Compact, High-Efficiency, Dual-Output

Step-Up and LCD Bias DC-DC Converter

8

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Pin Description

Dual Mode is a trademark of Maxim Integrated Products.

Detailed Description

The MAX1677 is a highly-efficient, dual-output power

supply for battery-powered devices. On-chip are two

complete step-up DC-DC converters to power main

logic and bias an LCD (Figure 1). The main boost con-

verter (MBC) has on-chip P-channel and N-channel

MOSFETs that provide synchronous-rectified voltage

conversion for maximum efficiency at loads up to

300mA. See Table 1 for available output current with

typical battery configurations. The output voltage of the

MBC is factory-preset to 3.3V, or can be set from 2.5V

to 5.5V with external resistors (dual-mode operation).

Either fixed-frequency PWM or low-operating-current

PFM operation can be selected for the MBC using the

CLK/SEL input (Table 2).

The LCD boost converter (LCD) includes an internal N-

channel DMOS switch to generate positive or negative

voltages up to ±28V. The polarity of the LCD output is

set by LCDPOL input (Table 3). Figure 2 shows the

MAX1677 configured for a positive LCD output voltage

with a 3.3V main output. Figure 3 shows the MAX1677

configured for a negative LCD output. LCDPOL also

allows the current limit of LCDLX to be reduced from

350mA to 225mA to allow minimum-size inductors in

low-current LCD applications (typically for LCD loads

<10mA).

Also included in the MAX1677 are a precision 1.25V

reference that sources up to 50µA, logic shutdown con-

trol for the MBC and LCD (the MBC must be on for the

LCD to operate), and a low-battery comparator.

NAME

FUNCTION

1

OUT

Output Sense Input. The device is powered from OUT. Bypass to GND with a 0.1µF ceramic capacitor.

Connect OUT to POUT through a 10

Ω series resistor.

2

FB

Dual Mode™ Main Boost Feedback Input. Connect to GND for 3.3V output. Connect a voltage-divider from

OUT to FB to adjust the output in the 2.5V to 5.5V range (Figure 5).

PIN

3

LBI

Low-Battery-Comparator Input. Threshold is 614mV. Set the low-battery trip-point with an external voltage

divider (Figure 7).

4

LBO

Open-Drain, Low-Battery Output.

LBO is low when LBI is below 614mV, otherwise it remains high.

Sync Clock and PWM Select Input.

CLK/SEL = low: low-power, low-quiescent-current PFM mode.

CLK/SEL = high: low-noise, high-power PWM mode at 300kHz.

CLK/SEL = driven with external clock of 200kHz to 400kHz, synchronized PWM high-power mode.

5

CLK/SEL

10

LCDFB

LCD Feedback Input. Threshold is 1.25V for positive with LCDPOL high, and 0 for negative with

LCDPOL low.

9

GND

Ground

8

REF

1.25V Reference Output. Bypass with 0.1µF.

7

LCDPOL

LCD Polarity Select Input. Sets LCD boost converter polarity and peak current output (Table 2).

6

LCDON

LCD Enable Input. Drive high to turn on LCD boost converter. Main DC-DC must also be on.

11

ON

I.C. Enable Input. Drive high to enable the MAX1677.

12

LCDLX

LCD Boost 28V Switch Drain

13

LCDGND

Source of the Internal N-Channel DMOS LCD Boost-Converter Switch

14

PGND

Source of the Internal N-Channel Main Boost-Converter Switch

15

LX

Main Output Boost Internal Switch Drain

16

POUT

Boost DC-DC Converter Power Output. Source of internal P-channel MOSFET main boost-converter

synchronous rectifier.