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19-4505; Rev 3; 8/97
KIT ATION EVALU LUDED ION INC ORMAT INF
5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters
____________________________Features
o High Efficiency for a Wide Range of Load Currents o 10A Quiescent Current o Output Currents Up to 225mA o Preset or Adjustable Output Voltage: 5.0V (MAX639) 3.3V (MAX640) 3.0V (MAX653) o Low-Battery Detection Comparator o Current-Limiting PFM Control Scheme
_______________General Description
The MAX639/MAX640/MAX653 step-down switching regulators provide high efficiency over a wide range of load currents, delivering up to 225mA. A current-limiting pulse-frequency-modulated (PFM) control scheme gives the devices the benefits of pulse-width-modulated (PWM) converters (high efficiency at heavy loads), while using only 10A of supply current (vs. 2mA to 10mA for PWM converters). The result is high efficiency over a wide range of loads. The MAX639/MAX640/MAX653 input range is 4V to 11.5V, and the devices provide lower preset output voltages of 5V, 3.3V, and 3V, respectively. Or, the output can be user-adjusted to any voltage from 1.3V to the input voltage. The MAX639/MAX640/MAX653 have an internal 1A power MOSFET switch, making them ideal for minimum-component, low- and medium-power applications. For increased output drive capability, use the MAX649/MAX651/MAX652 step-down controllers, which drive an external P-channel FET to deliver up to 5W.
MAX639/MAX640/MAX653
______________Ordering Information
PART MAX639CPA MAX639CSA MAX639C/D MAX639EPA MAX639ESA MAX639MJA 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
9V Battery to 5V, 3.3V, or 3V Conversion High-Efficiency Linear Regulator Replacement Portable Instruments and Handy-Terminals 5V-to-3.3V Converters
Ordering Information continued on last page. * Contact factory for dice specifications.
__________Typical Operating Circuit
INPUT 5.5V TO 11.5V
V+
__________________Pin Configuration
TOP VIEW
LX
OUTPUT 5V 225mA
VOUT 1 2 3 4 8 SHDN VFB V+
MAX639
ON/OFF LOW-BATTERY DETECTOR INPUT SHDN LBI VOUT LBO LOW-BATTERY DETECTOR OUTPUT
LBO LBI GND
MAX639 MAX640 MAX653
7 6
5 LX
DIP/SO
VFB GND
________________________________________________________________ Maxim Integrated Products
1
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5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters MAX639/MAX640/MAX653
ABSOLUTE MAXIMUM RATINGS
V+...........................................................................................12V LX .........................................................(V+ - 12V) to (V+ + 0.3V) LBI, LBO, VFB, SHDN, VOUT........................-0.3V to (V+ + 0.3V) LX Output Current (Note 1) ......................................................1A LBO Output Current ............................................................10mA 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: MAX639C_ _ .......................................................0C to +70C MAX639E_ _ ....................................................-40C to +85C MAX639MJA ..................................................-55C to +125C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
Note 1: Peak inductor current must be limited to 600mA by using an inductor of 100H or greater.
