SP4490 Datasheet, PDF (6/12 Pages) Sipex Corporation – Two Panel Electroluminscent Lamp Driver

H-Bridge. An external resistor from VDD to
ROSC sets the oscillator frequency. Typically a
464kΩ resistor sets the frequency to 44.0kHz.
The high frequency clock directly controls the
coil switch. This high frequency clock is divided
by 128 to generate a low frequency clock which
controls the EL H-Bridge and sets the EL lamp
frequency. The oscillator has low sensitivity to
temperature and supply voltage variations, in-
creasing the performance of the EL 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 ON/OFF to HIGH
activates the H-Bridge that is selected by EL1/
EL2. If the EL1/EL2 is low, EL1 is illuminated.
If EL1/EL2 is HIGH, EL2 is illuminated. 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 terminal polarity between the high
voltage supply and ground at a constant fre-
quency. This applies an AC voltage 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 illuminate the EL lamp.
Setting ON/OFF to LOW disables the outputs
and places the circuit in a low power state.
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. Inductor
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 resis-
tance.
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
Rev. 9/12/00
SP4490 Two Panel Electroluminescent Lamp Driver
6
© Copyright 2000 Sipex Corporation