SP4428DS/16

SP4428 Electroluminescent Lamp Driver

© Copyright 2000 Sipex Corporation

4

energy produced by the coil V

represent the largest voltage in the system (up to

the maximum tolerance of the coil) and the coil

should have low resistance; V

with a 470

µH/4.9Ω coil are typical. The majority

of the current goes through the coil and typically

less than 1mA is required for V

V

that V

core material and winding used -- performance

variances may be noticeable from different coil

suppliers. The Sipex SP4428 is final tested using

a 470

µH/4.9Ω coil from Sumida. For suggested

coil sources see page 9.

The f

end of the coil at pin 3 to ground or to open circuit.

The f

at the oscillator frequency. During the time when

the f

V

created in the coil. During the low part of f

ground connection is switched open, the field

collapses and the energy in the inductor is forced

to flow toward the lamp. f

charge pulses every half cycle (see figure 2

on page 7) to the lamp, each pulse increases the

voltage drop across the lamp in discrete steps. As

the voltage potential approaches its maximum, the

steps become smaller (see figure 1 on page 7).

The H-bridge consists of two SCR structures that

act as high voltage switches. These two switches

control the polarity of how the lamp is charged.

The SCR switches are controlled by the f

LAMP

signal which is the oscillator frequency divided by

64. For a 20kHz oscillator, f

Optional

1800pF CAP

LAMP

1

Pager

Application

C

OSC

V

SS

Coil

D1

HON

V

DD

EL1

EL2

0.1

µF

5 FtL

1.5 SQ. IN.

Fast

Recovery

Diode

1.5 Volt

Battery

180pF

470

µH

4.9 Ohms

Typical SP4428 Application Circuit

When the energy from the coil is released, a high

voltage spike is created triggering the SCR

switches. The direction of current flow is

determined by which SCR is enabled. One full

cycle of the H-bridge will create a voltage step

from ground to 80V (typical) on pins 5 and 6 which

are 180 degrees out of phase with each other

(see figure 3 on page 7). A differential view of the

outputs is shown in figure 4 on page 7. If Line

Noise is of concern it is advisable to add a

decoupling cap at V

Electroluminescent Technology

What is electroluminescence?

An EL lamp is basically a strip of plastic that is

coated with a phosphorous material which emits

light (fluoresces) when a high voltage (>40V)

which was first applied across it, is removed or

reversed. Long periods of DC voltages applied to

the material tend to breakdown the material and

reduce its lifetime. With these considerations in

mind, the ideal signal to drive an EL lamp is a high

voltage sine wave. Traditional approaches to

achieving this type of waveform included discrete

circuits incorporating a transformer, transistors,

and several resistors and capacitors. This approach

is large and bulky, and cannot be implemented in

most hand held equipment. Sipex now offers low

power single chip driver circuits specifically

designed to drive small to medium sized

electroluminescent panels if all that is required is

one external inductor fast recovery diode and two

capacitors.