3.3V/5V/Adjustable-Output,

Step-Up DC-DC Converters

6

_______________________________________________________________________________________

detector, R3 and R4 are the input divider resistors at

Since the LBI current is less than 100nA, large resistor

values (typically 10k

Ω to 200kΩ) 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

Ω

when driving CMOS circuits. Any pull-up resistor con-

nected to LBO should not be returned to a voltage

threshold, the LBO output is off. The low-battery com-

parator and reference voltage remain active when the

MAX756/MAX757 is in shutdown mode.

If the low-battery comparator is not used, connect LBI

Inductor Selection

The inductors should have a saturation (incremental)

current rating equal to or greater than the peak switch-

current limit, which is 1.2A worst-case. However, it’s

generally acceptable to bias the inductor into satura-

tion by 20%, although this will reduce the efficiency.

The 22µH inductor shown in the typical applications cir-

cuit 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 (10µH suggested).

The inductor’s DC resistance significantly affects effi-

ciency. For highest efficiency, limit L1’s DC resistance

to 0.03

Ω or less. See Table 1 for a list of suggested

inductor suppliers.

Table 1. Component Suppliers

AVX

USA:

(207) 282-5111, FAX (207) 283-1941

(800) 282-9975

CoilCraft

USA:

(708) 639-6400, FAX (708) 639-1969

Coiltronics

USA:

(407) 241-7876, FAX (407) 241-9339

Collmer

Semiconductor

USA:

(214) 233-1589

Motorola

USA:

(602) 244-3576, FAX (602) 244-4015

Nichicon

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 100µF, 10V surface-mount (SMT) tantalum capacitor

typically provides 50mV output ripple when stepping

up from 2V to 5V at 200mA. Smaller capacitors, down

to 10µF, are acceptable for light loads or in applica-

tions that can tolerate higher output ripple.

MAX757

REF

3

LX

7

C1

150

µF

GND

OUT

6

D1

1N5817

LBO

4

8

C3

0.1

µF

L1

22

µH

LBI

5

C2

100

µF

SHDN

1

FB

2

R1

R2

R3

R4

Figure 1. Standard Application Circuit

PRODUCTION

METHOD

INDUCTORS

CAPACITORS

Surface-Mount

AVX

TPS series

Sprague

595D series

Miniature

Through-Hole

Sumida

RCH654-220

Low-Cost

Through-Hole

Sumida

CD54-220 (22µH)

CoilCraft

DT3316-223

Coiltronics

CTX20-1

Sanyo OS-CON

OS-CON series

low-ESR organic

semiconductor

CoilCraft

PCH-27-223

Nichicon

PL series

low-ESR

electrolyic

United Chemi-Con

LXF series