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2003 Microchip Technology Inc. DS00804B-page 1
M
AN804
INTRODUCTION
Creating a negative DC bias voltage from a positive DC
supply often is required in battery-powered, portable,
hand-held instruments that use a Liquid Crystal Display
(LCD). Many LCDs require a relatively large negative
bias (on the order of -10V). Since many portable
systems (such as cellular subscriber units) often have
a regulated +5V DC bias available, the critical design
task involves converting this supply voltage to a
negative DC bias for the LCD. This application note
discusses the advantages and disadvantages of
several alternatives for implementing the -10V bias
required by the LCD.
TRADITIONAL TC682
IMPLEMENTATION
Figure 1 shows a circuit implementation using
Microchip Technology's TC682 to generate the -10V
LCD bias from a regulated +5V input. Assuming that a
single-cell Li-Ion battery (with a nominal terminal
voltage of 3.6V) is powering the system, a regulating
DC/DC boost converter is needed to generate the +5V
regulated input supply to the TC682.
Although the TC682 requires only three external
tantalum capacitors to generate the -10V bias, its
package is a large 8-pin SOIC, which occupies
approximately 31 square millimeters of circuit board
space. The TC682 active supply current is typically
185 µA, with the internal charge pump switching
frequency being 12 kHz. The TC682 has no shutdown
features, so it always consumes power when the +5V
input bias is active.
FIGURE 1: TC682 Circuit Implementation.
Author: Pat Maresca
Microchip Technology Inc.
7
5
8
6
1
C
3
V
IN
TC682
-10V
to LCD
C
1
GND
NC
3.6V
Li-Ion
Battery
2
3
C
2
4
V
OUT
+5V
+
-
+
+
+
DC/DC
Converter
10 µF
10 µF
10 µF
Notes:
1. Requires one 8-pin SOIC package, 31 sq. millimeters.
2. Requires three 10 µF tantalum capacitors.
3. TC682 supply current approximately 185 µA.
4. Charge pump switching frequency: 12 kHz.
5. No shutdown mode available.
Display
Design Alternatives To The TC682 For Performing
Inverting Voltage Doubler Functions