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+ = 9V for the MAX639, V+ = 5V for the MAX640/MAX653, ILOAD = 0mA, TA = TMIN to TMAX, typical values are at TA = +25C, unless otherwise noted.) PARAMETER Supply Voltage Supply Current Output Voltage (Note 2) Dropout Voltage SHDN = V+, no load MAX639, V+ = 6.0V to 11.5V, 0mA < IOUT < 100mA MAX640, V+ = 4.0V to 11.5V, 0mA < IOUT < 100mA MAX653, V+ = 4.0V to 11.5V, 0mA < IOUT < 100mA IOUT = 100mA, L = 100H MAX639 Efficiency MAX640 MAX653 MAX639 Switch On-Time MAX640 MAX653 MAX639 Switch Off-Time MAX640 MAX653 IOUT = 100mA, L = 100H IOUT = 25mA, L = 470H IOUT = 100mA, L = 100H IOUT = 25mA, L = 470H IOUT = 100mA, L = 100H IOUT = 25mA, L = 470H V+ = 9V, VOUT = 5V V+ = 6V, VOUT = 3V V+ = 9V, VOUT = 3.3V V+ = 4V, VOUT = 3.3V V+ = 9V, VOUT = 3V V+ = 4V, VOUT = 3V V+ = 9V, VOUT = 5V V+ = 6V, VOUT = 3V V+ = 9V, VOUT = 3.3V V+ = 4V, VOUT = 3.3V V+ = 9V, VOUT = 3V V+ = 4V, VOUT = 3V 10.6 14.2 7.5 60.7 7.1 42.5 9.0 16.6 13.3 13.3 14.6 14.6 4.80 3.17 2.88 CONDITIONS MIN 4.0 TYP 10 5.00 3.30 3.00 0.5 91 94 87 91 85 89 12.5 16.7 8.8 71.4 8.3 50.0 11.7 19.5 15.6 15.6 17.2 17.2 14.4 19.2 10.1 82.1 9.5 57.5 13.5 22.4 17.9 17.9 19.8 19.8 s s % MAX 11.5 20 5.20 3.43 3.12 V V UNITS V A
2
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5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 9V for the MAX639, V+ = 5V for the MAX640/MAX653, ILOAD = 0mA, TA = TMIN to TMAX, typical values are at TA = +25C, unless otherwise noted.) PARAMETER LX Switch On-Resistance CONDITIONS V+ = 9V, TA = +25C, MAX639/MAX640/MAX653 V+ = 6V, TA = TMIN to TMAX, MAX639 V+ = 4V, TA = TMIN to TMAX, MAX640/MAX653 LX Switch Leakage VFB Bias Current VFB Dual-Mode Trip Point VFB Threshold LBI Bias Current LBI Threshold LBO Sink Current LBO Leakage Current LBO Delay SHDN Threshold SHDN Pull-Up Current SHDN = 0V MAX6_ _C MAX6_ _E/M VLBI = 2V MAX6_ _C MAX6_ _E/M VLBO = 0.4V VLBO = 11.5V 50mV overdrive 0.80 0.10 MAX639 MAX640/MAX653 1.26 1.24 0.8 0.4 1.26 1.24 V+ = 11.5V, VLX = 0V VFB = 2V TA = +25C TA = TMIN to TMAX 4.0 50 1.28 1.28 2 1.28 1.28 2.5 1.2 0.001 25 1.15 0.20 2.00 0.40 0.1 1.30 1.32 10 1.30 1.32 0.003 MIN TYP 0.8 MAX 1.5 2.5 2.8 1.0 30.0 15.0 A nA mV V nA V mA A s V A UNITS
MAX639/MAX640/MAX653
Note 2: Output guaranteed by correlation to measurements of device parameters (i.e., switch on-resistance, on-times, off-times, and output voltage trip points).
__________________________________________Typical Operating Characteristics
(Circuit of Figure 3, internal feedback, L = 100H, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
MAX639-1
EFFICIENCY vs. OUTPUT CURRENT
L = 470H
MAX639-2
EFFICIENCY vs. OUTPUT CURRENT
L = 470H V+ = 9V
MAX639-3
100
L = 100H V+ = 9V
100
100
90 EFFICIENCY (%) EFFICIENCY (%)
90 EFFICIENCY (%)
90
80 MAX639 MAX640 MAX653
80
MAX639, V+ = 6V MAX640, V+ = 4.3V MAX653, V+ = 4V
80 MAX639 MAX640 MAX653
70
70
70
60
60
60
50 10 100 1m 10m 100m 1 OUTPUT CURRENT (A)
50 10 100 1m 10m 100m OUTPUT CURRENT (A)
50 10 100 1m 10m 100m OUTPUT CURRENT (A)
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5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters MAX639/MAX640/MAX653
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 3, internal feedback, L = 100H, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. INPUT VOLTAGE
MAX639-05 MAX639-4
EFFICIENCY vs. OUTPUT CURRENT
100 100 L = 100H
EFFICIENCY vs. INPUT VOLTAGE
L = 470H IOUT = 25mA 95 EFFICIENCY (%) MAX639
MAX639-06
IOUT = 100mA 95 MAX639
100
90 EFFICIENCY (%) EFFICIENCY (%)
80
70
MAX639, V+ = 6V MAX640, V+ = 4.3V MAX653, V+ = 4V
90 MAX640
90 MAX640 85 MAX653
85 60 MAX653 50 10 100 1m 10m 100m 1 OUTPUT CURRENT (A) 80 3 4 5 6 7 8 9 10 11 12 V+ (V)
80 3 4 5 6 7 8 9 10 11 12 V+ (V)
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
MAX639-07
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
MAX639-08
MAX639 OUTPUT VOLTAGE RIPPLE vs. INPUT VOLTAGE
