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19-0225; Rev 3; 9/97
K ATION EVALU BLE AVAILA
IT
5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers
_______________General Description
The MAX649/MAX651/MAX652 BiCMOS, step-down DCDC switching controllers provide high efficiency over three decades of load current. A unique, current-limited pulse-frequency-modulated (PFM) control scheme gives these devices the benefits of pulse-width-modulation (PWM) converters (high efficiency at heavy loads), while using only 100A of supply current (vs. 2mA to 10mA for PWM converters). The result is high efficiency over loads ranging from 10mA to more than 2.5A. These devices use miniature external components. Their high switching frequency (up to 300kHz) allows for less than 9mm diameter surface-mount inductors. The MAX649/MAX651/MAX652 have dropout voltages less than 1V and accept input voltages up to 16.5V. Output voltages are preset at 5V (MAX649), 3.3V (MAX651), and 3V (MAX652). These controllers can also be adjusted to any voltage from 1.5V to the input voltage by using two resistors. These step-down controllers drive external P-channel MOSFETs at loads greater than 10W. If less power is required, use the MAX639/MAX640/MAX653 step-down converters with on-chip FETs, which allow up to a 225mA load current.
____________________________Features
o o o o o o o More than 90% Efficiency (10mA to 1.5A Loads) More than 12.5W Output Power 100A Max Quiescent Supply Current 5A Max Shutdown Supply Current Less than 1.0V Dropout Voltage 16.5V Max Input Voltage 5V (MAX649), 3.3V (MAX651), 3V (MAX652), or Adjustable Output Voltage o Current-Limited Control Scheme o Up to 300kHz Switching Frequency
MAX649/MAX651/MAX652
______________Ordering Information
PART MAX649CPA MAX649CSA MAX649C/D MAX649EPA MAX649ESA MAX649MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP**
________________________Applications
5V-to-3.3V Green PC Applications High-Efficiency Step-Down Regulation Minimum-Component DC-DC Converters Battery-Powered Applications
Ordering Information continued at end of data sheet. * Dice are tested at TA = +25C. **Contact factory for availability and processing to MIL-STD-883.
__________Typical Operating Circuit
INPUT 4V TO 16.5V
__________________Pin Configuration
TOP VIEW
V+ OUT 1 2 8 7 GND EXT CS V+
MAX651
ON/OFF SHDN CS EXT P OUTPUT 3.3V
FB
SHDN 3 REF 4
MAX649 MAX651 MAX652
6 5
REF FB GND
OUT
DIP/SO
________________________________________________________________ Maxim Integrated Products
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, V+ to GND.......................................-0.3V, +17V REF, SHDN, FB, CS, EXT, OUT .......................-0.3V, (V+ + 0.3V) Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 9.09mW/C above +70C) .............727mW SO (derate 5.88mW/C above +70C) ..........................471mW CERDIP (derate 8.00mW/C above +70C) ..................640mW Operating Temperature Ranges MAX649C_A, MAX65_C_A ..................................0C to +70C MAX649E_A, MAX65_E_A ................................-40C to +85C MAX649MJA, MAX65_MJA ............................