High-Frequency, High-Power, Low-Noise,

Step-Up DC-DC Converter

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9

_______________Detailed Description

The MAX1708 step-up converter offers high efficiency

and high integration for high-power applications. It

operates with an input voltage as low as 0.7V and is

suitable for single- to 3-cell battery inputs, as well as

2.5V or 3.3V regulated supply inputs. The output volt-

age is preset to +3.3V or +5.0V or can be adjusted with

external resistors for voltages between +2.5V to +5.5V.

The MAX1708 internal N-channel MOSFET switch is

rated for 5A (RMS value) and can deliver loads to 2A,

depending on input and output voltages. For flexibility,

the current limit and soft-start rate are independently

programmable.

A 600kHz switching frequency allows for a small induc-

tor to be used. The switching frequency is also syn-

chronizable to an external clock ranging from 350kHz

to 1MHz.

ONA,

ONB

The logic levels at ONA and ONB turn the MAX1708 on

or off. When ONA = 1 or ONB = 0, the device is on.

When ONA = 0 and ONB = 1, the device is off (Table

1). Logic high on-control can be implemented by con-

necting ONB high and using ONA for shutdown.

Implement inverted single-line on/off control by ground-

ing ONA and toggling ONB. Implement momentary

pushbutton on/off as described in the Applications

Information section. Both inputs have approximately

0.15V of hysteresis.

Switching Frequency

The MAX1708 switches at the fixed-frequency internal

oscillator rate (600kHz) or can be synchronized to an

external clock. Connect CLK to OUT for internal clock

operation. Apply a clock signal to CLK to synchronize

to an external clock. The MAX1708 will synchronize to a

new external clock rate in two cycles and will take

approximately 40µs to revert to its internal clock fre-

quency once the external clock pulses stop and CLK is

driven high. Table 2 summarizes oscillator operation.

Operation

The MAX1708 switches at a constant frequency

(600kHz) and modulates the MOSFET switch pulse

width to control the power transferred per cycle and

regulate the voltage across the load. In low-noise appli-

cations, the fundamental and the harmonics generated

by the fixed switching frequency are easily filtered out.

Figure 2 shows the simplified functional diagram for the

MAX1708. Figure 3 shows the simplified PWM con-

Figure 1. Standard Operating Circuit

KEEP TRACES

SHORT AND WIDE

ON/OFF

CONTROL

MAX1708

L1

2.2

µH

C1

150

µF

GND

R1

C3

C4

0.22

µF

GND

REF

SS/LIM

PGND PGND PGND

C5

0.1

µF

FB

GND

OUT

3.3/5

CLK

D1

LX

LX

LX

R2

2

Ω

C2

150

µF

+5V

GND

ONA

ONB

KEEP TRACES

SHORT AND WIDE