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19-0113; Rev. 2; 1/95
UAL IT MAN TION K T VALUA A SHEE E T WS DA FOLLO
3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters
____________________________Features
o Operates Down to 0.7V Input Supply Voltage o 87% Efficiency at 200mA o 60A Quiescent Current o 20A Shutdown Mode with Active Reference and LBI Detector o 500kHz Maximum Switching Frequency o 1.5% Reference Tolerance Over Temperature o Low-Battery Detector (LBI/LBO) o 8-Pin DIP and SO Packages
_______________General Description
The MAX756/MAX757 are CMOS step-up DC-DC switching regulators for small, low input voltage or battery-powered systems. The MAX756 accepts a positive input voltage down to 0.7V and converts it to a higher pinselectable output voltage of 3.3V or 5V. The MAX757 is an adjustable version that accepts an input voltage down to 0.7V and generates a higher adjustable output voltage in the range from 2.7V to 5.5V. Typical full-load efficiencies for the MAX756/MAX757 are greater than 87%. The MAX756/MAX757 provide three improvements over previous devices. Physical size is reduced--the high switching frequencies (up to 0.5MHz) made possible by MOSFET power transistors allow for tiny (<5mm diameter) surface-mount magnetics. Efficiency is improved to 87% (10% better than with low-voltage regulators fabricated in bipolar technology). Supply current is reduced to 60A by CMOS construction and a unique constant-off-time pulse-frequency modulation control scheme.
MAX756/MAX757
______________Ordering Information
PART MAX756CPA MAX756CSA MAX756C/D MAX756EPA MAX756ESA MAX757CPA MAX757CSA MAX757C/D MAX757EPA MAX757ESA 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
3.3V to 5V Step-Up Conversion Palmtop Computers Portable Data-Collection Equipment Personal Data Communicators/Computers Medical Instrumentation 2-Cell & 3-Cell Battery-Operated Equipment Glucose Meters
* Dice are tested at TA = +25C only.
__________Typical Operating Circuit
INPUT 2V to VOUT
_________________Pin Configurations
TOP VIEW
SHDN 1 2 8 7 LX GND OUT LBI
150F 5 1 LBI SHDN LX 8
22H
OUTPUT 5V at 200mA or 1N5817 3.3V at 300mA 100F
3/5
REF 3 LBO 4
MAX756
6 5
DIP/SO
SHDN FB 1 2 8 7
2
MAX756
3/5 OUT
6
LX GND OUT LBI
3 0.1F
REF GND 7
LBO
4
LOW-BATTERY DETECTOR OUTPUT
REF 3 LBO 4
MAX757
6 5
DIP/SO
________________________________________________________________ Maxim Integrated Products
1
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3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters MAX756/MAX757
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (OUT to GND) ....................................-0.3V, +7V Switch Voltage (LX to GND) ........................................-0.3V, +7V Auxiliary Pin Voltages (SHDN, LBI, LBO, REF, 3/5, FB to GND) ........................................-0.3V, (VOUT + 0.3V) Reference Current (IREF) ....................................................2.5mA 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: MAX75_C_ _ ........................................................0C to +70C MAX75_E_ _......................................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range............................... -65to +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