IOUT = 25mA OUTPUT VOLTAGE RIPPLE (mV) 125 100 75 50 25 L = 470H 0 L = 100H
MAX639-09
250 L = 100H MAXIMUM OUTPUT CURRENT (mA)
75 L = 470H MAXIMUM OUTPUT CURRENT (mA) 65 MAX639 55 MAX640
150
200
MAX640 MAX653
45 MAX653
150 MAX639 100 3 4 5 6 7 8 9 10 11 12 V+ (V)
L = 220H
35 25 3 4 5
6
7
8
9
10
11
12
5
6
7
8
9
10
11
12
V+ (V)
INPUT VOLTAGE (V)
MAX640 OUTPUT VOLTAGE RIPPLE vs. INPUT VOLTAGE
MAX639-10
MAX653 OUTPUT VOLTAGE RIPPLE vs. INPUT VOLTAGE
ILOAD = 25mA OUTPUT VOLTAGE RIPPLE (mV) 125 100 75 50 25 L = 470H 0 L = 220H L = 100H
MAX639-11
150 IOUT = 25mA OUTPUT VOLTAGE RIPPLE (mV) 125 L = 100H 100 75 50 25 0 3 4 5 6 7 8 9 10 11 L = 220H L = 470H
150
12
3
4
5
6
7
8
9
10
11
12
V+ (V)
V+ (V)
4
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5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 3, internal feedback, L = 100H, TA = +25C, unless otherwise noted.)
MAX639 START-UP TIME vs. OUTPUT CURRENT
MAX639-12
MAX639/MAX640/MAX653
MAX640/MAX653 START-UP TIME vs. OUTPUT CURRENT
MEASURED FROM THE RISING EDGE OF V+ OR SHDN TO (VOUT = 3.3V)(MAX640) OR (VOUT = 3.0V)(MAX653). THE START-UP TIME DIFFERENCE BETWEEN THE MAX640 AND THE MAX653 IS NEGLIGIBLE. V+ = 5.0V 4 V+ = 9.0V
MAX639-13
MAX639 START-UP TIME vs. OUTPUT CURRENT
MEASURED FROM THE RISING EDGE OF V+ OR SHDN TO (VOUT = 5.0V) L = 470H. START-UP TIME (ms) 30 V+ = 5.5V 20 V+ = 9.0V
MAX639-14
12 10 START-UP TIME (ms) 8 6 4 2 0 0 V+ = 11.5V V+ = 9.0V MEASURED FROM THE RISING EDGE OF V+ OR SHDN TO (VOUT = 5V). V+ = 5.5V
10
40
8 START-UP TIME (ms)
6
10 V+ = 11.5V
2 0 L = 100H 0
V+ = 11.5V 0 0 5 10 15 20 25 30 OUTPUT CURRENT (mA) OUTPUT CURRENT (mA)
10 20 30 40 50 60 70 80 90 100 OUTPUT CURRENT (mA)
10 20 30 40 50 60 70 80 90 100
MAX640/MAX653 START-UP TIME vs. OUTPUT CURRENT
MAX639-15
NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE
MAX639-16
40
70 NO-LOAD SUPPLY CURRENT (A) 60 MAX639, VOUT = 5V 50 40 30 20 10 0
START-UP TIME (ms)
30
MEASURED FROM THE RISING EDGE OF V+ OR SHDN TO (VOUT = 3.3V)(MAX640) OR (VOUT = 3.0V)(MAX653). THE START-UP TIME DIFFERENCE BETWEEN THE MAX640 AND THE MAX653 IS NEGLIGIBLE. L = 470H V+ = 5.0V
20
MAX653, VOUT = 3V
10 V+ = 11.5V 0 0 5 10 15 20
V+ = 9.0V
25
30
01
2
34
5
6
78
9 10 11 12
OUTPUT CURRENT (mA)
INPUT VOLTAGE (V)
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5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters MAX639/MAX640/MAX653
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 3, internal feedback, L = 100H, TA = +25C, unless otherwise noted.) MAX653 LOAD-TRANSIENT RESPONSE MAX639 LOAD-TRANSIENT RESPONSE
A
A
B B
1ms/div A: ILOAD, 0mA TO 100mA, 50mA/div B: VOUT, 100mV/div, AC COUPLED VIN = 5V, VOUT = 3V
1ms/div A: ILOAD, 0mA TO 200mA, 100mA/div B: VOUT, 100mV/div, AC COUPLED VIN = 9V, VOUT = 5V
MAX653 LINE-TRANSIENT RESPONSE
MAX639 LINE-TRANSIENT RESPONSE
A
A
B
B
10ms/div A: VIN, 4V TO 8V, 2V/div B: VOUT, 100mV/div VOUT = 3V, ILOAD = 100mA
10ms/div A: VIN, 6V TO 11.5V, 2V/div B: VOUT, 100mV/div VOUT = 5V, ILOAD = 100mA
6
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5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters
______________________________________________________________Pin Description
PIN NAME VOUT LBO LBI GND LX V+ VFB FUNCTION Sense Input for regulated-output operation. Internally connected to an on-chip voltage divider and to the variable duty-cycle, on-demand oscillator. It must be connected to the external regulated output. Low-Battery Output. An open-drain N-channel MOSFET sinks current when the voltage at LBI drops below 1.28V. Low-Battery Input. When the voltage at LBI drops below 1.28V, LBO sinks current. Ground Drain of a PMOS power switch that has its source connected to V+. LX drives the external inductor, which provides current to the load. Positive Supply-Voltage Input. Should not exceed 11.5V Dual-Mode Feedback Pin. When VFB is grounded, the internal voltage divider sets the output to 5V (MAX639), 3.3V (MAX640) or 3V (MAX653). For adjustable operation, connect VFB to an external voltage divider. Shutdown Input -- active low. When pulled below 0.8V, the LX power switch stays off, shutting down the regulator. When the shutdown input is above 2V, the regulator stays on. Tie SHDN to V+ if shutdown mode is not used.
