®
SP4428
Electroluminescent Lamp Driver
High Drive Capability for Low Voltage Applications
s
Low Power +1.1V to +1.7V Single
Cell Operation
s
Low-Cost EL Driver Ideal for Backlighting
s
DC-to-AC Inverter Generates High
Voltage AC to Drive EL Lamps
s
Externally Adjustable Oscillator
s
Low Current Standby Mode
APPLICATIONS
s
Pagers
s
Watches
s
Backlit LCD Displays
DESCRIPTION
The
SP4428
is a high voltage output DC-AC converter that can operate from a single 1.5 VDC
power supply. The
SP4428
is capable of supplying 200 V
PP
(typical), making it ideal for driving
electroluminescent lamps. The device features 1µA (typical) standby current, for use in low
power portable products. One external inductor is required to generate the high voltage
charge, and one external capacitor is used to produce a clock signal that generates the coil
and lamp frequencies. The
SP4428
is ideal for PDAs, pagers, and other low power portable
applications using LCDs in dim or low light environments. The
SP4428
is offered in an 8-pin
narrow SOIC. For delivery in die form, please consult the factory.
C
OSC
V
SS
COIL
D1
1
2
3
4
SP4428
8
7
6
5
HON
V
DD
EL1
EL2
Block Diagram
SP4428DS/16
SP4428 Electroluminescent Lamp Driver
© Copyright 2000 Sipex Corporation
1
ABSOLUTE MAXIMUM RATINGS
These are stress ratings only and functional operation of the device at
these ratings or any other above those indicated in the operation sections
of the specifications below is not implied. Exposure to absolute maximum
rating conditions for extended periods of time may affect reliability.
TA= +25˚C unless otherwise noted
V
DD
............................................................................................................5V
Input Voltages/Currents
HON (pin1).........................................-0.5V to (V
DD
+0.5V)
COIL (pin3)............................................................100mA
Lamp Outputs.............................................................................230V
PP
Storage Temperature....................................................-65˚C to +150˚C
Power Dissipation Per Package
8-pin NSOIC (derate 6.14mW/
o
C above +70
o
C).................................500mW
The information furnished herein by Sipex has been carefully reviewed
for accuracy and reliability. Its application or use, however, is solely the
responsibility of the user. No responsibility for the use of this information
is assumed by Sipex, and this information shall not explicitly or implicitly
become part of the terms and conditions of any subsequent sales
agreement with Sipex. Specifications are subject to change without
prior notice. By the sale or transfer of this information, Sipex assumes
no responsibility for any infringement of patents or other rights of third
parties which may result from its use. No license or other proprietary
rights are granted by implication or otherwise under any patent or
patent rights of Sipex Corporation.
SPECIFICATIONS
(T= 25°C; V
DD
= 1.5V; Lamp Capacitance = 4.7nF; Coil = 470µH at 4.9 Ohms; C
OSC
= 180pF unless otherwise noted) C
INT
= 1800pF
PARAMETER
Supply Voltage, V
DD
Supply Current, I
COIL
+I
DD
Coil Voltage, V
COIL
HON Input Voltage, V
HON
LOW: EL off
HIGH: EL on
HON Current, EL on
Shutdown Current, I
SD
=I
COIL
+I
DD
INDUCTOR DRIVE
Coil Frequency, f
COIL
Coil Duty Cycle
Peak Coil Current, I
PK-COIL
EL LAMP OUTPUT
EL Lamp Frequency, f
LAMP
MIN.
1.1
TYP.
1.5
35
45
MAX.
1.7
50
80
1.7
UNITS
V
mA
V
CONDITIONS
V
DD
=1.1V, V
HON
=1.1V
V
DD
=1.5V, V
HON
=1.5V
V
DD
-0.25
V
DD
-0.25
0
V
DD
3
0.5
0.25V
V
DD
+0.25
15
10
V
µA
µA
internal pulldown, V
HON
=V
DD
=1.5V
V
HON
=0V
28.8
90
100
kHz
%
mA
Guaranteed by design.
