SP4425Q Datasheet, PDF (5/12 Pages) Sipex Corporation – Electroluminescent Lamp Driver for Low Noise Applications

Waveshaping
The SP4425Q allows the user to "tune" the output
waveform for specific application requirements.
External resistors, QR1 and QR2 (see SP4425QCU
schematic page 3) can be adjusted to remove any
sharp, high frequency edges present on the EL
output waveform. Typical values range from 5kΩ
to 20kΩ. The waveforms on page 9 show the effect
that the Q resistors have on the output. As the
sharp discharge edge is filtered, the available
noise from the vibration of the lamp is reduced.
The user must balance the noise performance
with the light output performance to achieve the
desired results.
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.
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 power 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_PP with a frequency of
500Hz can yield 15Cd/m². In the same application
a different EL driver could produce 170V with a
PP
frequency of 450Hz and can also yield 15Cd/m².
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).
SP4425QDS/12
SP4425Q Electroluminescent Lamp Driver
5
© Copyright 1998 Sipex Corporation