SP4480 Datasheet, PDF (6/13 Pages) Sipex Corporation – Dual Electroluminscent Lamp Driver

driver over the operating parameters.
Dual H-Bridge
The H-Bridge consists of two SCR structures
and two NPN transistors that control how the
lamp is charged. Setting ELEN1 to HIGH
activates the EL1 and EL2 outputs. Setting
ELEN2 to HIGH activates EL3 and EL4 out-
puts. The EL driver illuminates the lamp by
applying the high voltage supply of the boost
converter to the lamp terminals through the H-
Bridge and then switching the terminals polarity
between the high voltage supply and ground at
a constant frequency. This applies an AC volt-
age to the lamp that is twice the peak output
voltage of the boost driver. An AC voltage
greater than the 40V across the terminals of the
lamp is typically necessary to adequately illu-
minate the EL lamp.
Both EL drivers may be operated simultaneously
but with decreased light output from the EL
panels.
DESIGN CONSIDERATIONS
Inductor Selection
If limiting peak current draw from the power
supply is important, small coil values (<1mH)
may need a higher oscillator frequency. Induc-
tor current ramps faster in a lower inductance
coil than a higher inductance coil for a given coil
switch on time period, resulting in higher peak
coil currents.
It is important to observe the saturation current
rating of a coil. When this current is exceeded,
the coil is incapable of storing any more energy
and then ceases to act as an inductor. Instead,
the coil behaves according to its series DC
resistance. Since small coils (<1mH) have in-
herently low series DC resistance, the current
can peak dramatically through a small coil dur-
ing 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 brightness. 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 frequency of the EL driver. Green EL
lamps will emit a more blue light as EL lamp
frequency increases.
Noise Decoupling on Logic Inputs
If ELEN1 or ELEN2 are connected to traces
susceptible to noise, it may be necessary to
connect bypass capacitors of approximately 10nF
between ELEN1 and VSS and ELEN2 and VSS.
If these inputs are driven by a microprocessor
which provides a low impedance HIGH and
LOW signal, then noise bypassing may not 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 will vary depending
INT
on the lamp parameters and coil value. Lower
C values can decrease average supply current
INT
but higher C_INT values can increase lamp bright-
ness. This is best determined by experimenta-
tion. 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 ca-
pacitance of the integration capacitor, C_INT, will
integrate the sawtooth waveform making it ap-
pear more like a square wave.
SP4480DS/06
SP4480 Dual Electroluminescent Lamp Driver
6
© Copyright 2000 Sipex Corporation