SP4425DS/20
SP4425 Electroluminescent Lamp Driver
Copyright 2000 Sipex Corporation
1
SP4425
Electroluminescent Lamp Driver
Low Voltage Applications
s
Low Voltage, Single Battery Operation
(V
BATTERY
> 1.1 VDC)
s
DC to AC Inverter for EL Backlit
Display Panels
s
Externally Adjustable Internal Oscillator
s
Low Current Standby Mode
APPLICATIONS
s
Pagers
s
Digital Watches
s
Backlit LCD Displays
DESCRIPTION
The SP4425 is a high voltage output DC-AC converter that can operate from a single 1.5 VDC
power supply. The SP4425 is capable of supplying up to 220 V
PP
signals, making it ideal for
driving electroluminescent lamps. The device features 1
A (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 select the oscillator and lamp frequencies.
The SP4425 is offered in both an 8-pin narrow SOIC and 8-pin micro SOIC package.
For delivery in die form, please consult the factory.
SP4425 Block Diagram
HON
V
DD
D1
V
SS
COIL
EL2
EL1
SP4425
8
7
4
3
2
1
5
6
C
OSC
SP4425DS/20
SP4425 Electroluminescent Lamp Driver
Copyright 2000 Sipex Corporation
2
SPECIFICATIONS
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.
V
DD
...................................................................................................................5V
Input Voltages/Currents
HON (pin1)........................................-0.5V to (V
DD
+ 0.5V)
COIL (pin3)............................................................100mA
Lamp Outputs..............................................................................230V
PP
Storage Temperature....................................................-65C to +150C
Power Dissipation Per Package
8-pin NSOIC (derate 6.14mW/
o
C above +70
o
C).................................500mW
8-pin
SOIC (derate 4.85mW/
o
C above +70
o
C)..................................390mW
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.
(T= 25
C; V
DD
= 1.5V; Lamp Capacitance = 8200pFwith 1 00
Series resistance; Coil = 470
H at 4 Ohms; C
OSC
= 180pF unless otherwise noted); C
INT
=1800pF
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=
SP4425DS/20
SP4425 Electroluminescent Lamp Driver
Copyright 2000 Sipex Corporation
3
(T= 25
C; V
DD
= 3.0V; Lamp Capacitance = 4nF with 100
Series resistance; Coil = 2mH/44ohms; C
OSC
= 180pF, C
INT
= 470pF unless otherwise noted)
V
SS
HON
D1
Coil
C
OSC
73 x 46
MS666
EL2
V
DD
EL1
NOTES:
1. Dimensions are in Microns unless otherwise noted.
2. Bonding pads are 125x125 typical.
3. Outside dimensions are maximum, including scribe area.
4. Die thickness is 10 mils +/- 1.
5. Pad center coordinates are relative to die center.
6. Die size 74 x 44 mils.
This data sheet specifies environmental parameters, final test conditions and limits as well suggested operating conditions.
For applications which require performance beyond the specified condition and or limits please consult the factory.
Bonding Diagram:
PAD
X
Y
261.0
427.0
EL1
813.0
429.0
EL2
813.0
28.0
D1
813.0
-172.0
COIL
767.0
-381.0
143.5
-412.0
-790.0
-157.5
HON
-785.5
402.0
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V
SS
C
OSC
SP4425DS/20
SP4425 Electroluminescent Lamp Driver
Copyright 2000 Sipex Corporation
4
THEORY OF OPERATION
The SP4425 is made up of three basic circuit
elements, an oscillator, coil, 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.
The suggested oscillator frequency is 25.6kHz
(C
OSC
=180pF). The oscillator output is internally
divided to create the control signal for f
LAMP
. The
oscillator output is internally divided down by 6
flip flops, a 25.6kHz signal will be divided into 6
frequency levels: 12.8kHz, 6.4kHz, 3.2kHz,
1.6kHz, 800Hz, and 400Hz. The oscillator output
(25.6kHz) is used to drive the coil (see figure 2 on
page 11) 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 SP4425. V
BATTERY
= 1.5
VDC with a 470
H/4
coil are typical conditions.
