SP4425DS/20

SP4425 Electroluminescent Lamp Driver

© Copyright 2000 Sipex Corporation

6

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.

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. Both voltage and

frequency are directly related to light output. In

other words as the voltage or the frequency of the

EL output is increased the light output will also

increase. The voltage has a much larger impact on

light output than the frequency does. For example,

an output signal of 168V

a different EL driver could produce 170V

Variations in peak to peak voltage and variations

in lamp frequency are to be expected, light output

will also vary from device to device however

typical light output variations are usually not

visually noticeable.

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 7 and 8).