(Circuits of Figure 1 and Typical Operating Circuit, VIN = 2.5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER Output Voltage CONDITIONS MAX756, 3/5 = 0V, 0mA < ILOAD < 200mA 2V < VIN < 3V MAX756, 3/5 = 3V, 0mA < ILOAD < 300mA MAX757, VOUT = 5V, 0mA < ILOAD < 200mA Minimum Start-Up Supply Voltage ILOAD = 10mA Minimum Operating Supply Voltage (once started) Quiescent Supply Current in 3.3V Mode (Note 1) Battery Quiescent Current Measured at VIN in Figure 1 Shutdown Quiescent Current (Note 1) Reference Voltage Reference-Voltage Regulation LBI Input Threshold LBI Input Hysteresis LBO Output Voltage Low LBO Output Leakage Current SHDN, 3/5 Input Voltage Low SHDN, 3/5 Input Voltage High SHDN, 3/5, FB, LBI Input Current FB Voltage Output Voltage Range LBI = 1.25V, FB = 1.25V, SHDN = 0V or 3V, 3/5 = 0V or 3V MAX757 MAX757, ILOAD = 0mA (Note 2) 1.22 2.7 1.25 1.6 100 1.28 5.5 ISINK = 2mA LBO = 5V ILOAD = 20mA ILOAD = 0mA, 3/5 = 3V, LBI = 1.25V, VOUT = 3.47V, FB = 1.3V (MAX757 only) Output set for 3.3V SHDN = 0V, LBI = 1.25V, 3/5 = 3V, VOUT = 3.47V, FB = 1.3V (MAX757 only) No REF load, CREF = 0.1F 3/5 = 3V, -20A < REF load < 250A, CREF = 0.22F With falling edge 1.23 1.22 60 MIN 4.8 3.17 4.8 TYP 5.0 3.30 5.0 1.1 0.7 60 MAX 5.2 3.43 5.2 1.8 V V A UNITS V
A
20 1.25 0.8 1.25 25
40 1.27 2.0 1.28 0.4 1 0.4
A V % V mV V A V V nA V V
Note 1: Supply current from the 3.3V output is measured with an ammeter between the 3.3V output and OUT pin. This current correlates directly with actual battery supply current, but is reduced in value according to the step-up ratio and efficiency. Note 2: Minimum value is production tested. Maximum value is guaranteed by design and is not production tested.
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3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters
__________________________________________Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT 3.3V OUTPUT MODE
MAX756-1
MAX756/MAX757
EFFICIENCY vs. LOAD CURRENT 5V OUTPUT MODE
VIN = 3.3V
MAX756-2
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
700 600 500 400 300 200 100 3.3V MODE 5V MODE
MAX756-3
90 VIN = 2.0V 80 VIN = 1.2V EFFICIENCY (%)
90
800 MAXIMUM OUTPUT CURRENT (mA)
80 VIN = 2.5V EFFICIENCY (%) 70 VIN = 1.25V
70
60
60
50
50
40 0.1 1 10 100 1000 LOAD CURRENT (mA)
40 0.1 1 10 100 1000 LOAD CURRENT (mA)
0 0 1 2 3 4 5 INPUT VOLTAGE (V)
SWITCHING FREQUENCY vs. LOAD CURRENT
MAX756-4
QUIESCENT CURRENT vs. INPUT VOLTAGE
MAX756-5
SHUTDOWN QUIESCENT CURRENT vs. INPUT VOLTAGE
SHUTDOWN QUIESCENT CURRENT (A) CURRENT MEASURED AT VIN 40
MAX756-6
1M
500 CURRENT MEASURED AT VIN QUIESCENT CURRENT (A) 400
50
SWITCHING FREQUENCY (Hz)
100k
5V MODE
10k 3.3V MODE
300
VOUT = 5V
30
1k
200
20
100 VIN = 2.5V 10 10 100 1m 10m 100m 1 LOAD CURRENT (A)
100 VOUT = 3.3V 0 1 2 3 INPUT VOLTAGE (V) 4 5
10 0 1 2 3 INPUT VOLTAGE (V) 4 5
MINIMUM START-UP INPUT VOLTAGE vs. LOAD CURRENT
MAX756-7
REFERENCE VOLTAGE LOAD REGULATION
MAX756-8
1.8
10
START-UP INPUT VOLTAGE (V)
1.6
VREF LOAD REGULATION (mV)
8
1.4
6
1.2
4 VOUT = 3.3V
1.0 3.3V MODE 0.8 1 10 100 1000 LOAD CURRENT (mA)
2
0 0 50 100 150 200 250 LOAD CURRENT (A)
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3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters MAX756/MAX757
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.) LOAD-TRANSIENT RESPONSE START-UP DELAY
3V OUTPUT VOLTAGE 50mV/div VSHDN 2V/div 0V 5V OUTPUT CURRENT 0mA to 200mA VOUT 2V/div 0V
VIN = 2.5V HORIZONTAL = 50s/div 5V Mode
VIN = 2.5V HORIZONTAL = 5ms/div 5V Mode
______________________________________________________________Pin Description
PIN MAX756 MAX757 NAME FUNCTION Shutdown Input disables SMPS when low, but the voltage reference and low-battery comparator remain active. Selects the main output voltage setting; 5V when low, 3.3V when high. Feedback Input for adjustable output operation. Connect to an external voltage divider between OUT and GND. 1.25V Reference Voltage Output. Bypass with 0.22F to GND (0.1F if there is no external reference load). Maximum load capability is 250A source, 20A sink. Low-Battery Output. An open-drain N-channel MOSFET sinks current when the voltage at LBI drops below +1.25V. Low-Battery Input. When the voltage on LBI drops below +1.25V, LBO sinks current. Connect to VIN if not used. Connect OUT to the regulator output. It provides bootstrapped power to both devices, and also senses the output voltage for the MAX756. Power Ground. Must be low impedance; solder directly to ground plane. 1A, 0.5 N-Channel Power MOSFET Drain
