3.3V/5V or Adjustable-Output,

Step-Up DC-DC Converters

6

_______________________________________________________________________________________

______________________________________________________________Pin Description

N-Channel Power-MOSFET Drain

8

8

Low-Battery Output. An open-drain N-channel MOSFET sinks current when the voltage at

LBI drops below 1.25V.

4

4

Low-Battery Input. When the voltage on LBI drops below 1.25V, LBO sinks current.

5

5

Connect OUT to the regulator output. OUT provides bootstrap power to the IC.

6

6

Power Ground. Must be low impedance; solder directly to ground plane.

7

7

1.25V Reference Voltage Output. Bypass with 0.22µF to GND (0.1µF if there is no external

reference load). Maximum load capability is 250µA source, 20µA sink.

3

3

Feedback Input for adjustable-output operation. Connect to an external resistor voltage

divider between OUT and GND.

2

—

Selects the output voltage; connect to GND for 5V output, and to OUT for 3.3V

output.

—

2

forward voltage drop of the external Schottky rectifier.

1

1

FUNCTION

PIN

LX

LBO

LBI

OUT

GND

REF

FB

3/

–

5

–

–

S

—

H

—

D

—

N

–

NAME

MAX856

MAX858

MAX857

MAX859

_______________Detailed Description

Operating Principle

The MAX856–MAX859 combine a switch-mode regula-

tor, N-channel power MOSFET, precision voltage refer-

ence, 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 with low battery voltages (0.8V typ).

PFM Control Scheme

A unique minimum-off-time, current-limited pulse-fre-

quency modulation (PFM) control scheme is a key fea-

ture of the MAX856 series (Figure 1). This scheme

combines the high output power and efficiency of a

pulse-width modulation (PWM) device with the ultra-low

quiescent current of a traditional PFM pulse-skipper.

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 vary

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 (1µs) and a

maximum on-time (4µs). The switching frequency

depends upon the load and the input voltage, and can

range up to 500kHz.

The peak switch current of the internal MOSFET power

switch is fixed at 500mA ±100mA (MAX856/MAX857)

or 125mA ±25mA (MAX858/MAX859). The switch’s on-

resistance is typically 1

Ω (MAX856/MAX857) or 4Ω

(MAX858/MAX859), resulting in a switch voltage drop

Conventional PWM converters generate constant-fre-

quency switching noise, whereas the unique architec-

ture of the MAX856–MAX859 produces variable-fre-

quency switching noise. However, unlike conventional

pulse-skippers (where noise amplitude varies with input

voltage), noise in the MAX856 series does not exceed

the switch current limit times the filter-capacitor equiva-

lent series resistance (ESR).

Voltage Reference

The precision voltage reference is suitable for driving

external loads, such as an analog-to-digital converter.

The voltage-reference output changes less than ±2%

when sourcing up to 250µA and sinking up to 20µA. If

the reference drives an external load, bypass it with

0.22µF to GND. If the reference is unloaded, bypass it

with at least 0.1µF.