Compact, High-Efficiency, Dual-Output

Step-Up and LCD Bias DC-DC Converter

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9

Main Boost Converter (MBC)

The MBC operates either in PFM mode, 300kHz PWM

mode, or externally synchronized PWM mode as select-

ed by the CLK/SEL input (Table 2). PWM mode offers

fixed-frequency operation and maximum output power.

PFM mode offers the lowest IC operating current. LX

current limit is reduced in PFM mode to increase effi-

ciency and minimize output ripple.

PWM Mode

When CLK/SEL is high, the MAX1677 operates in its

high-power, low-noise PWM mode, switching at the

300kHz internal oscillator frequency. The MOSFET

switch pulse-width is modulated to control the power

transferred on each switching cycle and regulate the

Table 1. Main Boost Converter Available

Output Current

ON

OUT

START-UP OSC

IN

POL

SUCLK

MAIN DC-DC

839k

QP

501k

QN

CLK/SEL

OUT

POUT

LX

PGND

50% REF

FB

LBO

LBI

LCDON

LCDLX

LCDGND

ON

CLK

EA

ST

EA

LCDDRV

CL

POL

LCD BIAS

ON

ON

REF

ON

LCDPOL

LCDFB

GND

REF

90% REF

2.25V

START-UP

LCD ON

REFERENCE

ISET/POL SENSE

MAX1677

Figure 1. Functional Block Diagram

NUMBER OF

CELLS

1 Alk/NiCd/NiMH

140/150

MBC OUTPUT

CURRENT

(mA)

PWM/PFM

1 Alk/NiCd/NiMH

1.2

1.2

INPUT

VOLTAGE

(V)

MBC

OUTPUT

VOLTAGE

(V)

3.3

5

100/70

2 Alk/NiCd/NiMH

2.4

3.3

350/170

2 Alk/NiCd/NiMH

2.4

5

260/125

1 Alk/NiCd/NiMH

or 1 Li-Ion

3.6

5

350/170