Compact, High-Efficiency, Dual-Output
Step-Up and LCD Bias DC-DC Converter
MAX1677
OUT
MAX1677
START-UP OSC
OUT
ON
839k
MAIN DC-DC
CLK
SUCLK
EA
CLK/SEL
POUT
QP
LX
QN
PGND
ON
REF
ON
ON
REF
501k
90% REF
REFERENCE
FB
2.25V
LCD ON
ISET/POL SENSE
POL
LCDPOL
IN
I
LCDLX
START-UP
I
LCDLX
ST
LCDDRV
LCDFB
EA
CL
LCD BIAS
GND
POL
ON
LBI
LCDON
LCDLX
LBO
50% REF
LCDGND
Figure 1. Functional Block Diagram
Table 1. Main Boost Converter Available
Output Current
NUMBER OF
CELLS
1 Alk/NiCd/NiMH
1 Alk/NiCd/NiMH
2 Alk/NiCd/NiMH
2 Alk/NiCd/NiMH
1 Alk/NiCd/NiMH
or 1 Li-Ion
MBC
MBC OUTPUT
INPUT
OUTPUT
CURRENT
VOLTAGE
VOLTAGE
(mA)
(V)
(V)
PWM/PFM
1.2
1.2
2.4
2.4
3.6
3.3
5
3.3
5
5
140/150
100/70
350/170
260/125
350/170
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
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