MAX639/MAX640/MAX653
1
2 3 4 5 6 7
8
SHDN
____________________Getting Started
Designing power supplies with the MAX639/MAX640/ MAX653 is easy. The few required external components are readily available. The most general applications use the following components: (1) Capacitors: For the input and output filter capacitors, try using electrolytics in the 100F range, or use low-ESR capacitors to minimize output ripple. Capacitor values are not critical. (2) Diode: Use the popular 1N5817 or equivalent Schottky diode. (3) Inductor: For the highest output current, choose a 100H inductor with an incremental saturation current rating of at least 600mA. To obtain the highest efficiencies and smallest size, refer to the Inductor Selection section.
L VL V+ COUT VOUT IL
Figure 1. Simplified Step-Down Converter
IL AT 200mA/div
_______________Detailed Description
Figure 1 shows a simplified, step-down DC-DC converter. When the switch is closed, a voltage equal to (V+ - V OUT) is applied to the inductor. The current through the inductor ramps up, storing energy in the inductor's magnetic field. This same current also flows into the output filter capacitor and load. When the switch opens, the current continues to flow through the inductor in the same direction, but must also flow through the diode. The inductor alone supplies current to the load when the switch is open. This current decays to zero as the energy stored in the inductor's magnetic field is transferred to the output filter capacitor and the load.
0A
0V
VL AT 5V/div
SWITCH ON SWITCH OFF
SWITCH ON SWITCH OFF
Figure 2. Simplified Step-Down Converter Operation
7
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MAX639 FG02
5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters MAX639/MAX640/MAX653
Figure 2 shows what happens to the ideal circuit of Figure 1 if the switch turns on with a 66% duty cycle and V+ = 3/2 VOUT. The inductor current rises more slowly than it falls because the magnitude of the voltage applied during tON is less than that applied during tOFF. Varying the duty cycle and switching frequency keeps the peak current constant as input voltage varies. The MAX639/MAX640/MAX653 control the switch (tON and tOFF) according to the following equations: Equation (1) tON = 50sV / (V+ - VOUT) Equation (2) tOFF 50sV / VOUT Equation (3) IPEAK = 50sV / L These three equations ensure constant peak currents for a given inductor value, across all input voltages (ignoring the voltage drop across the diode (D1) and the resistive losses in the switch and inductor). The variable duty cycle also ensures that the current through the inductor discharges to zero at the end of each pulse. Figure 3 shows the MAX639/MAX640/MAX653 block diagram and a typical connection in which 9V is converted to 5V (MAX639), 3.3V (MAX640), or 3.0V (MAX653). The sequence of events in this application is as follows:
INPUT, +5.5V TO +11.5V (MAX639), +3.8V TO +11.5V (MAX640), +3.5V TO +11.5V (MAX653) 8 CIN 33F +1.28V BANDGAP REFERENCE SHDN 6 V+ 5V, 3.3V OR 3.0V AT 100mA L = 100H VARIABLE FREQUENCY AND DUTY-CYCLE OSCILLATOR VOUT 1 R1 1N5817
When the output dips: (1) The error comparator switches high. (2) The internal oscillator starts (15s start-up time) and connects to the gate of the LX output driver. (3) LX turns on and off according to t ON and tOFF, charging and discharging the inductor, and supplying current to the output (as described above). When the output voltage recovers: (1) The comparator switches low. (2) LX turns off. (3) The oscillator shuts down to save power.
