SP4428 Datasheet, PDF (4/11 Pages) Sipex Corporation – Electroluminescent Lamp Driver High Drive Capability for Low Voltage Applications

energy produced by the coil V_BATTERY should
represent the largest voltage in the system (up to
the maximum tolerance of the coil) and the coil
should have low resistance; V_BATTERY= 1.5 VDC
with a 470µH/4.9Ω coil are typical. The majority
of the current goes through the coil and typically
less than 1mA is required for V of the SP4428.
DD
V
DD
can
range
from
1.5V
to
1.7V;
it
is
not
necessary
that V_DD=V_BATTERY. Coils are also a function of the
core material and winding used -- performance
variances may be noticeable from different coil
suppliers. The Sipex SP4428 is final tested using
a 470µH/4.9Ω coil from Sumida. For suggested
coil sources see page 9.
The f signal controls a switch that connects the
COIL
end of the coil at pin 3 to ground or to open circuit.
The f_COIL signal is a 90% duty cycle signal switching
at the oscillator frequency. 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 lamp. f_COIL will send 32 of these
charge pulses every half cycle (see figure 2
on page 7) to the lamp, 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 on page 7).
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_LAMP
signal which is the oscillator frequency divided by
64. For a 20kHz oscillator, f_LAMP=300Hz.
Pager
Application
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 voltage step
from ground to 80V (typical) on pins 5 and 6 which
are 180 degrees out of phase with each other
(see figure 3 on page 7). A differential view of the
outputs is shown in figure 4 on page 7. If Line
Noise is of concern it is advisable to add a
decoupling cap at V_DD.
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 if all that is required is
one external inductor fast recovery diode and two
capacitors.
1.5 Volt
Battery
0.1µF
180pF
Fast
Recovery
Diode
470µH
4.9 Ohms
1
C_OSC
V_SS
Coil
D1
HON
V_DD
EL1
EL2
Optional
1800pF CAP
5 FtL
LAMP
1.5 SQ. IN.
Typical SP4428 Application Circuit
SP4428DS/16
SP4428 Electroluminescent Lamp Driver
4
© Copyright 2000 Sipex Corporation