1
2 -
1
- 2
SHDN 3/5 FB
3
3
REF
4
4
LBO
5
5
LBI
6 7 8 4
6 7 8
OUT GND LX
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3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters
_______________Detailed Description
Operating Principle
The MAX756/MAX757 combine a switch-mode regulator with an N-channel MOSFET, precision voltage reference, and power-fail detector in a single monolithic device. The MOSFET is a "sense-FET" type for best efficiency, and has a very low gate threshold voltage to ensure start-up under low-battery voltage conditions (1.1V typ).
MAX756/MAX757
voltage, the diode current should be limited by a series resistor (1M suggested). The logic input threshold level is the same (approximately 1V) in both 3.3V and 5V modes. Do not leave the control inputs floating.
__________________Design Procedure
Output Voltage Selection
The MAX756 output voltage can be selected to 3.3V or 5V under logic control, or it can be left in one mode or the other by tying 3/5 to GND or OUT. Efficiency varies depending upon the battery and the load, and is typically better than 80% over a 2mA to 200mA load range. The device is internally bootstrapped, with power derived from the output voltage (via OUT). When the output is set at 5V instead of 3.3V, the higher internal supply voltage results in lower switch-transistor on resistance and slightly greater output power. Bootstrapping allows the battery voltage to sag to less than 1V once the system is started. Therefore, the battery voltage range is from VOUT + VD to less than 1V (where VD is the forward drop of the Schottky rectifier). If the battery voltage exceeds the programmed output voltage, the output will follow the battery voltage. In many systems this is acceptable; however, the output voltage must not be forced above 7V. The output voltage of the MAX757 is set by two resistors, R1 and R2 (Figure 1), which form a voltage divider between the output and the FB pin. The output voltage is set by the equation: VOUT = (VREF) [(R2 + R1) / R2] where VREF = 1.25V. To simplify resistor selection: R1 = (R2) [(VOUT / VREF) - 1] Since the input bias current at FB has a maximum value of 100nA, large values (10k to 200k) can be used for R1 and R2 with no significant loss of accuracy. For 1% error, the current through R1 should be at least 100 times FB's bias current.
Pulse-Frequency Modulation Control Scheme
A unique minimum off time, current-limited, pulse-frequency modulation (PFM) control scheme is a key feature of the MAX756/MAX757. This PFM scheme combines the advantages of pulse-width modulation (PWM) (high output power and efficiency) with those of a traditional PFM pulse-skipper (ultra-low quiescent currents). There is no oscillator; at heavy loads, switching is accomplished through a constant peak-current limit in the switch, which allows the inductor current to self-oscillate between this peak limit and some lesser value. At light loads, switching frequency is governed by a pair of one-shots, which set a minimum off-time (1s) and a maximum on-time (4s). The switching frequency depends on the load and the input voltage, and can range as high as 500kHz. The peak switch current of the internal MOSFET power switch is fixed at 1A 0.2A. The switch's on resistance is typically 0.5, resulting in a switch voltage drop (VSW) of about 500mV under high output loads. The value of VSW decreases with light current loads. Conventional PWM converters generate constant-frequency switching noise, whereas this architecture produces variable-frequency switching noise. However, the noise does not exceed the switch current limit times the filter-capacitor equivalent series resistance (ESR), unlike conventional pulse-skippers.