-55C to +125C 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+ = 5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER V+ Input Voltage Range Supply Current SYMBOL V+ V+ = 16.5V, SHDN 0.4V (operating, switch off) IQ V+ = 16.5V, SHDN 1.6V (shutdown) V+ = 10V, SHDN 1.6V (shutdown) MAX649C, MAX65_C FB Trip Point MAX649E, MAX65_E MAX649M, MAX65_M MAX649C, MAX65_C FB Input Current IFB MAX649E, MAX65_E MAX649M, MAX65_M MAX649, V+ = 6V to 16.5V Output Voltage VOUT Circuit of Figure 1 MAX651, V+ = 4V to 16.5V MAX652, V+ = 4V to 16.5V MAX649C, MAX65_C, IREF = 0 Reference Voltage VREF MAX649E, MAX65_E, IREF = 0 MAX649M, MAX65_M, IREF = 0 REF Load Regulation REF Line Regulation 0 IREF 100A, sourcing only 4V V+ 16.5V MAX649, 6V V+ 16V, ILOAD = 1A Output Voltage Line Regulation Circuit of Figure 1 MAX651, 4.5V V+ 16V, ILOAD = 1A MAX652, 4V V+ 16V, ILOAD = 1A MAX649C/E, MAX65_C/E MAX649M, MAX65_M 4.80 3.17 2.88 1.470 1.4625 1.455 5.0 3.3 3.0 1.5 1.5 1.5 4 4 40 2.6 1.7 1.9 mV/V 1.470 1.4625 1.455 CONDITIONS MIN 4.0 80 4 2 1.5 1.5 1.5 5 1.530 1.5375 1.545 50 70 90 5.20 3.43 3.12 1.530 1.5375 1.545 10 15 100 mV V/V V V nA V TYP MAX 16.5 100 A UNITS V
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER SYMBOL CONDITIONS MAX649, 0 ILOAD 1.5A, VIN = 10V Output Voltage Load Regulation Circuit of Figure 1 MAX651, 0 ILOAD 1.5A, VIN = 5V MAX652, 0 ILOAD 1.5A, VIN = 5V MAX649, V+ = 10V, ILOAD = 1A Efficiency Circuit of Figure 1 MAX651, V+ = 5V, ILOAD = 1A MAX652, V+ = 5V, ILOAD = 1A SHDN Input Current SHDN Input Voltage High SHDN Input Voltage Low Current-Limit Trip Level (V+ to CS) CS Input Current Switch Maximum On-Time Switch Minimum Off-Time EXT Rise Time EXT Fall Time tON (max) tOFF (min) VIH VIL VCS V+ = 16.5V, SHDN = 0V or V+ 4V V+ 16.5V 4V V+ 16.5V 4V V+ 16.5V 4V V+ 16.5V V+ = 12V V+ = 12V CEXT = 0.001F, V+ = 12V CEXT = 0.001F, V+ = 12V 12 1.8 16 2.3 50 50 MAX649C/E, MAX65_C/E MAX649M, MAX65_M 180 160 210 210 1.6 0.4 240 260 1 20 2.8 MIN TYP -47 -45 MAX UNITS
MAX649/MAX651/MAX652
mV/A
-45 92 89
%
88 1 A V V mV A s s ns ns
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
__________________________________________Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE
MAX649-A01
SHUTDOWN CURRENT vs. TEMPERATURE
MAX649-A02
MAX649 MAXIMUM LOAD CURRENT vs. SUPPLY VOLTAGE
MAXIMUM LOAD CURRENT (mA)
MAX649-A03
80 78 76 I+ (mA) V+ = 10V V+ = 16.5V
4.0 3.5 3.0 V+ = 16.5V 2.5 I+ (mA) 2.0 1.5 V+ = 8V
2500
2000
74 72 70
1500
1000 VOUT = 5V CIRCUIT OF FIGURE 1
V+ = 4V 68 66 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
1.0 0.5 V+ = 4V 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
500
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 1415 INPUT VOLTAGE (V)
MAX649 EFFICIENCY vs. LOAD CURRENT
MAX649-A04
MAX651 EFFICIENCY vs. LOAD CURRENT
MAX649-A05
MAX652 EFFICIENCY vs. LOAD CURRENT
90 80 TOP TO BOTTOM: VIN = 4.3V VIN = 5V VIN = 8V VIN = 10V VIN = 12V VIN = 15V
MAX649-A06
100 90 80 EFFICIENCY (%) TOP TO BOTTOM: VIN = 6V VIN = 8V VIN = 10V VIN = 12V VIN = 15V
100 90 80 EFFICIENCY (%) TOP TO BOTTOM: VIN = 4.3V VIN = 5V VIN = 8V VIN = 10V VIN = 12V VIN = 15V
100
60 50 40 30 20 10 0 100 1m 10m VOUT = 5V
