Microchip TC682 Handleiding

Microchip Niet gecategoriseerd TC682

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 2003 Microchip Technology Inc. DS00804B-page 1
MAN804
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
1C
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
AN804
DS00804B-page 2  2003 Microchip Technology Inc.
DUAL TCM828 IMPLEMENTATION
Figure 2 shows a two-chip inverting doubler solution
using Microchip’s TCM828. Like the TC682 example, a
regulating DC/DC boost converter is needed to provide
the +5V regulated input voltage to the first TCM828,
assuming the system is powered from a single-cell Li-
Ion battery.
The second TCM828 is level-shifted (i.e., powered
from ground and -5V) to generate the -10V LCD bias.
Although this implementation requires four external
tantalum capacitors, the total package size of the two
TCM828s occupies only 18 square millimeters of board
space, which is considerably less than the TC682
solution.
The dual-TCM828 approach consumes only 100 µA
(again, considerably less than the TC682) and has the
same internal charge pump switching frequency as the
TC682 (i.e., 12 kHz). As in the TC682 example, the
dual TCM828 solution has no shutdown feature.
FIGURE 2: Dual TCM828 implementation.
5
4
4
1
22
1
3
C
4
V
IN
TCM828 TCM828
-10V to
LCD Display
V
IN
C
1
GND GND
5
3
C
3
C
2
OUT OUT
-5V
3.6V
Li-Ion
Battery
DC/DC
Converter
+5V
+
+
+
+
+
-
10 µF
10 µF
10 µF
10 µF
Notes:
1. Requires two SOT-23A-5 packages, 18 sq. millimeters total.
2. Requires four 10 µF tantalum capacitors.
3. Combined TCM828 supply current approximately 100 µA.
4. Charge pump switching frequency: 12 kHz.
5. No shutdown mode available.
 2003 Microchip Technology Inc. DS00804B-page 3
AN804
DUAL TCM829 IMPLEMENTATION
Figure 3 shows another two-chip inverting doubler
solution using Microchip’s TCM829. This
implementation is almost identical to the dual TCM828
approach. The primary difference being that the two
TC829s switch at a higher frequency (35 kHz) than the
dual TCM828 or TC682. This allows the designer to
use smaller external capacitors with the dual TCM829
solution at the expense of a higher supply current
(230 µA) than the dual TCM828 approach. All other
features of the dual TCM828 solution apply to the dual
TCM829 solution.
FIGURE 3: Dual TCM829 implementation.
5
44
1
22
1
3
C
4
V
IN
TCM829 TCM829
-10V to
LCD Display
V
IN
C
1
GND GND
5
3
C
3
C
2
OUT OUT
-5V
3.6V
Li-Ion
Battery
DC/DC
Converter
+5V
+
+
+
+
+
-
3.3 µF
3.3 µF 3.3 µF
3.3 µF
Notes:
1. Requires two SOT-23A-5 packages, 18 sq. millimeters total.
2. Requires four 3.3 µF tantalum capacitors.
3. Combined TCM829 supply current approximately 230 µA.
4. Charge pump switching frequency: 35 kHz.
5. No shutdown mode available.

Product specificaties

Merk: Microchip
Categorie: Niet gecategoriseerd
Model: TC682

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