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19-0305; Rev 2; 9/95
5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers
_______________General Description
The MAX1649/MAX1651 BiCMOS, step-down, DC-DC switching controllers provide high efficiency over loads ranging from 1mA to more than 2.5A. A unique, currentlimited 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). Dropout performance down to 300mV is provided by a high switch duty cycle (96.5%) and a low current-sense threshold (110mV). A high switching frequency (up to 300kHz) allows these devices to use miniature external components. The MAX1649/MAX1651 have dropout voltages less than 0.3V at 500mA and accept input voltages up to 16V. Output voltages are preset at 5V (MAX1649), or 3.3V (MAX1651). They 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 12.5W. 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 Less than 0.3V Dropout Voltage at 500mA 100A Max Quiescent Supply Current 5A Max Shutdown Supply Current 16V Max Input Voltage 5V (MAX1649), 3.3V (MAX1651), or Adjustable Output Voltage o Current-Limited Control Scheme o Up to 300kHz Switching Frequency o Up to 96.5% Duty Cycle
MAX1649/MAX1651
______________Ordering Information
PART MAX1649CPA MAX1649CSA MAX1649C/D MAX1649EPA MAX1649ESA MAX1651CPA MAX1651CSA MAX1651C/D MAX1651EPA MAX1651ESA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO
________________________Applications
PDAs High-Efficiency Step-Down Regulation 5V-to-3.3V Green PC Applications Battery-Powered Applications
* Dice are tested at TA = +25C.
__________Typical Operating Circuit
INPUT 3.6V TO 16V
__________________Pin Configuration
TOP VIEW
V+ OUT 1 2 8 7 GND EXT CS V+
MAX1651
ON/OFF SHDN CS EXT P OUTPUT 3.3V
FB
SHDN 3 REF 4
MAX1649 MAX1651
6 5
REF FB GND
OUT
DIP/SO
________________________________________________________________ Maxim Integrated Products
1
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5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649/MAX1651
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 Operating Temperature Ranges MAX1649C_A, MAX1651C_A ..............................0C to +70C MAX1649E_A, MAX1651E_A ............................-40C to +85C 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+ I+ VOUT < V+ V+ = 16V, SHDN 0.4V (operating, switch off) V+ = 16V, SHDN 1.6V (shutdown) V+ = 10V, SHDN 1.6V (shutdown) FB Trip Point FB Input Current Output Voltage Reference Voltage REF Load Regulation REF Line Regulation IFB VOUT VREF MAX1649C, MAX1651C MAX1649E, MAX1651E MAX1649C, MAX1651C MAX1649E, MAX1651E Circuit of Figure 1 MAX1649, V+ = 5.5V to 16V MAX1651, V+ = 3.6V to 16V 4.80 3.17 1.470 1.4625 5.0 3.3 1.5 1.5 4 40 MAX1649, 5.5V V+ 16V, ILOAD = 1A MAX1651, 3.6V V+ 16V, ILOAD = 1A MAX1649, 0A ILOAD 1.5A, VIN = 10V MAX1651, 0A ILOAD 1.5A, VIN = 5V MAX1649, V+ = 10V, ILOAD = 1A MAX1651, V+ = 5V, ILOAD = 1A 1.6 0.4 2.6 mV/V 1.7 -47 mV/A -45 90 % 90 1 A V V 1.470 1.4625 CONDITIONS MIN 3.0 78 2 1 1.5 1.5 5 1.530 1.5375 50 70 5.20 3.43 1.530 1.5375 10 100 V nA V V mV V/V TYP MAX 16 100 A UNITS V
MAX1649C, MAX1651C, IREF = 0A MAX1649E, MAX1651E, IREF = 0A 0A IREF 100A, sourcing only 3V V+ 16V Circuit of Figure 1
Output Voltage Line Regulation
Output Voltage Load Regulation
Circuit of Figure 1
Efficiency
Circuit of Figure 1
SHDN Input Current SHDN Input Voltage High SHDN Input Voltage Low VIH VIL
V+ = 16V, SHDN = 0V or V+ 3V V+ 16V 3V V+ 16V
2
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5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Current-Limit Trip Level (V+ to CS) CS Input Current Switch Maximum On-Time tON (max) Switch Minimum Off-Time tOFF (min) EXT Rise Time EXT Fall Time Maximum Duty Cycle SYMBOL VCS 3V V+ 16V 3V V+ 16V V+ = 12V V+ = 12V CEXT = 0.001F, V+ = 12V CEXT = 0.001F, V+ = 12V tON x 100% tON + tOFF 95 24 0.8 32 1.1 25 25 96.5 CONDITIONS MIN 80 TYP 110 MAX 140 1 40 1.8 UNITS mV A s s ns ns %
MAX1649/MAX1651
__________________________________________Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE
MAX1649-TOC06
SHUTDOWN CURRENT vs. TEMPERATURE
MAX1649-TOC05