EFFICIENCY (%)
70
70 60 50 40 30 20 10 VOUT = 3.3V 100 1m 10m
70 60 50 40 30 20 10 0 VOUT = 3V 100 1m 10m
100m
1
0 100m 1 LOAD CURRENT (A)
100m
1
LOAD CURRENT (A)
LOAD CURRENT (A)
SWITCH ON-TIME vs. TEMPERATURE
MAX649-A07
SWITCH OFF-TIME vs. TEMPERATURE
MAX649-A08
SWITCH ON-TIME/OFF-TIME RATIO vs. TEMPERATURE
V+ = 5V 7.8 7.6 7.4 tON/tOFF RATIO
MAX649-A9
17 V+ = 5V
2.5 V+ = 5V
8.0
16
tOFF (ms)
tON (ms)
7.2 7.0 6.8 6.6 6.4 6.2
2.0
15 -60 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
1.5 -60 -40 -20 0 20 40 60 TEMPERATURE (C) 80 100 120
6.0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers
____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
EXT RISE AND FALL TIMES vs. TEMPERATURE (1nF)
MAX649-A10
MAX649/MAX651/MAX652
EXT RISE AND FALL TIMES vs. TEMPERATURE (5nF)
MAX649-A11
DROPOUT VOLTAGE vs. LOAD CURRENT
900 DROPOUT VOLTAGE (mV) 800 700 600 500 400 300 200 MAX651, VOUT = 3.3V MAX649, VOUT = 5V MAX652, VOUT = 3V
MAX649-A12
130 120 110 100 tRISE & tFALL (ns) 90 80 70 60 50 40 30 20 -60 -40 -20 0 V+ = 12V, tFALL V+ = 12V, tRISE V+ = 5V, tFALL CEXT = 1nF V+ = 5V, tRISE
500 450 400 tRISE & tFALL (ns) 350 300 250 200 150 100 50 -60 -40 -20 0 V+ = 12V, tFALL V+ = 5V, tFALL V+ = 12V, tRISE CEXT = 5nF V+ = 5V, t RISE
1000
100 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
20 40 60 80 100 120 140
20 40 60 80 100 120 140
TEMPERATURE (C)
TEMPERATURE (C)
LOAD CURRENT (A)
DROPOUT VOLTAGE vs. TEMPERATURE
MAX649-A13
CS TRIP LEVEL vs. TEMPERATURE
230 225 REFRENCE OUTPUT (V) CS TRIP LEVEL (mV) 220 215 210 205 200 195 1.502 1.500 1.498 1.496 1.494 1.492 -60 -40 -20 0 20 40 60 80 100 120 140
MAX649-A14
REFERENCE OUTPUT VOLTAGE vs. TEMPERATURE
MAX649-A15
1100 MAX649
235
1.506 1.504
DROPOUT VOLTAGE (mV)
1000
900
MAX652 MAX651
800
700 ILOAD = 1A CIRCUIT OF FIGURE 1 600 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
190 185 TEMPERATURE (C) -60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (C)
REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE
REFRENCE OUTPUT RESISTANCE ()
MAX649-A16
250
200
IREF = 10A
150 IREF = 50A 100
50
IREF = 100A
0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
_____________________________Typical Operating Characteristics (continued)
MAX649 LINE-TRANSIENT RESPONSE
MAX649 LOAD-TRANSIENT RESPONSE
A
A
B
B
250s/div ILOAD = 1A A: INPUT VOLTAGE (7V & 12V), 5V/div B: 5V OUT, AC COUPLED, 100mV/div
250s/div A: LOAD CURRENT (100mA & 1A), 500mA/div B: 5V OUTPUT VOLTAGE, AC COUPLED, 50mV/div
MAX649 SHUTDOWN RESPONSE TIME
A
B
______________________________________________________________Pin Description
PIN NAME OUT FB SHDN REF V+ CS EXT GND FUNCTION Sense input for fixed 5V, 3.3V, or 3V output operation. OUT is internally connected to the on-chip voltage divider. Although it is connected to the output of the circuit, the OUT pin does not supply current. Feedback input. Connect to GND for fixed-output operation. Connect a resistor divider between OUT, FB, and GND for adjustable-output operation. See Setting the Output Voltage section. Active-high TTL/CMOS logic-level input. Part is placed in shutdown when SHDN is driven high. In shutdown mode, the reference and the external MOSFET are turned off, and OUT = 0V. Connect to GND for normal operation. 1.5V reference output that can source 100A. Bypass with 0.1F. Positive power-supply input Current-sense input. Connect current-sense resistor between V+ and CS. When the voltage across the resistor equals the current-limit trip level, the external MOSFET is turned off. Gate drive for external P-channel MOSFET. EXT swings between V+ and GND. Ground