Fixed or Adjustable Output
For operation at the preset output voltage, connect VFB to GND; no external resistors are required. For other output voltages, use an external voltage divider. Set the output voltage using R3 and R4 as determined by the following formula: R3 = R4 [(VOUT / VFB Threshold) - 1] where R4 is any resistance in the 10k to 1M range (typically 100k), and the VFB threshold is typically 1.28V.
LX
5
ERROR COMPARATOR
LOW-BATTERY COMPARATOR MODE-SELECT COMPARATOR 3 LBI 2 LBO 50mV
COUT 100F
R2 VFB 7 GND 4
MAX639 MAX640 MAX653
Figure 3. Block Diagram
8 _______________________________________________________________________________________
5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters
Low-Battery Detector
The low-battery detector compares the voltage on the LBI input with the internal 1.28V reference. LBO goes low whenever the input voltage at LBI is less than 1.28V. Set the low-battery detection voltage with resistors R1 and R2 (Figure 3) as determined by the following formula: R1 = R2 [(VLB / LBI Threshold) - 1] where R2 is any resistance in the 10k to 1M range (typically 100k), the LBI threshold is typically 1.28V, and VLB is the desired low-battery detection voltage. The low-battery comparator remains active in shutdown mode.
MAX639/MAX640/MAX653
Table 1. Component Suppliers
INDUCTORS -- THROUGH HOLE
PART NUMBER SIZE (inches) VALUE (H) IMAX (A) SERIES R ()
MAXL001* 7300-13** 7300-15** 7300-17** 7300-19** 7300-21** 7300-25**
0.65 x 0.33 dia. 0.63 x 0.26 dia. 0.63 x 0.26 dia. 0.63 x 0.26 dia. 0.63 x 0.26 dia. 0.63 x 0.26 dia. 0.63 x 0.26 dia.
100 100 150 220 330 470 1000
1.75 0.89 0.72 0.58 0.47 0.39 0.27
0.2 0.27 0.36 0.45 0.58 0.86 2.00
Shutdown Mode
Bringing SHDN below 0.8V places the MAX639/ MAX640/MAX643 in shutdown mode. LX becomes high impedance, and the voltage at VOUT falls to zero. The time required for the output to rise to its nominal regulated voltage when brought out of shutdown (start-up time) depends on the inductor value, input voltage, and load current (see the Start-Up Time vs. Output Current graph in the Typical Operating Characteristics). The low-battery comparator remains active in shutdown mode.
* Maxim Integrated Products **Caddell-Burns 258 East Second Street Mineola, NY 11501-3508 (516) 746-2310
INDUCTORS -- SURFACE MOUNT
PART NUMBER SIZE (mm) VALUE (H) IMAX (A) SERIES R ()
CD54 CD54 CDR74 CDR74 CDR105 CDR105
5.2 x 5.8 x 4.5 5.2 x 5.8 x 4.5 7.1 x 7.7 x 4.5 7.1 x 7.7 x 4.5 9.2 x 10.0 x 5.0 9.2 x 10.0 x 5.0
100 220 100 220 100 220
0.52 0.35 0.52 0.35 0.80 0.54
0.63 1.50 0.51 0.98 0.35 0.69
__________Applications Information
Inductor Selection
When selecting an inductor, consider these four factors: peak-current rating, inductance value, series resistance, and size. It is important not to exceed the inductor's peak-current rating. A saturated inductor will pull excessive currents through the MAX639/MAX640/MAX653's switch, and may cause damage. Avoid using RF chokes or air-core inductors since they have very low peak-current ratings. Electromagnetic interference must not upset nearby circuitry or the regulator IC. Ferrite-bobbin types work well for most digital circuits; toroids or pot cores work well for EMI-sensitive analog circuits. Recall that the inductance value determines IPEAK for all input voltages (Equation 3). If there are no resistive losses and the diode is ideal, the maximum average current that can be drawn from the MAX639/MAX640/MAX653 will be one-half IPEAK. With the real losses in the switch, inductor, and diode taken into account, the real maximum output current typically varies from 90% to 50% of the ideal. The following steps describe a conservative way to pick an appropriate inductor. Step 1: Decide on the maximum required output current, in amperes: IOUTMAX. Step 2: IPEAK = 4 x IOUTMAX.
