SP4427DS/01

SP4427 Electroluminescent Lamp Driver

© Copyright 1997 Sipex Corporation

3

THEORY OF OPERATION

The SP4427 is made up of three basic circuit

elements: an oscillator, coil, and switched H-bridge

network. The oscillator portion of the circuit is

divided into two precision oscillators. The

oscillators set the coil and lamp frequencies

independently. This allows for the arbitrary setting

of the coil/lamp frequency ratio. The coil frequency

can be selected for maximum energy transfer per

cycle for a given external coil value. The coil

oscillator is set at 75% duty cycle and the lamp

oscillator is set at 50% duty cycle for minimum DC

offset across the EL lamp.

An external capacitor connected between pin 8

and V

frequency from 75Hz to 350Hz. Likewise, an

external capacitor connected between pin 1 and

V

SS

allows the user to vary the coil oscillator

frequency from 3.5kHz to 50kHz. The graphs on

page 6 show the relationship between oscillators

(L

values.

The coil is an external component connected from

V

in the coil according to the equation E

2

where the current I is defined as I = (V

V

by the coil V

PIN DESCRIPTION

Pin 1 – Cosc- Connect Cap1 from Vss to pin 1 to

set oscillator frequency.

Pin 2 – Vss- Power supply common, connect to

ground.

Pin 3 – Coil- Coil input, connect coil from

Vbattery to pin 3.

Pin 4 – EL2- Lamp driver output1, connect to

EL lamp.

Pin 5 – EL1- Lamp driver output2, connect to

EL lamp.

Pin 6 – Vdd- Power supply for driver, connect to

system Vdd.

Pin 7 – HON- Enable for driver operation,

high = active; low = inactive.

Pin 8 – Losc- Connect Cap2 from Vss to pin 8 to

set lamp frequency.

1

2

3

4

8

7

6

5

OSC 1

Coil

EL Lamp

0.1

µF

1

200 pF

OSC 2

Lamp

8

1800 pF

7

6

Vbat

HON

EL

DRIVER

3

5

4

2

+

Vbat

+

L1

20 mH/70

Ω

SP4427

Place as close

as possible to

IC pin 6

Low

ESR CAP

SP4427 Schematic