1ms/div ILOAD = 1A A: SHDN INPUT VOLTAGE (0V & 5V), 2V/div B: 5V OUTPUT VOLTAGE, 2V/div
1
2 3 4 5 6 7 8 6
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers
VIN C4 0.1F R1 0.1 CS EXT 6 7 P1 Si9430* L1 22H** OUTPUT @ 1.5A C1 100F
V+
5
MAX649 MAX651 MAX652
3 SHDN
4
REF FB GND 8 2
OUT
1
C3 0.1F
D1 NSQ03A02L
C2 330F
*SILICONIX SURFACE-MOUNT MOSFET **SUMIDA CDR125-220
Figure 1. Test Circuit
_______________Detailed Description
The MAX649/MAX651/MAX652 are BiCMOS, stepdown, switch-mode power-supply controllers that provide fixed outputs of 5V, 3.3V, and 3V, respectively. Their unique control scheme combines the advantages of pulse-frequency-modulation (low supply current) and pulse-width-modulation (high efficiency at high loads). An external P-channel power MOSFET allows peak currents in excess of 3A, increasing the output current capability over previous PFM devices. Figure 2 is the block diagram. The MAX649/MAX651/MAX652 offer three main improvements over prior solutions: 1) The converters operate with tiny (less than 9mm diameter) surface-mount inductors, due to their 300kHz switching frequency. 2) The current-limited PFM control scheme allows greater than 90% efficiencies over a wide range of load currents (1.0mA to 1.5A). 3) The maximum supply current is only 100A.
is out of regulation. However, unlike traditional PFM converters, switching is accomplished through the combination of a peak current limit and a pair of oneshots that set the maximum switch on-time (16s) and minimum switch off-time (2.3s). Once off, the minimum off-time one-shot holds the switch off for 2.3s. After this minimum time, the switch either 1) stays off if the output is in regulation, or 2) turns on again if the output is out of regulation. The MAX649/MAX651/MAX652 also limit the peak inductor current, which allows them to run in continuous-conduction mode and maintain high efficiency with heavy loads (Figure 3a). This current-limiting feature is a key component of the control circuitry. Once turned on, the switch stays on until either 1) the maximum on-time one-shot turns it off (16s later), or 2) the current limit is reached. To increase light-load efficiency, the current limit for the first two pulses is set to half the peak current limit. If those pulses bring the output voltage into regulation, the voltage comparator holds the MOSFET off and the current limit remains at half its peak. If the output voltage is still out of regulation after two pulses, the current limit for the next pulse is raised to its peak (Figure 3b). Calculate the peak current limit by dividing the Current-Limit Trip Level (see Electrical Characteristics) by the value of the current-sense resistor.
