SP4403 Datasheet, PDF (5/14 Pages) Sipex Corporation – Low Voltage Electroluminescent Lamp Driver

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_COIL) and the H-bridge network (f_LAMP and f_LAMP).
Although the oscillator frequency can be varied
to optimize the lamp output, the ratio of f_COIL/
f_LAMP will always equal 128.
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_OSC = 450kΩ) for f_COIL.
The oscillator output is internally divided down
by 7 flip flops to create a second internal control
signal at 390Hz for f .
LAMP
The Coil
The supply V can range from +2.2V to
BATT
+4.5V. V_BATT should 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 coil is an external component connected
from V_BATT to pin 3 of the SP4403. Energy is
stored in the coil according to the equation
VBATT
ELEN
1
ROSC
450kΩ
VDD
6
ROSC
8
SP4403
COIL
3
7
CINT
C1
0.1µF
C2
10nF
2
VSS
45
EL1
EL2
L1
470µH
1ND41148*
CINT
1800pF
*
EL Lamp
*optional device
Figure 2. Typical Application Circuit for the SP4403, Set
for
a
Square
Wave
Output
with
C
INT
=
0.1µF
E_L=1/2LI_P2 where I_P, to the first approximation,
is the product I_P = (tON)(VBATT - V_CE)/L),
where t_ON is the time it takes for the coil to reach
its peak current, V is the voltage drop across
CE
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_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 switching at the oscillator frequency,
50kHz. During the time when the f_COIL signal is
HIGH, the coil is connected from V_BATT to ground
and a charged magnetic field is created in the
coil. When the f signal is LOW, the ground
COIL
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 will send an array of charge pulses (see
COIL
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).
SP4403DS/05
SP4403 Electroluminescent Lamp Driver
5
© Copyright 2000 Sipex Corporation