EXT RISE AND FALL TIMES vs. TEMPERATURE (1nF)
55 50 tRISE & tFALL (ns) CEXT = 1nF
MAX1649/51-01
80 78 76 I+ (A) V+ = 16V
4.0 3.5 3.0 V+ = 16V 2.5 I+ (A)
60
45 40 35 30 25
V+ = 5V, tRISE
74 72 70
V+ = 10V
2.0 1.5 V+ = 8V 1.0
V+ = 5V, tFALL
V+ = 4V 68 66 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) 0.5 V+ = 4V 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
V+ = 15V, tRISE V+ = 15V, tFALL -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
20 15
EXT RISE AND FALL TIMES vs. TEMPERATURE (5nF)
MAX1649/51-02
EFFICIENCY vs. LOAD CURRENT (VOUT = 5V)
VOUT = 5V CIRCUIT OF FIGURE 1
MAX1649/51-A1
220 200 tRISE & tFALL (ns) 180 160 140 120 100 80 60 40
CEXT = 5nF V+ = 5V, tRISE
90 EFFICIENCY (%) 80 70 60 50
90 EFFICIENCY (%) 80 70 60 50 40 0.1
VOUT = 3.3V CIRCUIT OF FIGURE 1
V+ = 5V, tFALL
V+ = 15V, tRISE
TOP TO BOTTOM:
TOP TO BOTTOM:
V+ = 15V, tFALL -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
VIN = 6V VIN = 8V VIN = 10V VIN = 12V VIN = 15V 0.1 1 10 100 1k LOAD CURRENT (mA) 10k
VIN = 4.3V VIN = 5V VIN = 8V VIN = 10V VIN = 12V VIN = 15V 1 10 100 1k LOAD CURRENT (mA) 10k
40
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MAX1649/51-A2
240
100
EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
100
5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649/MAX1651
____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
SWITCH ON-TIME vs. TEMPERATURE
MAX1649/51-03
SWITCH OFF-TIME vs. TEMPERATURE
MAX1649/51-04
MAXIMUM DUTY CYCLE vs. TEMPERATURE
MAX1649/51-05
34.0 33.5 33.0
1.5 1.4 1.3
100 99 DUTY CYCLE (%) 98 97 96 95 94 93
1.2 tOFF (s) -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) tON (s) 32.5 32.0 31.5 31.0 30.5 30.0 1.1 1.0 0.9 0.8 0.7 0.6 0.5 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
-60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
CS TRIP LEVEL vs. TEMPERATURE
MAX1649/51-06
DROPOUT VOLTAGE vs. LOAD CURRENT
CIRCUIT OF FIGURE 1
MAX1649/51-A3
120
600 500 DROPOUT VOLTAGE (mV) 400 300
115 CS TRIP LEVEL (mV)
VOUT = 4.80V
110
105
VOUT = 3.17V 200 100 0
100
95 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
0
0.5
1.0
1.5
2.0
LOAD CURRENT (A)
REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE
REFERENCE OUTPUT RESISTANCE ()
MAX1649-TOC07
REFERENCE OUTPUT VOLTAGE vs. TEMPERATURE
MAX1649-TOC01
250
1.506 REFERENCE OUTPUT VOLTAGE (V) 1.504 1.502 1.500 1.498 1.496 1.494 1.492
200
IREF = 10A
IREF = 10A
150 IREF = 50A 100
50
IREF = 100A
0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (C)
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5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers
____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.) MAX1649 LINE-TRANSIENT RESPONSE MAX1649 LOAD-TRANSIENT RESPONSE
MAX1649/MAX1651
A
A 16V B 6V 1.6A B 0A
5ms/div CIRCUIT OF FIGURE 1, ILOAD = 1A A: VOUT = 5V, 100mV/div, AC-COUPLED B: V+ = 6V TO 16V, 5V/div
200s/div CIRCUIT OF FIGURE 1, V+ = 10V A: VOUT = 5V, 100mV/div, AC-COUPLED B: ILOAD = 30mA TO 1.6A, 1A/div
MAX1649 SHDN RESPONSE TIME
5V OUTPUT 0V 4V SHDN INPUT 0V
1ms/div CIRCUIT OF FIGURE 1, V+ = 10V, ILOAD = 1A
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5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649/MAX1651
______________________________________________________________Pin Description
PIN NAME FUNCTION Sense Input for fixed 5V or 3.3V 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. Leave OUT unconnected for adjustable-output operation. 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 Shutdown Input. Part is placed in shutdown when SHDN is driven high. In shutdown mode, the reference, output, and external MOSFET are turned off. 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
1
2 3 4 5 6 7 8
OUT FB SHDN REF V+ CS EXT GND
VIN C4 0.1F C1 100F
V+
5 R1 0.05 6 7
MAX1649 MAX1651
CS 3 SHDN EXT
P1 Si9430* L1 47H**
OUTPUT @ 1.5A
The MAX1649/MAX1651 offer four main improvements over prior solutions: 1) The converters operate with miniature 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 (10mA to 1.5A). 3) Dropout voltage has been reduced to less than 300mV for many applications. 4) The quiescent supply current is only 100A.
