sp4403 Exar Corporation, sp4403 Datasheet - Page 5

• Manufacturer Part Number: sp4403
• Description: Low Voltage Electroluminescent Lamp Driver
• Manufacturer: Exar Corporation

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 for the EL driver circuitry is
an external inductor and an external capacitor.
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 typically 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 SP4403 is a DC-AC inverter made up of:
1. The Oscillator/Frequency Divider, 2. The
Coil, and 3. The Switched H-bridge Network.
Further details of each element follow.
The Oscillator/Frequency Divider
The oscillator provides the SP4403 with an
on-chip clock used to control the coil switch
(f
Although the oscillator frequency can be varied
to optimize the lamp output, the ratio of f
f
Figure 1 shows the oscillator output driving the
coil and the output of the oscillator with 7 flip
flops driving the lamp. The suggested oscillator
frequency is 50kHz (R
The oscillator output is internally divided down
by 7 flip flops to create a second internal control
signal at 390Hz for f
The Coil
The supply V
+4.5V. V
coil current specification. The majority of the
current goes through the coil and is typically
much greater than I
The coil is an external component connected
from V
stored in the coil according to the equation
SP4403DS/05
LAMP
COIL
) and the H-bridge network (f
will always equal 128.
BATT
BATT
to pin 3 of the SP4403. Energy is
should not exceed the maximum
BATT
DD
can range from +2.2V to
LAMP
.
OSC
.
= 450k ) for f
LAMP
SP4403 Electroluminescent Lamp Driver
and f
LAMP
COIL
COIL
).
/
.
5
Figure 2. Typical Application Circuit for the SP4403, Set
for a Square Wave Output with C
E
is the product I
where t
its peak current, V
the internal NPN switch transistor, and L is the
inductance of the coil. When the NPN transistor
switch is off, the energy is forced through an
internal diode which drives the switched H-
bridge network. This energy recovery is directly
related to the brightness of the EL lamp output.
There are many variations among coils; magnetic
material differences, winding differences and
parasitic capacitances. For suggested coil
suppliers, refer to Page 7.
The f
the end of the coil at pin 3 to ground or to open
circuit. The f
signal switching at the oscillator frequency,
50kHz. During the time when the f
HIGH, the coil is connected from V
and a charged magnetic field is created in the
coil. When the f
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
Figure 4) 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 3).
COIL
0.1 F
L
C1
=1/2LI
V
BATT
COIL
will send an array of charge pulses (see
ON
450k
ELEN
R
OSC
10nF
C2
P
is the time it takes for the coil to reach
signal controls a switch that connects
2
where I
R
V
COIL
OSC
DD
EL Lamp
P
COIL
1
6
8
= (tON)(VBATT - V
signal is a 90% duty cycle
CE
2
P
V
SS
, to the first approximation,
signal is LOW, the ground
SP4403
is the voltage drop across
© Copyright 2000 Sipex Corporation
EL1
4
INT
5
EL2
= 0.1 F
3
7
COIL
C
BATT
INT
*
optional device
COIL
to ground
signal is
1800pF
CE
470 H
1N4148
C
L1
D1
INT
)/L),
*
*