Voltage Reference
The precision voltage reference is suitable for driving external loads such as an analog-to-digital converter. It has guaranteed 250A source-current and 20A sink-current capability. The reference is kept alive even in shutdown mode. If the reference drives an external load, bypass it with 0.22F to GND. If the reference is unloaded, bypass it with at least 0.1F.
Low-Battery Detection
The MAX756/MAX757 contain on-chip circuitry for lowbattery detection. If the voltage at LBI falls below the regulator's internal reference voltage (1.25V), LBO (an opendrain output) sinks current to GND. The low-battery monitor's threshold is set by two resistors, R3 and R4 (Figure 1), which forms a voltage divider between the input voltage and the LBI pin. The threshold voltage is set by R3 and R4 using the following equation: R3 = [(VIN / VREF) - 1] (R4)
Control-Logic Inputs
The control inputs (3/5, SHDN) are high-impedance MOS gates protected against ESD damage by normally reverse-biased clamp diodes. If these inputs are driven from signal sources that exceed the main supply
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3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters MAX756/MAX757
VIN C1 150F R3 5 8
L1 22H D1 1N5817
The inductor's DC resistance significantly affects efficiency. For highest efficiency, limit L1's DC resistance to 0.03 or less. See Table 1 for a list of suggested inductor suppliers.
LBI
LX
VOUT C2 100F
Table 1. Component Suppliers
PRODUCTION METHOD Surface-Mount INDUCTORS Sumida CD54-220 (22H) CoilCraft DT3316-223 Coiltronics CTX20-1 Sumida RCH654-220 CAPACITORS AVX TPS series Sprague 595D series
R4 1 3 C3 0.1F
MAX757 OUT 6
SHDN REF GND 7 FB LBO 2 4
R1
R2
Miniature Through-Hole
Figure 1. Standard Application Circuit
Sanyo OS-CON OS-CON series low-ESR organic semiconductor Nichicon PL series low-ESR electrolyic United Chemi-Con LXF series
where VIN is the desired threshold of the low-battery detector, R3 and R4 are the input divider resistors at LBI, and VREF is the internal 1.25V reference. Since the LBI current is less than 100nA, large resistor values (typically 10k to 200k) can be used for R3 and R4 to minimize loading of the input supply. When the voltage at LBI is below the internal threshold, LBO sinks current to GND. A pull-up resistor of 10k or more connected from LBO to V OUT can be used when driving CMOS circuits. Any pull-up resistor connected to LBO should not be returned to a voltage source greater than V OUT . When LBI is above the threshold, the LBO output is off. The low-battery comparator and reference voltage remain active when the MAX756/MAX757 is in shutdown mode. If the low-battery comparator is not used, connect LBI to VIN and leave LBO open.
Low-Cost Through-Hole
CoilCraft PCH-27-223
AVX CoilCraft Coiltronics Collmer Semiconductor Motorola Nichicon
USA: USA: USA:
(207) 282-5111, FAX (207) 283-1941 (800) 282-9975 (708) 639-6400, FAX (708) 639-1969 (407) 241-7876, FAX (407) 241-9339
Inductor Selection
The inductors should have a saturation (incremental) current rating equal to or greater than the peak switchcurrent limit, which is 1.2A worst-case. However, it's generally acceptable to bias the inductor into saturation by 20%, although this will reduce the efficiency. The 22H inductor shown in the typical applications circuit is sufficient for most MAX756/MAX757 application circuits. Higher input voltages increase the energy transferred with each cycle, due to the reduced input/output differential. Minimize excess ripple due to increased energy transfer by reducing the inductor value (10H suggested).