4
REF FB GND 8 2
OUT
1
PFM Control Scheme
The MAX1649/MAX1651 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 is out of regulation. However, unlike traditional PFM converters, switching is accomplished through the combination of a peak current limit and a pair of one-shots that set the maximum switch on-time (32s) and minimum switch off-time (1.1s). Once off, the off-time one-shot holds the switch off for 1.1s. 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 MAX1649/MAX1651 also limit the peak inductor current, which allows them to run in continuous-conduction mode and maintain high efficiency with heavy loads (Figure 3). 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 (32s later), or 2) the current limit is reached. EXT swings from V+ to GND and provides the drive output for an external P-channel power MOSFET.
C3 0.1F
D1 NSQ03A02L
C2 330F
*SILICONIX SURFACE-MOUNT MOSFET **SUMIDA CDRH125-470
Figure 1. Test Circuit
_______________Detailed Description
The MAX1649/MAX1651 are BiCMOS, step-down, switch-mode power-supply controllers that provide adjustable and fixed outputs of 5V and 3.3V, 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.
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5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649/MAX1651
V+ FB DUAL-MODETM COMPARATOR
MAX1649 MAX1651
SHDN ERROR COMPARATOR
50mV OUT
REF 1.5V REFERENCE N Q MINIMUM OFF-TIME TRIG ONE-SHOT
FROM V+ EXT
S F/F MAXIMUM TRIG ON-TIME Q ONE-SHOT R CURRENT COMPARATOR
Q
CS
110mV FROM V+ GND
TM Dual-Mode is a trademark of Maxim Integrated Products
Figure 2. Block Diagram
Shutdown Mode
When SHDN is high, the MAX1649/MAX1651 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 logic-level input. Connect SHDN to GND for normal operation.
Quiescent Current
In normal operation, the device's typical quiescent current is 78A. 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, typical no-load supply current for the entire circuit is 90A.
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7
5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649/MAX1651
VIN C4 0.1F C1 100F
V+
5 R1 0.05 6 7 P1 Si9430
MAX1649 MAX1651
1.5A 1A 3 SHDN CS EXT
L1 47H
OUTPUT @ 1.5A
0A
4
REF GND
OUT FB
1 2 R2 C2 330F D1 1N5820 R3 150k
C3 0.1F 2s/div V+ = 10V, ILOAD = 1.3A CIRCUIT OF FIGURE 1, R1 = 75m
8
VOUT -1 R2 = R3 VREF VREF = 1.5V
(
)
Figure 3. MAX1649 Continuous-Conduction Mode, Heavy Load-Current Waveform (500mA/div)
Figure 4. Adjustable-Output Operation
Modes of Operation
When delivering high output currents, the MAX1649/ MAX1651 operate in continuous-conduction mode. In this mode, current always flows in the inductor, and the control circuit adjusts the switch duty cycle to maintain regulation without exceeding the switch current capability (Figure 3). This provides excellent load-transient response and high efficiency. In discontinuous-conduction mode, 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 waveform exhibits ringing (at the inductor's self-resonant frequency). This ringing is to be expected and poses no operational problems.