Sumida Electric (USA) 637 East Golf Road Arlington Heights, IL 60005 (708) 956-0666
CAPACITORS -- LOW ESR
PART NUMBER SIZE (inches) VALUE (F) ESR () VMAX (V)
MAXC001* 267 Series** 267 Series**
0.49 x 0.394 dia. D SM packages E SM packages
150 47 100
0.2 0.2 0.2
35 10 6.3
* Maxim Integrated Products **Matsuo Electronics 2134 Main Street Huntington Beach, CA 92648 (714) 969-2491
SCHOTTKY DIODES -- SURFACE MOUNT
PART NUMBER SIZE VF (V) IMAX (A)
SE014 SE024
SOT89 SOT89
0.55 0.55
1 0.95
Collmer Semiconductor 14368 Proton Road Dallas, TX 75244 (214) 233-1589 NOTE: This list does not constitute an endorsement by Maxim Integrated Products and is not intended to be a comprehensive list of all manufacturers of these components.
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5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters MAX639/MAX640/MAX653
INPUT +4.0V TO +11.5V 6 8 CIN 100F V+ SHDN LX 5 L = 100H 1N5817 OUTPUT COUT 100F R3
MAX639 VOUT 1 MAX640 MAX653
VFB 7 GND 4 LBI 3
MAX639 MAX640 MAX653
R4
Figure 4. Adjustable-Output Operation
Figure 5. Through-Hole PC Layout and Component Placement Diagram for Standard Step-Down Application (Top-Side View)
Step 3: L = 50 / IPEAK. L will be in H. Do not use an inductor of less than 100H. Step 4: Make sure that IPEAK does not exceed 0.6A or the inductor's maximum current rating, whichever is lower. Inductor series resistance affects both efficiency and dropout voltage. A high series resistance severely limits the maximum current available at lower input voltages. Output currents up to 225mA are possible if the inductor has low series resistance. Inductor and series switch resistance form an LR circuit during tON. If the L/R time constant is less than the oscillator tON, the inductor's peak current will fall short of the desired IPEAK. To maximize efficiency, choose the highest-value inductor that will provide the required output current over the whole range of your input voltage (see Typical Operating Characteristics). Inductors with peak currents in the 600mA range do not need to be very large. They are about the size of a 1W resistor, with surfacemount versions less than 5mm in diameter. Table 1 lists suppliers of inductors suitable for use with the MAX639/MAX640/MAX653.
It decreases with larger inductance, but increases as the input voltage lessens. As a general rule, a smaller amount of charge delivered in each pulse results in less output ripple. With low-cost aluminum electrolytic capacitors, the ESR-induced ripple can be larger than that caused by the charge variation. Consequently, high-quality aluminum-electrolytic or tantalum filter capacitors will minimize output ripple. Best results at reasonable cost are typically achieved with an aluminum-electrolytic capacitor in the 100F range, in parallel with a 0.1F ceramic capacitor (Table 1).
External Diode
In most MAX639/MAX640/MAX653 circuits, the current in the external diode (D1, Figure 3) changes abruptly from zero to its peak value each time LX switches off. To avoid excessive losses, the diode must have a fast turn-on time. For low-power circuits with peak currents less than 100mA, signal diodes such as the 1N4148 perform well. The 1N5817 diode works well for highpower circuits, or for maximum efficiency at low power. 1N5817 equivalent diodes are also available in surfacemount packages (Table 1). Although the 1N4001 and other general-purpose rectifiers are rated for high currents, they are unacceptable because their slow turnoff times result in excessive losses.
