Detailed Description

The MAX1795/MAX1796/MAX1797 compact step-up

DC-DC converters start up with voltages as low as

0.85V and operate with an input voltage down to +0.7V.

Consuming only 25µA of quiescent current, these

devices have an internal synchronous rectifier that

reduces cost by eliminating the need for an external

diode and improves overall efficiency by minimizing

losses in the circuit (see Synchronous Rectification sec-

tion for details). The internal N-channel MOSFET power

switch resistance is typically 0.17

Ω, which minimizes

losses. The LX switch current limits of the MAX1795/

MAX1796/MAX1797 are 0.25A, 0.5A, and 1A, respec-

tively.

All three devices offer Maxim’s proprietary True-

Shutdown circuitry, which disconnects the output from

the input in shutdown and puts the output in a high

impedance state. These devices also feature Maxim’s

proprietary LX-damping circuitry, which reduces EMI in

noise-sensitive applications. For additional in-system

flexibility, the LBI/LBO comparator remains active in

shutdown. (Figure 1 is a typical application circuit).

Control Scheme

A unique minimum-off-time, current-limited control

scheme is the key to the MAX1795/MAX1796/

MAX1797s’ low operating current and high efficiency

over a wide load range. The architecture combines the

high output power and efficiency of a pulse-width-mod-

ulation (PWM) device with the ultra-low quiescent cur-

rent of a traditional pulse-skipping controller (Figure 2).

Switching frequency depends upon the load current

and input voltage, and can range up to 500kHz. Unlike

conventional pulse-skipping DC-DC converters (where

ripple amplitude varies with input voltage), ripple in

these devices does not exceed the product of the

switch current limit and the filter-capacitor equivalent

series resistance (ESR).

Low Supply Current, Step-Up DC-DC Converters

with True-Shutdown

8

_______________________________________________________________________________________

Figure 1. Typical Application Circuit

GND

LBO

LBI

*SEE TABLE 1 FOR COMPONENT VALUES.

BATT

FB

LX

OUT

MAX1795

MAX1796

MAX1797

SHDN

22

µH

47

µF

1M

PIN

NAME

FUNCTION

1

LBI

Low-Battery Comparator Input. Internally set to trip at +0.85V. This function remains operational in

shutdown.

2FB

Dual-Mode™ Feedback Input. Connect to GND for preset 5.0V output. Connect to OUT for preset

3.3V output. Connect a resistive voltage-divider from OUT to GND to adjust the output voltage from

2V to 5.5V.

3

LBO

remains operational in shutdown.

4

SHDN

Shutdown Input. If SHDN is high, the device is in shutdown mode, OUT is high impedance, and

LBI/LBO are still operational. Connect shutdown to GND for normal operation.

5

GND

Ground

6

LX

Inductor Connection

7

OUT

Power Output. OUT provides bootstrap power to the IC.

8

BATT

Battery Input and Damping Switch Connection

Pin Description

Dual Mode is a trademark of Maxim Integrated Products.