150
300
150
90
120
450
500
550
750
Hz
T
AMB
=+25
O
C, V
DD
=1.1V
T
AMB
=+25
O
C, V
DD
=1.5V
T
AMB
=-40
O
C to +85
O
C, V
DD
=1.5V
V
DD
=1.1V
V
DD
=1.5V
Peak to Peak Output Voltage
120
160
V
PP
SP4428DS/16
SP4428 Electroluminescent Lamp Driver
© Copyright 2000 Sipex Corporation
2
PIN DESCRIPTION
THEORY OF OPERATION
The
SP4428
is made up of three basic circuit
elements, an oscillator, coil, driver and switched
H-bridge network. The oscillator provides the
device with an on-chip clock source used to control
the charge and discharge phases for the coil and
lamp. An external capacitor connected between
pins 1 and V
SS
allows the user to vary the oscillator
frequency. For a given choice of coil inductance
there will be an optimum C
OSC
Capacitor value that
gives the maximum light output in a given lamp.
The suggested oscillator frequency is 28.8kHz
(C
OSC
=180pF). The oscillator output is internally
divided to create the control signal f
LAMP
.
The oscillator output is internally divided down
by 6 flip flops, a 28.8kHz signal will be divided
into 6 frequency levels; 14.4kHz, 7.2kHz, 3.6kHz,
1.8kHz, 900Hz, and 450Hz. The oscillator output
(28.8kHz) is used to drive the coil (see
figure 2
on
page 7)
and the sixth flip flop output (300Hz) is
used to drive the lamp. Although the oscillator
frequency can be varied to optimize the lamp
output, the ratio of f
COIL
/f
LAMP
will always equal 64.
The coil is an external component connected from
V
BATTERY
to pin 3 of the
SP4428.
Energy is
developed in the coil according to the equation
E
L
=1/2LI
2
where the current I is defined as
I=(V
BATTERY
-IR-V
OL
)/R
T
. In order to maximize the
1
2
3
4
8
7
6
5
Pin 1 – C
OSC
- Connect CAP from V
SS
to Pin 1 to
set Oscillator frequency.
Pin 2 – V
SS
- Power supply common, connect to
ground.
Pin 3 – Coil- Coil input, connect coil from
V
BATTERY
to Pin 3.
Pin 4 – D1- Diode cathode connection.
Pin 5 – EL2- Lamp driver output1, connect to
EL lamp.
Pin 6 – EL1- Lamp driver output2, connect to
EL lamp.
Pin 7 – V
DD
- Power supply for driver, connect to
system V
DD
.
Pin 8 – HON- Enable for driver operation,
high = active; low = inactive.
SP4428
V
DD
7
V
DD
V
BATTERY
470µH/4.9Ω
IN4148
8
HON
1MΩ
3
Coil
4
Optional
1,800pF
.1µF
D1
1 C
OSC
180pF
OSC
Cap1
f
COIL
SCR1
SCR2
f
LAMP
Q
FF1
FF6
Q
f
LAMP
EL1
5
EL2
6
V
SS
2
EL Lamp
SP4428 Schematic
SP4428DS/16
SP4428 Electroluminescent Lamp Driver
© Copyright 2000 Sipex Corporation
3
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
DD
of the
SP4428.
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
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. 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.
0.1µF
180pF
1.5 Volt
Battery
1
C
OSC
V
SS
Coil
D1
Fast
Recovery
Diode
HON
V
DD
EL1
EL2
5 FtL
Coil
Typical SP4428 Application Circuit
SP4428DS/16
SP4428 Electroluminescent Lamp Driver
© Copyright 2000 Sipex Corporation
+
-
LAMP
1.5 SQ. IN.
470µH
4.9 Ohms
Optional
1800pF CAP
4
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 than LEDs
or bulbs making them ideal for battery powered
products. Also, EL lamps are able to evenly light
an area without creating "hot spots" in the display.
The amount of light emitted is a function of the
voltage applied to the lamp, the frequency at which
it is applied, the lamp material used and its
size. There are many variables which can be
optimized for specific applications.
Sipex
supplies
characterization charts to aid the designer in
selecting the optimum circuit configuration
(see
page 6
and
7).
SP4428DS/16
SP4428 Electroluminescent Lamp Driver
© Copyright 2000 Sipex Corporation
5
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