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

STARTUP 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