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USA: (214) 233-1589 USA: (602) 244-3576, FAX (602) 244-4015 USA: (708) 843-7500, FAX (708) 843-2798 Japan: +81-7-5231-8461, FAX (+81-) 7-5256-4158 Nihon USA: (805) 867-2555, FAX (805) 867-2556 Japan: +81-3-3494-7411, FAX (+81-) 3-3494-7414 Sanyo OS-CON USA: (619) 661-6835 Japan: +81-720-70-1005, FAX (+81-720-) 70-1174 Sprague USA: (603) 224-1961, FAX (603) 224-1430 Sumida USA: (708) 956-0666 Japan: +81-3-3607-5111, FAX (+81-3-) 3607-5428 United Chemi-Con USA: (708) 696-2000, FAX (708) 640-6311
Capacitor Selection
A 100F, 10V surface-mount (SMT) tantalum capacitor typically provides 50mV output ripple when stepping up from 2V to 5V at 200mA. Smaller capacitors, down to 10F, are acceptable for light loads or in applications that can tolerate higher output ripple.
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3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters
Rectifier Diode
The ESR of both bypass and filter capacitors affects efficiency. Best performance is obtained by using specialized low-ESR capacitors, or connecting two or more filter capacitors in parallel. The smallest low-ESR SMT tantalum capacitors currently available are Sprague 595D series, which are about half the size of competing products. Sanyo OS-CON organic semiconductor through-hole capacitors also exhibit very low ESR, and are especially useful for operation at cold temperatures. Table 1 lists suggested capacitor suppliers. For optimum performance, a switching Schottky diode, such as the 1N5817, is recommended. 1N5817 equivalent diodes are also available in surface-mount packages from Collmer Semiconductor in Dallas, TX, phone (214) 233-1589. The part numbers are SE014 or SE024. For low output power applications, a pn junction switching diode, such as the 1N4148, will also work well, although efficiency will suffer due to the greater forward voltage drop of the pn junction diode.
MAX756/MAX757
VIN MINIMUM OFF-TIME ONE-SHOT Q TRIG ONE-SHOT LX F/F S 3/5 R Q N VOUT
SHDN
GND MAXIMUM ON-TIME ONE-SHOT TRIG Q OUT
ONE-SHOT
MAX756
LBO N
REF
LBI
REFERENCE
Figure 2. MAX756 Block Diagram
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3.3V/5V/Adjustable-Output, 3.3V/5V/Adjustable-Output Step-Up DC-DC Converters MAX756/MAX757
PC Layout and Grounding
The MAX756/MAX757 high peak currents and high-frequency operation make PC layout important for minimizing ground bounce and noise. The distance between the MAX756/MAX757's GND pin and the ground leads of C1 and C2 in Figure 1 must be kept to less than 0.2" (5mm). All connections to the FB and LX pins should also be kept as short as possible. To obtain maximum output power and efficiency and minimum output ripple voltage, use a ground plane and solder the MAX756/MAX757 GND (pin 7) directly to the ground plane.
___________________Chip Topography
SHDN LX
3/5 (MAX756) FB (MAX757)
GND 0.122" (3.10mm) REF GND
OUT
LBI
LBO
0.080" (2.03mm)
TRANSISTOR COUNT: 758 SUBSTRATE CONNECTED TO OUT
________________________________________________________Package Information
DIM A A1 B C D E e H h L INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.197 0.189 0.157 0.150 0.050 BSC 0.244 0.228 0.020 0.010 0.050 0.016 8 0 MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 4.80 5.00 3.80 4.00 1.27 BSC 5.80 6.20 0.25 0.50 0.40 1.27 0 8
21-325A
E
H
D A e B
0.127mm 0.004in.
h x 45
A1
C
L
8-PIN PLASTIC SMALL-OUTLINE PACKAGE
8
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