__________________Design Procedure
Setting the Output Voltage
The MAX1649/MAX1651 are preset for 5V and 3.3V 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--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
(
)
Dropout
The MAX1649/MAX1651 are 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.
where VREF = 1.5V. When using external resistors, it does no harm to connect OUT and the output together, or to leave OUT unconnected.
Current-Sense Resistor Selection
The current-sense resistor limits the peak switch current to 110mV/RSENSE, where RSENSE is the value of the current-sense resistor, and 110mV is the currentlimit trip level (see Electrical Characteristics).
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5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649/MAX1651
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. 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 or 5b, 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 sense-resistor (5%) and inductor (47H 10%) values, the diode drop (0.4), and the IC's current-sense trip level (85mV); an external MOSFET on-resistance of 0.07 is assumed for VGS = -5V. 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 U-shaped 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). trollers' high switching frequency. With a high inductor value, the MAX1649/MAX1651 will begin continuous-current operation (see Detailed Description) at a lower fraction of full-load current. In general, smaller values produce higher ripple (see below) while larger values require larger size for a given current rating. In both the continuous and discontinuous modes, the lower limit of the inductor is important. With a too-small inductor value, the current rises faster and overshoots the desired peak current limit because the current-limit comparator has a finite response time (300ns). This reduces efficiency and, more importantly, could cause the current rating of the external components to be exceeded. Calculate the minimum inductor value as follows: (V+(max) - VOUT) x 0.3s L(min) = -------------------------- I x ILIM where I is the inductor-current overshoot factor, ILIM = VCS/RSENSE, and 0.3s is the time it takes the comparator to switch. Set I = 0.1 for an overshoot of 10%. For highest efficiency, use a coil with low DC resistance; a value smaller than 0.1V/ILIM 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 inductor'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).
Inductor Selection
The MAX1649/MAX1651 operate with a wide range of inductor values, although for most applications coils between 10H and 68H take best advantage of the con-
1649 Fig05a
VOUT = 5V rs = 0.030 rs = 0.040 rs = 0.050
VOUT = 3.3V rs = 0.030 rs = 0.040 rs = 0.050
MAXIMUM OUTPUT CURRENT (A)
2.5 2.0 1.5 1.0 0.5 0 5.0 5.4 5.8
MAXIMUM OUTPUT CURRENT (A)
2.5 2.0 1.5 1.0 0.5 0
rs = 0.060 rs = 0.080 rs = 0.100
rs = 0.060 rs = 0.080 rs = 0.100
6.2
6.6
16.0
3.0
3.4
3.8
4.2
4.6
16.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 5a. MAX1649 Current-Sense Resistor Graph
Figure 5b. MAX1651 Current-Sense Resistor Graph
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1651 Fig05b
3.0
3.0
5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649/MAX1651
Table 1. Component Selection Guide
PRODUCTION METHOD INDUCTORS Sumida CDRH125-470 (1.8A) CDRH125-220 (2.2A) CoilCraft DO3316-473 (1.6A) DO3340-473 (3.8A) Sumida RCH875-470M (1.3A) CAPACITORS DIODES CURRENT-SENSE RESISTORS MOSFETS
Surface Mount
AVX TPS series Sprague 595D series Sanyo OS-CON series low-ESR organic semiconductor
Motorola MBRS340T3 Nihon NSQ series
Dale WSL Series IRC LRC series
Siliconix Little Foot series Motorola medium-power surface-mount products
Miniature Through-Hole
IRC OAR series
Motorola
Low-Cost Through-Hole
CoilCraft PCH-45-473 (3.4A)
Nichicon PL series Motorola low-ESR electrolytics 1N5817 to 1N5823 United Chemi-Con LXF series
Motorola TMOS power MOSFETs
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 2.7A. Using a slightly underrated inductor can sometimes reduce size and cost, with only a minor impact on efficiency. 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.
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 onresistance must be low for the minimum available VGS, which equals V+(min). Select a transistor with an onresistance between 50% and 100% of the currentsense resistor. The Si9430 transistor chosen for the Typical Operating Circuit has a drain-to-source rating of -20V and a typical on-resistance of 0.070 at 2A with VGS = -4.5V. Tables 1 and 2 list suppliers of switching transistors suitable for use with these devices.
Diode Selection
The MAX1649/MAX1651's high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5817 through 1N5823 (and their surfacemount equivalents), are recommended. Choose a diode with an average current rating equal to or greater than I LIM (max) and a voltage rating higher than V+(max).