Output Filter Capacitor
The MAX639/MAX640/MAX653's output ripple has two components. One component results from the variation in stored charge on the filter capacitor with each LX pulse. The other is the product of the current into the capacitor and the capacitor's equivalent series resistance (ESR). The amount of charge delivered in each oscillator pulse is determined by the inductor value and input voltage.
10
Minimum Load
Under no-load conditions, because of leakage from the PMOS power switch (see the LX Leakage Current vs. Temperature graph in the Typical Operating Characteristics) and from the internal resistor from V+ to VOUT, leakage current may be supplied to the output
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5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters MAX639/MAX640/MAX653
VIN CIN 100F L = 100H
MAXIMUM OUTPUT CURRENT (mA)
6 V+
8 SHDN LX 5
140 120 100 80 60 40 20 0 0 1 2
TA = +25C L = 100H MAX639
MAX639 MAX640 1 MAX653 VOUT
1N5817
COUT 100F GND 4 VFB 7 -5V -3.3V OR -3V
3 V+ (V)
4
5
Figure 6. Inverting Configuration
Figure 7. Maximum Current Capability of Figure 6 Circuit
EFFICIENCY (%)
capacitor, even when the switch is off. This will usually not be a problem for a 5V output at room temperature, since the diode's reverse leakage current and the feedback resistors' current typically drain the excess. However, if the diode leakage is very low (which can occur at low temperatures and/or small output voltages), charge may build up on the output capacitor, making VOUT rise above its set point. If this happens, add a small load resistor (typically 1M) to the output to pull a few extra microamps of current from the output capacitor.
86.5 86.0 85.5 85.0 84.5 84.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 V+ (V) TA = +25C VOUT = -5V L = 470H IOUT = 10mA
Layout
Several of the external components in a MAX639/ MAX640/MAX653 circuit experience peak currents up to 600mA. Wherever one of these components connects to ground, there is a potential for ground bounce. Ground bounce occurs when high currents flow through the parasitic resistances of PC board traces. What one component interprets as ground can differ from the IC's ground by several millivolts. This may increase the MAX639/MAX640/MAX653's output ripple, since the error comparator (which is referenced to ground) will generate extra switching pulses when they are not needed. It is essential that the input filter capacitor's ground lead, the MAX639/MAX640/MAX653's GND pin, the diode's anode, and the output filter capacitor's ground lead are as close together as possible, preferably at the same point. Figure 5 shows a suggested through-hole printed circuit layout that minimizes ground bounce.
Figure 8. Efficiency of Figure 6 Circuit
-3.3V (MAX640), or -3V (MAX653). Avoid exceeding the maximum differential voltage of 11.5V from V+ to VOUT. Other negative voltages can be generated by placing a voltage divider across COUT and connecting the tap point to VFB in the same manner as the normal stepdown configuration.
Two AA Batteries to 5V, 3.3V, or 3V
For battery-powered applications, where the signal ground does not have to correspond to the power-supply ground, the circuit in Figure 6 generates 5V (MAX639), 3.3V (MAX640), or 3V (MAX653) from a pair of AA batteries. Connect the VIN ground point to your system's input, and connect the output to your system's ground input. This configuration has the added advantage of reduced on resistance, since the IC's internal power FET has VIN + VOUT of gate drive (Figures 7 and 8).
11
Inverter Configuration
Figure 6 shows the MAX639/MAX640/MAX653 in a floating ground configuration. By tying what would normally be the output to the supply-voltage ground, the IC's GND pin is forced to a regulated -5V (MAX639),
______________________________________________________________________________________
MAX639 FG02
87.0
MAX639 FG02
160
5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters MAX639/MAX640/MAX653
_Ordering Information (continued)
PART MAX640CPA MAX640CSA MAX640C/D MAX640EPA MAX640ESA MAX640MJA MAX653CPA MAX653CSA MAX653C/D MAX653EPA MAX653ESA MAX653MJA 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
LBO VFB
___________________Chip Topography
VOUT SHDN
LBI
0.083" (2.108mm) V+
GND
LX 0.072" (1.828mm)
* Contact factory for dice specifications.
TRANSISTOR COUNT: 221 SUBSTRATE CONNECTED TO V+
________________________________________________________Package Information
SOICN.EPS
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) 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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