SP4491 Datasheet, PDF (7/13 Pages) Sipex Corporation – Two Panel Electroluminscent Lamp Driver

Since small coils (<1mH) have inherently low
series DC resistance, the current can peak dra-
matically through a small coil during saturation.
This situation results in wasted energy not stored
in the magnetics of the coil but expressed as
heating which could lead to failure of the coil.
Generally, selecting a coil with lower series DC
resistance will result in a system with higher
efficiency and lamp brightness.
Lamp Effects
EL lamp parameters vary between manufactur-
ers. Series DC resistance, lighting efficiency
and lamp capacitance per area differ the most
overall. Larger lamps require more energy to
illuminate. Lowering the oscillator frequency
allows more energy to be stored in the coil during
each coil switch cycle and increases lamp bright-
ness. The oscillator frequency can be lowered to
a point where the lamp brightness then begins to
drop because the lamp frequency must be above
a critical frequency (approx. 100Hz) to light.
Lamp color is affected by the switching fre-
quency of the EL driver. Green EL lamps will
emit a more blue light as EL lamp frequency
increases.
Noise Decoupling on Logic Inputs
If EL1/EL2 or ON/OFF are connected to traces
susceptible to noise, it may be necessary to
connect bypass capacitor of approximately 10nF
between EL1/EL2 and VSS, and ON/OFF and
VSS. If these inputs are driven by a micropro-
cessor which provides a low impedance HIGH
and LOW signal, then noise bypassing may not
ber be necessary.
Increasing Light Output
EL lamp light output can be improved by con-
necting a fast recovery diode from the COIL pin
to the CAP pin. The internal diode is bypassed
resulting in an increase in light output at the EL
lamp. We suggest a fast recovery diode such as
the industry standard 1N4148.
The optimal value of C_INT will vary depending on
the lamp parameters and coil value. Lower C_INT
values can decrease average supply current but
higher C_INT values can increase lamp brightness.
This is best determined by experimentation. A
rule of thumb is larger coils (1mH) are paired
with a smaller C_INT (680pF) and smaller coils
(470µH) are paired with a larger C_INT (1800pF).
Changing the EL lamp Output Voltage
Waveform
Designers can alter the sawtooth output voltage
waveform to the EL lamp. Increasing the capaci-
tance of the integration capacitor, C will inte-
INT,
grate the sawtooth waveform making it appear
more like a square wave.
Printed Circuit Board Layout Sugges-
tions
The EL driver's high-frequency operation makes
PCB layout important for minimizing electrical
noise. Keep the IC's GND pin and the ground
leads of C1 and C_INT less than 0.2in (5mm) apart.
Also keep the connections to the COIL pin as
short as possible. To maximize output power
and efficiency and minimize output ripple volt-
age, use a ground plane and solder the IC's VSS
pin directly to the ground plane.
EL Lamp Driver Design Challenges
There are many variables which can be opti-
mized for specific applications. The amount of
light emitted is a function of the voltage applied
to the lamp, the frequency at which is applied, the
lamp material, the lamp size, and the inductor
used. Sipex supplies characterization charts to
aid the designer in selecting the optimum circuit
configuration.
Sipex will perform customer application evalu-
ations, using the customer's actual EL lamp to
determine the optimum operating conditions for
specific applications. For customers consider-
ing an EL backlighting solution for the first time,
Sipex is able to provide retrofits to non-backlit
products for a thorough electrical and cosmetic
evaluation. Please contact your local Sipex sales
Representative or the Sipex factory directly to
initiate this valuable service.
Rev. 12/19/00
SP4491 Two Panel Electroluminescent Lamp Driver
7
© Copyright 2000 Sipex Corporation