SP4491 Datasheet, PDF (6/13 Pages) Sipex Corporation – Two Panel Electroluminscent Lamp Driver

Electroluminescent backlighting is ideal when
used with LCD displays, keypads or other back-
lit readouts. EL lamps uniformly light an area
without creating any undesirable "hot spots" in
the display. Also, an EL lamp typically con-
sumes less power that LED's or incandescent
bulbs in similar lighting situations. These fea-
tures make EL ideal for attractive, battery pow-
ered products.
THEORY OF OPERATION
An external resistor from VDD to ROSC sets the
oscillator frequency. Typically a 560kΩ resistor
sets the frequency to 44.0kHz. The high fre-
quency 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 fre-
quency. The oscillator has low sensitivity to
temperature and supply voltage variations, in-
creasing the performance of the EL driver over
the operating parameters.
Coil Switch
The SP4491 has an inductor-based boost con-
verter to generate the high voltage used to drive
the EL lamp. Energy is stored in the inductor
according to the equation E_L = 1/2 (LI_pk2) where
I_pk = (t_ON) (V_BATT - V_CEsat) /L. An internal oscilla-
tor controls the coil switch. During the time the
coil switch is on, the coil is connected between
V and the saturation voltage of the coil switch.
DD
While the coil is connected between V_DD and coil
switch and a magnetic field develops in the coil.
When the coil switch turns off, the switch opens,
the magnetic field collapses and the voltage
across the coil rises. The internal diode forward
biases when the coil voltage rises above the H-
Bridge voltage and the energy enters the EL
lamp. Each pulse increases the voltage across
the lamp in discrete steps.
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.
As the voltage approaches its maximum, the
steps become smaller. (see figure 4).
The brightness of the EL lamp output is directly
related to energy recovery in the boost converter.
There are many variations among coils such as
magnetic core differences, winding differences
and parasitic capacitances. For suggested coil
suppliers refer to page 10.
Oscillator
The internal oscillator generates a high frequency
clock used by the boost converter and H-Bridge.
And
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.
Rev. 12/19/00
SP4490 Two Panel Electroluminescent Lamp Driver
6
© Copyright 2000 Sipex Corporation