SP4438 Datasheet, PDF (7/19 Pages) Sipex Corporation – Ultra-Quiet Electroluminescent Lamp Driver For Cellular Phone Applications

DESCRIPTION
The SP4438 Electroluminescent Lamp Driver is
a low-cost low voltage device ideal for the
replacement of LED backlighting designs in
cell phones, PDAs and other portable designs
desiring low acoustic noise. The SP4438
contains a DC-AC inverter that can produce an
AC output of 145V_P-P (typical) from a +2.7V to
+3.3V input voltage. An internal block diagram
of the SP4438 can be found in Figure 2.
The SP4438 is built on Sipex's dielectrically
isolated BiCMOS process that provides the
isolation required to separate the high voltage
AC signal used to drive the EL lamp from the
low voltage logic and signal processing circuitry.
This ensures latch-up free operation in the
interface between the low voltage CMOS
circuitry and the high voltage bipolar circuitry.
The SP4438 is ideal for applications driving EL
lamps to backlight LCD displays and keypads,
used in cellular radios.
A total of only six external components are
required for the standard operation of the SP4438:
an inductor, a fast recovery diode, three
capacitors and a resistor. A diagram of the
SP4438 in a typical application can be found in
Figure 1.
Electroluminescent Technology
An EL lamp is a strip of plastic that is coated
with a phosphorous material which emits light
(fluoresces) when a (>40V) AC signal is applied
across it. Long periods of DC voltages applied
to the lamp tends 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 offers low power single chip driver circuits
specifically designed to drive small to medium
sized electroluminescent panels.
Market Applications
Electroluminescent backlighting is ideal when
used with LCD displays, keypads, or other backlit
readouts. Its main use is to illuminate displays
in dim to dark conditions for momentary periods
of time. EL lamps consume less power than
LEDs or incandescent bulbs making them ideal
for battery powered products. Also, EL lamps
are able to evenly light an area without creating
any undesirable "hot spots" in the display.
THEORY OF OPERATION
The SP4438 is a DC-AC inverter made up of:
1. The Oscillator/Frequency Divider, 2. The
Coil/Boost Converter, 3. The Switched H-
bridge Network, and 4. The Precision Bridge
Control Logic. Further details of each element
follow.
The Oscillator/Frequency Divider
The oscillator provides the SP4438 with an on-
chip clock used to control the coil switch (f_COIL)
and the H-bridge network (f_LAMP ). Although the
oscillator frequency can be varied to optimize
the lamp output, the ratio of f /f will
COIL LAMP
always equal 128.
Figure 2 shows the oscillator output driving the
coil and through 7 flip flops, driving the lamp.
The suggested oscillator frequency is 32kHz for
f_COIL. The oscillator output is internally divided
down by 7 flip flops to create a second internal
control signal at 250Hz for f_LAMP.
The Coil/Boost Converter
The supply V_COIL can range from +2.7V to +9V.
See figure 4 on page 6. V_COILshould be chosen
such that I_COIL does not exceed the maximum
coil current specification. The majority of the
current goes through the coil and is typically
much greater than I .
DD
The inductor is an external component connected
from V to the COIL pin of the SP4438.
COIL
Energy is stored in the coil according to the
equation
E = 1/2 x L x I ²
L
P
where I_P, to the first approximation, is the product
I_P = (t_ON) x ((V_BATT - V_CE)/L)
where t_ON is the time it takes for the coil to reach
its peak current, V_CE is the voltage drop across
the internal NPN transistor and L is the
Rev.11/13/00
SP4438 Ultra-Quiet Electroluminescent Lamp Driver
7
© Copyright 2000 Sipex Corporation