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AVAILABLE

EVALUATION KIT AVAILABLE

Functional Diagrams

Pin Configurations appear at end of data sheet.

Functional Diagrams continued at end of data sheet.

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3.3V/5V/Adjustable-Output,

Step-Up DC-DC Converters

_______________General Description

The MAX756/MAX757 are CMOS step-up DC-DC switch-

ing regulators for small, low input voltage or battery-pow-

ered systems. The MAX756 accepts a positive input

voltage down to 0.7V and converts it to a higher pin-

selectable 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 60µA

by CMOS construction and a unique constant-off-time

pulse-frequency modulation control scheme.

________________________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

____________________________Features

♦ Operates Down to 0.7V Input Supply Voltage

♦ 87% Efficiency at 200mA

♦ 60µA Quiescent Current

♦ 20µA Shutdown Mode with Active Reference and

LBI Detector

♦ 500kHz Maximum Switching Frequency

♦ ±1.5% Reference Tolerance Over Temperature

♦ Low-Battery Detector (LBI/LBO)

♦ 8-Pin DIP and SO Packages

______________Ordering Information

* Dice are tested at T

= +25°C only.

GND

OUT

LBI

LBO

REF

3/5

SHDN

MAX756

DIP/SO

TOP VIEW

GND

OUT

LBI

LBO

REF

SHDN

MAX757

DIP/SO

_________________Pin Configurations

MAX756

SHDN

3/5

REF

LBI

150μF

GND

OUT

INPUT

2V to V

OUT

1N5817

OUTPUT

5V at 200mA

3.3V at 300mA

100μF

LBO

0.1μF

22μH

LOW-BATTERY

DETECTOR OUTPUT

__________Typical Operating Circuit

PART TEMP. RANGE PIN-PACKAGE

MAX756CPA

0°C to +70°C 8 Plastic DIP

MAX756CSA 0°C to +70°C 8 SO

MAX756C/D 0°C to +70°C Dice*

MAX756EPA -40°C to +85°C 8 Plastic DIP

MAX756ESA -40°C to +85°C 8 SO

MAX757CPA

0°C to +70°C 8 Plastic DIP

MAX757CSA 0°C to +70°C 8 SO

MAX757C/D 0°C to +70°C Dice*

MAX757EPA -40°C to +85°C 8 Plastic DIP

MAX757ESA -40°C to +85°C 8 SO

3.3V/5V/Adjustable-Output,

Step-Up DC-DC Converters

19-0113; Rev. 2; 1/95

_______________General Description

The MAX756/MAX757 are CMOS step-up DC-DC switch-

ing regulators for small, low input voltage or battery-pow-

ered systems. The MAX756 accepts a positive input

voltage down to 0.7V and converts it to a higher pin-

selectable 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 60µA

by CMOS construction and a unique constant-off-time

pulse-frequency modulation control scheme.

________________________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

____________________________Features

♦ Operates Down to 0.7V Input Supply Voltage

♦ 87% Efficiency at 200mA

♦ 60µA Quiescent Current

♦ 20µA Shutdown Mode with Active Reference and

LBI Detector

♦ 500kHz Maximum Switching Frequency

♦ ±1.5% Reference Tolerance Over Temperature

♦ Low-Battery Detector (LBI/LBO)

♦ 8-Pin DIP and SO Packages

______________Ordering Information

* Dice are tested at T

= +25°C only.

MAX756/MAX757

Summary of content (9 pages)

PAGE 1
EVALUATION KIT AVAILABLE MAX756/MAX757 LE AVAILAB 3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters _______________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.
PAGE 2
MAX756/MAX757 3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters 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.
PAGE 3
MAX756/MAX757 3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters __________________________________________Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) 80 80 VIN = 2.5V EFFICIENCY (%) VIN = 1.2V EFFICIENCY (%) 800 70 60 VIN = 1.25V 70 60 50 50 MAX756-3 VIN = 3.3V MAXIMUM OUTPUT CURRENT (mA) VIN = 2.0V MAX756-2 90 MAX756-1 90 MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE EFFICIENCY vs. LOAD CURRENT 5V OUTPUT MODE EFFICIENCY vs.
PAGE 4
MAX756/MAX757 3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters _____________________________Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, 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 = 5ms/div 5V Mode VIN = 2.
PAGE 5
MAX756/MAX757 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).
PAGE 6
MAX756/MAX757 3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters The inductor’s DC resistance significantly affects efficiency. For highest efficiency, limit L1’s DC resistance to 0.03Ω or less. See Table 1 for a list of suggested inductor suppliers. VIN C1 150μF L1 22μH R3 5 3 C3 0.1μF D1 1N5817 MAX757 OUT 6 R1 LBI R4 1 8 LX SHDN FB REF LBO VOUT C2 100μF Table 1.
PAGE 7
MAX756/MAX757 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.
PAGE 8
MAX756/MAX757 3.3V/5V/Adjustable-Output, 3.3V/5V/Adjustable-Output Step-Up DC-DC Converters ___________________Chip Topography 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.
PAGE 9
MAX756/MAX757 3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters 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. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.