DN34

Design Note 34
Issue 1 April 1996
High Efficiency Converter for LCD Bias Contrast
Supplies
DC Input
+2.0V to +6.0V
22µF/10V
C3
C2
0.1µF
R2
V+
CS
Maxim 749
DHI
ADJ
TR1
FMMT720
R1
DLOW
680R
GND
FB
47µH
1M
RFB
L1
RV1
1M
D1
1N5819
C1
22µF/30V
Vout = -17.0V
COMP
High Efficiency Negative Boost Converter.
A high efficiency power supply based on
the Maxim MAX749 for generation of the
LCD bias contrast voltage is presented in
this design note. The circuit requires an
input of +2V to +6V and delivers an
output of -17V at 50mA.
Efficiency in the order of 80% is achieved
using the FMMT720 - one of the very low
VCE(sat) SOT23 PNP transistors available
from the Zetex SuperSOT series.
It is ideally suited for portable
equipment and can be powered by two
to five ‘AA’ size Ni-Cd batteries.
DN34 - 1
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