SP4422ADS/15

SP4422A Electroluminescent Lamp Driver

© Copyright 2000 Sipex Corporation

5

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 16 voltage steps

from ground to 80V (typical) on pins 4 and 5 which

are 180 degrees out of phase from each other (see

figure 3 on page 9. A differential representation of

the outputs is shown in figure 4 on page 9.

Layout Considerations

The SP4422A circuit board layout must observe

careful analog precautions. For applications with

noisy voltage power supplies a 0.1

µF low ESR

decoupling capacitor must be connected from V

DD

to ground. Any high voltage traces should be

isolated from any digital clock traces or enable

lines. A solid ground plane connection is strongly

recommended. All traces to the coil or to the high

voltage outputs should be kept as short as possible

to minimize capacitive coupling to digital clock

lines and to reduce EMI emissions.

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. All that is required is

one external inductor and capacitor.

Electroluminescent backlighting is ideal when used

with LCD displays, keypads, or other backlit

readouts. Its main use is to illuminate displays in

dim to dark conditions for momentary periods of

time. EL lamps typically consume less than LEDs

or bulbs making them ideal for battery powered

products. Also, EL lamps are able to evenly light

an area without creating "hot spots" in the display.

The amount of light emitted is a function of the

voltage applied to the lamp, the frequency at which

it is applied, the lamp material used and its size,

and lastly, the inductor used. There are many

variables which can be optimized for specific

applications. Sipex supplies characterization charts

to aid the designer in selecting the optimum circuit

configuration (see page 6).

SP4422A Test Circuit

0.1

µF Low ESR

Decoupling

Capacitor

NOTE:

Keep high voltage traces

and clock lines

Cap2

Cap1

EL1

HON

Coil

EL2

HON=V

HON=0V=OFF

SP4422A

NOTE:

Keep coil as close to the

SP4422A as possible

+

–

100

Ω

17nF