MAX649/MAX651/MAX652
Shutdown Mode
When SHDN is high, the MAX649/MAX651/MAX652 enter shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference) and the supply current drops to less than 5A. EXT goes high, turning off the external MOSFET. SHDN is a TTL/CMOS logic-level input. Connect SHDN to GND for normal operation.
Quiescent Current
In normal operation, the quiescent current is less than 100A. However, this current is measured by forcing the external transistor switch off. In an actual application, even with no load, additional current is drawn to supply external feedback resistors (if used) and the diode and capacitor leakage currents. In the circuit of Figure 1, with V+ at 5V and VOUT at 3.3V, the typical quiescent current is 90A.
EXT Drive Voltage Range
EXT swings from V+ to GND and provides the drive output for an external P-channel power MOSFET.
PFM Control Scheme
The MAX649/MAX651/MAX652 use a proprietary, current-limited PFM control scheme. As with traditional PFM converters, the external power MOSFET is turned on when the voltage comparator senses that the output
Modes of Operation
When delivering high output currents, the MAX649/ MAX651/MAX652 operate in continuous-conduction mode (CCM). In this mode, current always flows in the
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
V+
FB DUAL-MODETM COMPARATOR
SHDN
MAX649 MAX651 MAX652
ERROR COMPARATOR OUT
REF
1.5V REFERENCE N MINIMUM OFF-TIME TRIG ONE-SHOT
Q
FROM V+
S
Q
EXT
MAXIMUM TRIG ON-TIME Q ONE-SHOT
R CURRENT COMPARATOR CS
CURRENT CONTROL CIRCUITS
0.2V (FULL CURRENT)
0.1V (HALF CURRENT) FROM V+
GND
Figure 2. Block Diagram
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
1.5A 1A
2.5A 2.0A 1.5A
0A
1.0A 0.5A 0A
2s/div V+ = 10V, ILOAD = 1.3A CIRCUIT OF FIGURE 1, R1 = 150m
5s/div V+ = 10V, ILOAD = 1.4A CIRCUIT OF FIGURE 1, R1 = 100m
Figure 3a. MAX649 Continuous-Conduction Mode, Heavy Load-Current Waveform (500mA/div)
Figure 3b. MAX649 Light/Medium Load-Current Waveform (500mA/div)
inductor, and the control circuit adjusts the switch duty cycle to maintain regulation without exceeding the switch current capability (Figure 3a). This provides excellent load-transient response and high efficiency. In discontinuous-conduction mode (DCM), current through the inductor starts at zero, rises to a peak value, then ramps down to zero. Although efficiency is still excellent, the output ripple increases slightly, and the switch waveforms exhibit ringing (the self-resonant frequency of the inductor). This ringing is to be expected and poses no operational problems.
VIN C4 0.1F R1 0.1 CS EXT 6 7 P1 Si9430 L1 22H OUTPUT @ 1.5A C1 100F
V+
5
MAX649 MAX651 MAX652
3 SHDN
4
REF GND
OUT FB
1 2 R2 C2 330F D1 1N5820 R3 150k
Dropout
The MAX649/MAX651/MAX652 are said to be in dropout when the input voltage (V+) is low enough that the output drops below the minimum output voltage specification (see Electrical Characteristics). The dropout voltage is the difference between the input and output voltage when dropout occurs. See the Typical Operating Characteristics for the Dropout Voltage vs. Load Current and Dropout Voltage vs. Temperature graphs.