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 output filter capacitor's ESR 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 = 0.15 is 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. Again, low-ESR capacitors perform best. Table 1 lists some suppliers of low-ESR capacitors.
External Switching Transistor
The MAX1649/MAX1651 drive P-channel enhancementmode MOSFET transistors only. The choice of power transistor is primarily dictated by the input voltage and the peak current. The transistor's on-resistance, gatesource threshold, and gate charge must also be appropriately chosen. The drain-to-source and gate-tosource breakdown voltage ratings must be greater than V+. The total gate-charge specification is normally not
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5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers
Table 2. Component Suppliers
COMPANY AVX Coiltronics CoilCraft Dale International Rectifier IRC Motorola Nichicon Nihon Sanyo Siliconix Sprague Sumida United Chemi-Con USA USA USA USA USA USA USA USA Japan USA Japan USA Japan USA USA USA Japan USA PHONE (207) 282-5111 or (800) 282-4975 (407) 241-7876 (708) 639-6400 (402) 564-3131 (310) 322-3331 (512) 992-7900 (602) 244-3576 or (602) 244-5303 (708) 843-7500 81-7-5231-8461 (805) 867-2555 81-3-3494-7411 (619) 661-6835 81-7-2070-6306 (408) 988-8000 or (800) 554-5565 (603) 224-1961 (708) 956-0666 81-3-3607-5111 (714) 255-9500 FAX (207) 283-1941 (407) 241-9339 (708) 639-1469 (402) 563-1841 (310) 322-3332 (512) 992-3377 (602) 244-4015 (708) 843-2798 81-7-5256-4158 (805) 867-2556 81-3-3494-7414 (619) 661-1055 81-7-2070-1174 (408) 970-3950 (603) 224-1430 (708) 956-0702 81-3-3607-5144 (714) 255-9400
OUT GND
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 rectifier, 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 the V+ and GND pins.
MAX1649/MAX1651
MAX1649/MAX1651 vs. MAX649/MAX651
The MAX1649 and MAX1651 are pin compatible with the MAX649 and MAX651, but have been optimized for improved dropout performance and efficiency--particularly with low input voltages. The MAX1649/MAX1651 feature increased maximum switch duty cycle (96.5%) and reduced current-limit sense voltage (110mV). Their predecessors, the MAX649/MAX651, use a higher two-step (210mV/110mV) current-limit sense voltage to provide tighter current-sense accuracy and reduced inductor peak current at light loads.
___________________Chip Topography
Input Bypass Capacitor The input bypass capacitor reduces peak currents drawn from the voltage source, and also reduces the amount of noise at the voltage source caused by the switching action of the MAX1649/MAX1651. 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. Reference Capacitor Bypass REF with a 0.1F or larger capacitor.
EXT FB 0.106" (2.692mm) CS SHDN
REF
V+
0.081" (2.057mm)
TRANSISTOR COUNT: 428 SUBSTRATE CONNECTED TO V+
______________________________________________________________________________________ 11
5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649/MAX1651
________________________________________________________Package Information
DIM INCHES MAX MIN 0.200 - - 0.015 0.175 0.125 0.080 0.055 0.020 0.016 0.065 0.045 0.012 0.008 0.090 0.050 0.625 0.600 0.575 0.525 - 0.100 - 0.600 0.700 - 0.150 0.120 MILLIMETERS MIN MAX - 5.08 0.38 - 3.18 4.45 1.40 2.03 0.41 0.51 1.14 1.65 0.20 0.30 1.27 2.29 15.24 15.88 13.34 14.61 2.54 - 15.24 - - 17.78 3.05 3.81
E A2 A D A3 E1
A A1 A2 A3 B B1 C D1 E E1 e eA eB L
0-15 A1 L B D1 e B1 eA eB C
Plastic DIP PLASTIC DUAL-IN-LINE PACKAGE (0.600 in.)
PKG. DIM PINS P P P D D D 24 28 40
INCHES MILLIMETERS MIN MAX MIN MAX 1.230 1.270 31.24 32.26 1.430 1.470 36.32 37.34 2.025 2.075 51.44 52.71
21-0044A
DIM
D A e B
0.101mm 0.004in.
0-8
A1
C
L
A A1 B C E e H L
INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016
MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27
E
H
Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.)
DIM PINS D D D 8 14 16
INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00
21-0041A
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) 1995 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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