Microchip TC682 Handleiding

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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 12kHz. 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 10µF tantalum capacitors.

3.TC682 supply current approximately 185µA.

4.Charge pump switching frequency: 12kHz.

5.No shutdown mode available.

Display

Design Alternatives To The TC682 For Performing

Inverting Voltage Doubler Functions

Product specificaties