C3 0.1F 8
VOUT R2 = R3 -1 VREF VREF = 1.5V
(
)
Figure 4. Adjustable-Output Operation
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
MAX649-A25 MAX649-A26 MAX649-A27
3.0 MAXIMUM OUTPUT CURRENT (A) 2.5 2.0 1.5 1.0 0.5 0 34 567 RS = 0.06 RS = 0.07 RS = 0.08 RS = 0.10 RS = 0.12 RS = 0.14 MAX649 VOUT = 5V
3.0 RS = 0.06 MAXIMUM OUTPUT CURRENT (A) 2.5 RS = 0.07 2.0 1.5 1.0 0.5 0 345 INPUT VOLTAGE (V) RS = 0.08 RS = 0.10 RS = 0.12 RS = 0.14
MAX651 VOUT = 3.3V 6 7 8 9 10 11 12 13 14 15 16
8 9 10 11 12 13 14 15 16
INPUT VOLTAGE (V)
Figure 5a. MAX649 Current-Sense Resistor Graph
Figure 5b. MAX651 Current-Sense Resistor Graph
__________________Design Procedure
Setting the Output Voltage
The MAX649/MAX651/MAX652 are preset for 5V, 3.3V, and 3V output voltages, respectively. Tie FB to GND for fixed-output operation. They may also be adjusted from 1.5V (the reference voltage) to the input voltage, using external resistors R2 and R3 configured as shown in Figure 4. For adjustable-output operation, 150k is recommended for resistor R3. 150k is a good value--high enough to avoid wasting energy, yet low enough to avoid RC delays caused by parasitic capacitance at FB. R2 is given by: VOUT R2 = R3 x ------ -1 VREF
MAXIMUM OUTPUT CURRENT (A) 3.0 RS = 0.06 2.5 RS = 0.07 2.0 1.5 1.0 0.5 0 345 INPUT VOLTAGE (V) RS = 0.08 RS = 0.10 RS = 0.12 RS = 0.14 MAX652 VOUT = 3.0V 6 7 8 9 10 11 12 13 14 15 16
[
]
where VREF = 1.5V. When using external resistors, it does no harm to connect OUT and the output together, or to leave OUT unconnected.
Figure 5c. MAX652 Current-Sense Resistor Graph
Current-Sense Resistor Selection
The current-sense resistor limits the peak switch current to 210mV/RSENSE, where RSENSE is the value of the current-sense resistor, and 210mV is the currentlimit trip level (see Electrical Characteristics). To maximize efficiency and reduce the size and cost of external components, minimize the peak current. However, since the available output current is a function of the peak current, the peak current must not be too low.
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To choose the proper current-sense resistor for a particular output voltage, determine the minimum input voltage and the maximum load current. Next, referring to Figures 5a, 5b, or 5c, using the minimum input voltage, find the curve with the largest sense resistor that provides sufficient output current. It is not necessary to perform worst-case calculations. These curves take into account the worst-case values for sense resistor (5%), inductor (22H 10%), diode drop (0.6V), and the IC's current-sense trip level; an external MOSFET on-resistance of 0.13 is assumed for VGS = -4.5V.
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers
Standard wire-wound and metal-film resistors have an inductance high enough to degrade performance. Surface-mount (chip) resistors have very little inductance and are well suited for use as current-sense resistors. A wire resistor made by IRC works well in through-hole applications. Because this resistor is a band of metal shaped as a "U", its inductance is less than 10nH (an order of magnitude less than metal film resistors). Resistance values between 5m and 0.1 are available (see Table 1). tor's saturation-current rating is greater than ILIM(max). However, it is generally acceptable to bias the inductor into saturation by about 20% (the point where the inductance is 20% below its nominal value). The peak current of Figure 1 is 2.35A for a 1.5A output. The inductor used in this circuit is specified to drop by 10% at 2.2A (worst case); a curve provided by the manufacturer shows that the inductance typically drops by 20% at 3.1A. Using a slightly underrated inductor can sometimes reduce size and cost, with only a minor impact on efficiency. The MAX649/MAX651/MAX652 current limit prevents any damage from an underrated inductor's low inductance at high currents. Table 1 lists inductor types and suppliers for various applications. The efficiencies of the listed surfacemount inductors are nearly equivalent to those of the larger size through-hole versions.
