, and also reduces noise at the voltage source caused by the MAX747's fast switching action (this is especially important when other circuitry is operated from the same source). The input capacitor ripple current rating must exceed the RMS input current. IRMS = RMS AC input current
V OUT (VIN - VOUT ) = ILOAD VIN For load currents up to 2.5A, 100F (C2) in parallel with a 0.1F (C3) is adequate. Smaller bypass capacitors may be acceptable for lighter loads. The input voltage source impedance determines the capacitor size
10
Compensation Capacitor With a fixed +5V output, connect the compensation capacitor (C6) between CC and GND to optimize transient response. Appropriate compensation is determined by the ESR of the output filter capacitor (C1) and the feedback voltage-sense resistor network. 270pF is adequate for applications where V+ 9V. Over the full input voltage range, increase C6 to 470pF. C6 also depends on the load current, so for light loads, C6's value can be reduced. If appropriate compensation is not obtained using 470pF, use the following equations to determine C6: For fixed 5V output operation,
C6 = (C1) (ESRC1) 24k
For adjustable-output operation, FB becomes the compensation input pin and CC is left unconnected. Connect C6 between FB and GND in parallel with R4 (Figure 5). C6 is determined by: C6 = (C1) (ESRC1) R4 II R5
For example, with a fixed 5V output, C1 = 330F and an ESRC1 of 0.04 (at a 100kHz frequency), C6 = (C1) (ESRC1) = 783pF 24k
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High-Efficiency PWM, Step-Down P-Channel DC-DC Controller
Setting the Low-Battery Detector Voltage
Select R1 between 10k and 1M. (V - VREF ) R2 = R1 TRIP VREF Connect a pull-up resistor (e.g., 100k) between LBO and VOUT (Figure 4).
VIN V+ AV+ KELVIN SENSE CONNECTION
MAX747
RSENSE
MAX747
__________Applications Information
Layout Considerations
Due to high current levels and fast switching waveforms, which radiate noise, proper MAX747 PC board layout is essential. Protect sensitive analog grounds by using a star ground configuration. Use an adequate ground plane and minimize ground noise by connecting GND, the anode of the steering Schottky diode, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (star ground configuration). Also, minimize lead lengths to minimize stray capacitance, trace resistance, and radiated noise. Place bypass capacitor C3 as close as possible to V+ and GND. AV+ and CS are the inputs to the differential-input current-sense amplifier. Use a Kelvin connection across the sense resistor as shown in Figure 6. Note that even though AV+ also functions as the supply voltage for sensitive analog circuitry, a separate AV+ bypass capacitor should not be used. By not using a capacitor, any noise appearing at the CS input will also appear at the AV+ input and will appear as a commonmode signal to the current-sense amplifier. A separate AV+ capacitor causes the noise to appear only on one input, and this differential noise will be amplified, adversely affecting circuit operation. Similarly, CC (or FB in adjustable-output operation) is a sensitive input that should not be shorted to any node. Avoid shorting CC when probing the circuit, as this may damage the device.
CS
EXT
P L1 VOUT
Figure 6. Kelvin Connection for Current-Sense Amplifier
between" mode (due to the changing duty cycle inherent with DCM), when the device is at maximum duty cycle, EXT turns off at V+ - V GS . But it is not always pulled to V+ because the switch sometimes turns on again after a minimum off-time before EXT can be pulled to V+. The result is short spikes that appear on the EXT waveform in the Typical Operating Characteristics.
AC Stability with Low Input/Output Differentials
At low input/output differentials, the inductor current cannot slew quickly to respond to load changes, so the output filter capacitor must hold up the voltage as the load transient is applied. In Figure 1a's circuit, for V+ = 6.5V, increase the output filter capacitor to 700F (Sprague 595D low-ESR capacitors) to obtain a transient response less than 250mV with a load step from 200mA to 2.5A. For V+ = 6V and V OUT = 5V, increase the output filter capacitor to approximately 1000F. As V+ increases, the device will no longer be operating near full duty cycle with light loads, allowing it to adjust to full duty cycle when the load transient is applied and, in turn, allowing smaller output filter capacitors to be used.
Switching Waveforms
A region exists between CCM and DCM where the inductor current operates in both modes, as shown in the Idle-Mode Moderate current EXT waveform in the Typical Operating Characteristics. As the output voltage varies, it is fed back into CC and the duty cycle is adjusted to compensate for this change. The switch is considered off when V EXT the P-FET's V GS threshold voltage. Once the switch is off, the voltage at EXT is pulled to V+ and the P-FET drain voltage is a Schottky diode drop below GND. However, in this "in-
Dual-Mode Operation
The MAX747 is designed in either fixed-output mode (5V-output, FB = GND) or in adjustable mode (FB = 2V) using a resistor divider. It is not designed to be switched from one mode to another when powered up; however, in adjustment mode, switching between two different resistor dividers is acceptable.
11
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High-Efficiency PWM, Step-Down P-Channel DC-DC Controller MAX747
Additional Notes
When probing the MAX747 circuit, avoid shorting AV+ to GND (the two pins are adjacent to each other) as this may cause the IC to malfunction due to large ground currents. Also, the MAX747 may continue to operate with AV+ disconnected, but erratic switching waveforms will appear at EXT. Finally, due to its fast switching and high drive capability requirements, EXT is a low-impedance point that is not short-circuit protected. Therefore, do not short EXT to any node (including AGND and V+, which are adjacent to EXT) to prevent damaging the device. Table 1. Component Suppliers
REF
___________________Chip Topography
MAX747
LBI LBO GND
SS
V+ EXT 0.130" (3.30mm)
SUPPLIER INDUCTORS Coiltronics Gowanda Sumida USA Sumida Japan CAPACITORS Kemet Matsuo Nichicon Sprague Sanyo USA Sanyo Japan United Chemi-Con DIODES Motorola Nihon USA Nihon Japan POWER TRANSISTORS Harris International Rectifier Siliconix RESISTORS IRC
PHONE (305) 781-8900 (716) 532-2234 (708) 956-0666 81-3-3607-511 (803) 963-6300 (714) 969-2491 (708) 843-7500 (603) 224-1961 (619) 661-6322 81-3-3837-6242 (714) 255-9500 (800) 521-6274 (805) 867-2555 81-3-3494-7411 (407) 724-3739 (213) 772-2000 (408) 988-8000 (512) 992-7900
FAX (305) 782-4163 (716) 532-2702 (708) 956-0702 81-3-3607-5428 (803) 963-6322 (714) 960-6492 (708) 843-2798 (603) 224-1430
SHDN
AGND
FB CC
AV+ OUT 0.080" (2.03mm)
CS
(714) 255-9400 SUBSTRATE CONNECTED TO V+; TRANSISTOR COUNT: 508. (805) 867-2698 81-3-3494-7414 (407) 724-3937 (213) 772-9028 (408) 727-5414 (512) 992-3377
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 (c) 1993 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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