SP4427 Datasheet, PDF (4/9 Pages) Sipex Corporation – Electroluminescent Lamp Driver Dual Oscillators with High Drive Capability

voltage in the system (up to the maximum tolerance
of the coil) and the coil should have low resistance
and high inductance; V_battery= 1.5 VDC with a
20mH/70Ω coil are typical. The majority of the
current goes through the coil and typically less
than 1mA is required for Vdd of the SP4427. Vdd
can range from 1.1V to 5V; it is not necessary that
Vdd=V . Coils are also a function of the core
battery
material and winding used -- performance variances
may be noticeable from different coil suppliers.
The Sipex SP4427 is final tested using a 20mH/
70Ω coil from CTC. For suggested coil sources see
page 7.
The F_coil signal controls a switch that connects the
end of the coil at pin 3 to ground or to open circuit.
The F_coil signal is a 90% duty cycle signal. During
the time when the F_coil signal is high, the coil is
connected from V_battery to ground and a charged
magnetic field is created in the coil. During the low
part of F_coil , the ground connection is switched
open, the field collapses and the energy in the
inductor is forced to flow toward the
high voltage H-bridge switches. F_coil will send
as many charge pulses as possible in 1 Lamp Cycle
{Number of Coil pulses in 1 lamp cycle =
Coil Freq
Lamp Freq (2)
}(see figure 2 page 6). Each pulse
increases the voltage drop across the lamp
in discrete steps. As the voltage potential
approaches its maximum, the steps become smaller
(see figure 1 page 6).
The H-bridge consists of two SCR structures that
act as high voltage switches. These two switches
control the polarity of how the lamp is charged.
The SCR switches are controlled by the F signal
lamp
which is the oscillator frequency divided by 256.
For a 64kHz oscillator, F =256Hz.
lamp
When the energy from the coil is released, a high
voltage spike is created triggering the SCR
switches. The direction of current flow is
determined by which SCR is enabled. One full
cycle of the H-bridge will create a number of
voltage steps from ground to 55V (typ) on pins 4
and 5 which are 180 degrees out of phase with each
other (see figure 3 page 6). A differential view of
the outputs is shown in figure 4 on page 6. If Line
Noise is of concern it is advisable to add a
decoupling cap at Vdd (low ESR type).
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. All that is required is
one external inductor and capacitor.
Electroluminescent backlighting is ideal when used
with LCD displays, keypads, or other backlit
readouts. Its main use is to illuminate displays in
1.5 Volt
Application
-
0.1µF
+
1
Cosc Losc
Vss Hon
Coil Vdd
EL2 EL1
3.FtL
LAMP
1.6 SQ. IN.
Typical SP4427CN Application Circuit
SP4427DS/01
SP4427 Electroluminescent Lamp Driver
4
© Copyright 1997 Sipex Corporation