MAX649/MAX651/MAX652
Inductor Selection
Practical inductor values range from 10H to 50H or more. The circuit operates in discontinuous-conduction mode if: VOUT x (R + 1) VD V + ---------------- + ---- + VSW R R R, the switch on-time/off-time ratio, equals 6.7. VD is the diode's drop, and VSW is the voltage drop across the P-channel FET. To get the full output capability in discontinuous-conduction mode, choose an inductor value no larger than: RSENSE x 12s x (V+ - VSW - VOUT) L(max) = ---------------------------------- VCS where VCS is the current-sense voltage. In both the continuous and discontinuous modes, the lower limit of the inductor is more important. With a small inductor value, the current rises faster and overshoots the desired peak current limit because the current-limit comparator cannot respond fast enough. This reduces efficiency slightly and, more importantly, could cause the current rating of the external components to be exceeded. Calculate the minimum inductor value as follows: (V+(max) - VSW - VOUT) x 0.3s L(min) = ------------------------------ x ILIM(min) I where is the percentage of inductor-current overI shoot, where ILIM = VCS/RSENSE and 0.3s is the time it takes the comparator to switch. An overshoot of 10% is usually not a problem. Inductance values above the minimum work well if the maximum value defined above is not exceeded. Smaller inductance values cause higher output ripple because of overshoot. Larger values tend to produce physically larger coils. For highest efficiency, use a coil with low DC resistance; a value smaller than 0.1V/I LIM works best. To minimize radiated noise, use a toroid, pot core, or shielded-bobbin inductor. Inductors with a ferrite core or equivalent are recommended. Make sure the induc-
Diode Selection
The MAX649/MAX651/MAX652's high switching frequency demands a high-speed rectifier (commonly called a catch diode when used in switching-regulator circuits). Schottky diodes, such as the 1N5817 through 1N5822 families (and their surface-mount equivalents), are recommended. Choose a diode with an average current rating equal to or greater than ILIM(max) and a voltage rating higher than V+(max). For high-temperature applications, where Schottky diodes can be inadequate because of high leakage currents, use high-speed silicon diodes instead. At heavy loads and high temperatures, the disadvantages of a Schottky diode's high leakage current may outweigh the benefits of its low forward voltage. Table 1 lists diode types and suppliers for various applications.
External Switching Transistor
The MAX649/MAX651/MAX652 drive P-channel enhancement-mode MOSFET transistors only. The choice of power transistor is primarily dictated by the input voltage and the peak current. The transistor's on-resistance, gate-source threshold, and gate capacitance must also be appropriately chosen. The drain-to-source and gate-to-source breakdown voltage ratings must be greater than V+. The total gate-charge specification is normally not critical, but values should be less than 100nC for best efficiency. The MOSFET should be capable of handling the peak current and, for maximum efficiency, have a very low on-resistance at that current. Also, the on-resistance must be low for the minimum available VGS , which equals V+(min). Select a transistor with an on-resistance between 50% and 100% of the current-sense resistor. The Si9430 transistor chosen for the Typical Operating Circuit has
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
a drain-to-source rating of -20V and a typical on-resistance of 0.115 at 2A with VGS = -4.5V. Tables 1 and 2 list suppliers of switching transistors suitable for use with these devices. amount of noise at the voltage source caused by the switching action of the MAX649/MAX651/MAX652. The input voltage source impedance determines the size of the capacitor required at the V+ input. As with the output filter capacitor, a low-ESR capacitor is recommended. Bypass the IC separately with a 0.1F ceramic capacitor placed close to the V+ and GND pins.