Energy is stored in the coil according to the equation
E
L
=1/2LI
2
, where I is the peak current flowing in
PIN DESCRIPTION
Pin 1 C
OSC
- Capacitor input 1, connect Capacitor
from V
SS
to Pin 1 to set C
OSC
frequency.
Pin 2 V
SS
- Power supply common, connect to
ground.
Pin 3 Coil- Coil input, connect coil from V
DD
to pin 3.
Pin 4 D1- Diode Cathode connection.
C
INT
- Integrator capacitor, connect capacitor
from pin 4 to ground to minimize coil glitch energy.
Pin 5 Lamp- Lamp driver output2, connect to
EL lamp.
Pin 6 Lamp- Lamp driver output1, 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.
1
2
3
4
8
7
6
5
SP4425
SP4425 Schematic
Q
Q
HON
SCR1
SCR2
OSC
FF1
FF6
EL Lamp
1M
V
DD
Coil
5
6
EL2
EL1
1
Cap1
2
V
SS
V
BATTERY
V
DD
7
3
470
H/4
8
180pF
IN4148
4
D1
.1
F
C
OSC
Low ESR
decoupling
capacitor
C
INT
=1800pF typ
f
COIL
f
LAMP
f
LAMP
SP4425DS/20
SP4425 Electroluminescent Lamp Driver
Copyright 2000 Sipex Corporation
5
the inductor. The current in the inductor is time
dependent and is set by the "ON" time of the coil
switch: I=(V
L
/L)t
ON
, where V
L
is the voltage across
the inductor. At the moment the switch closes, the
current in the inductor is zero and the entire supply
voltage (minus the V
SAT
of the switch) is across the
inductor. The current in the inductor will then
ramp up at a linear rate. As the current in the
inductor builds up, the voltage across the inductor
will decrease due to the resistance of the coil and
the "ON" resistance of the switch: V
L
=V
BATTERY
-
IR
L
-V
SAT
. Since the voltage across the inductor is
decreasing, the current ramp-rate also decreases
which reduces the current in the coil at the end of
t
ON
the energy stored in the inductor per coil cycle
and therefore the light output. The other important
issue is that maximum current (saturation current)
in the coil is set by the design and manufacturer of
the coil. If the parameters of the application such
as V
BATTERY
, L, R
L
or t
ON
cause the current in the coil
to increase beyond its rated I
SAT
, excessive heat
will be generated and the power efficiency will
decrease with no additional light output.
The majority of the current goes through the coil
and typically less than 2mA is required for V
DD
of
the SP4425. V
DD
can range from 1.5V to 3.0V; 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 SP4425 is final
tested at 1.5V using a 470
H/4
coil from Toko,
and a 2mH/44
coil from Matsushita at 3V.
For suggested coil sources see page 12.
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 (see figure 2 on page 11) 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 11).
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 25.6kHz oscillator, f
LAMP
=400Hz.
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 11). A differential view of
the outputs is shown in figure 4 on page 11.
Layout Considerations
The SP4425 circuit board layout must observe
careful analog precautions. For applications with
noisy power supply voltages, a 0.1
F low ESR
decoupling capacitor must be connected from Vdd
to ground. Any high voltage traces should be
isolated from any digital clock traces or enable
lines. A solid ground plane connection is strongly
recommended. All traces to the coil or to the high
voltage outputs should be kept as short as possible
to minimize capacitive coupling to digital clock
lines and to reduce EMI emissions.
Integrator Capacitor
An integrating capacitor must be placed from pin
4 (D1) to ground in order to minimize glitches
associated with switching the coil. A capacitor at
this point will collect the high voltage spikes and
will maximize the peak to peak voltage output.
High resistance EL lamps will produce more
pronounced spiking on the EL output waveform;
adding the C
INT
capacitor will minimize the peaking
and increase the voltage output at each coil step.
The value of the integrator capacitor is application
specific typical values can range from 500pF to
0.1
F. No integrator capacitor or very small values
(500pF) will have a minor effect on the output,
whereas a 0.1
F capacitor will cause the output to
charge and discharge rapidly creating a square
wave output. For most applications an 1800pF
integrator capacitor is suitable.
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