SP4425DS/20

SP4425 Electroluminescent Lamp Driver

© Copyright 2000 Sipex Corporation

4

THEORY OF OPERATION

The SP4425 is made up of three basic circuit

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

network. The oscillator provides the device with

an on-chip clock source used to control the charge

and discharge phases for the coil and lamp. An

external capacitor connected between pins 1 and

V

For a given choice of coil inductance there will be

an optimum C

maximum light output.

The suggested oscillator frequency is 25.6kHz

(C

divided to create the control signal for f

oscillator output is internally divided down by 6

flip flops, a 25.6kHz signal will be divided into 6

frequency levels: 12.8kHz, 6.4kHz, 3.2kHz,

1.6kHz, 800Hz, and 400Hz. The oscillator output

(25.6kHz) is used to drive the coil (see figure 2 on

page 11) and the sixth flip flop output (300Hz) is

used to drive the lamp. Although the oscillator

frequency can be varied to optimize the lamp

output, the ratio of f

The coil is an external component connected from

V

VDC with a 470

µH/4Ω coil are typical conditions.

Energy is stored in the coil according to the equation

E

PIN DESCRIPTION

Pin 1 – C

from V

Pin 2 – V

ground.

Pin 3 – Coil- Coil input, connect coil from V

DD

to pin 3.

Pin 4 – D1- Diode Cathode connection.

– C

from pin 4 to ground to minimize coil glitch energy.

Pin 5 – Lamp- Lamp driver output2, connect to

EL lamp.

Pin 6 – Lamp- Lamp driver output1, connect to

EL lamp.

Pin 7 – V

system V

Pin 8 – HON- Enable for driver operation,

high = active; low = inactive.

1

2

3

4

8

7

6

5

SP4425 Schematic

Q

Q

HON

SCR1

SCR2

OSC

FF1

FF6

EL Lamp

1M

Ω

Coil

5

6

EL2

EL1

1

Cap1

2

7

3

470

µH/4Ω

8

180pF

IN4148

4

D1

.1

µF

Low ESR

decoupling

capacitor