Capacitor Selection
Output Filter Capacitor The primary criterion for selecting the output filter capacitor is low equivalent series resistance (ESR), rather than high capacitance. An electrolytic capacitor with low enough ESR will automatically have high enough capacitance. The product of the inductor-current variation and the ESR of the output filter capacitor determines the amplitude of the high-frequency ripple seen on the output voltage. When a 330F, 10V Sprague surface-mount capacitor (595D series) with ESR = 0.15is used, 40mV of output ripple is typically observed when stepping down from 10V to 5V at 1A. The output filter capacitor's ESR also affects efficiency. Use low-ESR capacitors for best performance. The smallest low-ESR SMT tantalum capacitors currently available are from the Sprague 595D series. Sanyo OSCON organic semiconductor through-hole capacitors and the Nichicon PL series also exhibit very low ESR. Table 1 lists some suppliers of low-ESR capacitors. Input Bypass Capacitor The input bypass capacitor reduces peak currents drawn from the voltage source, and also reduces the
Reference Capacitor Bypass REF with a 0.1F or larger capacitor. REF can source at least 100A.
Layout Considerations
Proper PC board layout is essential because of high current levels and fast switching waveforms that radiate noise. Minimize ground noise by connecting the anode of the catch diode, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point ("star" ground configuration). A ground plane is recommended. Also minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. In particular, the traces connected to FB (if an external resistor divider is used) and EXT must be short. Place the 0.1F ceramic bypass capacitor as close as possible to V+ and GND.
Table 1. Component Selection Guide
PRODUCTION METHOD INDUCTORS CAPACITORS DIODES CURRENT-SENSE RESISTORS MOSFETS Siliconix Little Foot series Nihon NSQ series IRC LRC series Motorola medium-power surface-mount products
Sumida Matsuo CDR125-220 (22H) 267 series Surface Mount Coiltronics CTX 100 series Sprague 595D series Sanyo OS-CON series low-ESR organic semiconductor
Miniature Through-Hole
Sumida RCH855-220M
IRC OAR series
Motorola
Low-Cost Through-Hole
Renco RL 1284-22
Nichicon PL series Motorola low-ESR electrolytics 1N5820, 1N5823 United Chemi-Con LXF series
Motorola TMOS power MOSFETs
12
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5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
Table 2. Component Suppliers
COMPANY Coiltronics Harris International Rectifier IRC Matsuo Motorola Nichicon Nihon Renco Sanyo Siliconix Sprague Sumida United Chemi-Con USA USA USA USA USA Japan USA USA Japan USA Japan USA USA Japan USA USA USA Japan USA PHONE (407) 241-7876 (800) 442-7747 (310) 322-3331 (704) 264-8861 (714) 969-2491 81-6-337-6450 (800) 521-6274 (708) 843-7500 81-7-5231-8461 (805) 867-2555 81-3-3494-7411 (516) 586-5566 (619) 661-6835 81-7-2070-6306 (408) 988-8000 (603) 224-1961 (708) 956-0666 81-3-3607-5111 (714) 255-9500 FAX (407) 241-9339 (407) 724-3937 (310) 322-3332 (704) 264-8866 (714) 960-6492 81-6-337-6456 (602) 244-4015 (708) 843-2798 81-7-5256-4158 (805) 867-2556 81-3-3494-7414 (516) 586-5562 (619) 661-1055 81-7-2070-1174 (408) 970-3950 (603) 224-1430 (708) 956-0702 81-3-3607-5144 (714) 255-9400
__Ordering Information (continued)
PART MAX651CPA MAX651CSA MAX651C/D MAX651EPA MAX651ESA MAX651MJA MAX652CPA MAX652CSA MAX652C/D MAX652EPA MAX652ESA MAX652MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP** 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP**
___________________Chip Topography
OUT GND
EXT FB 0.109" (2.769mm) CS SHDN
* Dice are tested at TA = +25C. **Contact factory for availability and processing to MIL-STD-883.
REF
V+
0.080" (2.032mm)
TRANSISTOR COUNT: 442; SUBSTRATE CONNECTED TO V+.
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13
5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
________________________________________________________Package Information
PDIPN.EPS
14
______________________________________________________________________________________
SOICN.EPS
5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers
___________________________________________Package Information (continued)
CDIPS.EPS
MAX649/MAX651/MAX652
______________________________________________________________________________________
15
5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652
NOTES
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.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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