SP4414 Datasheet, PDF (5/14 Pages) Sipex Corporation – Low Cost Electroluminescent Lamp Driver

Electroluminescent Technology
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 can be
difficult to implement in some smaller hand
held equipment. Sipex now 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 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 SP4414 is a DC-AC inverter made up of:
1. The Frequency Divider, 2. The Coil, and
3. The Switched H-bridge Network. Further
details of each element follow.
The Oscillator/Frequency Divider
The external oscillator provides the SP4414
with a clock signal used to control the coil
switch (f_COIL) and the H-bridge network (f_LAMP
and f_LAMP). The ratio of f_COIL/f_LAMP will always
equal 32 for the SP4414.
The external clock frequency is 32kHz. Figure 1
shows the external clock is internally divided
VBATTERY
C1
10nF
VDD ₇
ELEN
CLK
SP4414
4
COIL
8
1
2
5
CINT
63
VSS
EL1
EL2
L1
2.7mH
*D1
1N4148
CINT
100pF
EL Lamp
*optional device
Figure 2. Typical Application Circuit for the SP4414
down by a flip flop to drive the coil at 8kHz.
This signal is divided down by 5 more flip flops
to drive the lamp at 250Hz.
The Coil
The supply V_BATTERY can range from +2.2V to
+4.5V. V_BATTERY and the coil should be chosen
such that I does not exceed the maximum
COIL
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_BATTERY to pin 4 of the SP4414. Energy is
stored in the coil according to the equation
E_L=1/2LI_P2 where I_P, to the first approximation,
is the product I_P = (tON) (VBATTERY - 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 for f_COIL, and
L
is
the inductance of the coil. When the NPN
transistor switch is off, the energy is forced
through a 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.
SP4414DS/17
SP4414 Electroluminescent Lamp Driver
5
© Copyright 2000 Sipex Corporation