SP4425Q
SIGNAL PROCESSING EXCELLENCE
Electroluminescent Lamp Driver for
Low Noise Applications
s
Low Noise Waveform
s
Tunable Waveshaping
s
DC to AC Inverter for EL Backlit
Display Panels
s
Externally Adjustable Internal Oscillator
s
Low Current Standby Mode
APPLICATIONS
s
Cellular Phones
s
Cordless Phones
s
Handsets
s
Backlit LCD Displays
DESCRIPTION
The
SP4425Q
is a high voltage output DC-AC converter that can operate from a single 3.0
V
DC
power supply. The
SP4425Q
is capable of supplying up to 220 V
PP
signals, making it ideal
for driving electroluminescent lamps. The device features 100 nA (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
SP4425Q
is offered in an 8-pin
µSOIC
package. For delivery in die form,
please consult the factory.
C
OSC
V
SS
COIL
D1
1
2
3
4
SP4425Q
8
7
6
5
HON
V
DD
EL1
EL2
SP4425Q Block Diagram
SP4425QDS/12
SP4425Q Electroluminescent Lamp Driver
© Copyright 1998 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.
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
µSOIC
(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.
SPECIFICATIONS
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
=f
LAMP
x64
Coil Duty Cycle
Peak Coil Current, I
PK-COIL
EL LAMP OUTPUT
EL Lamp Frequency, f
LAMP
300
225
90
140
90
450
28.8
90
V
DD
-0.25
V
DD
-0.25
0
V
DD
5
0.1
MIN.
2.2
TYP.
3.0
28
(T= 25°C; V
DD
= 3.0V; see test circuit schematic page 6; Coil = 2mH/44ohms; C
OSC
= 180pF, C
INT
= 820pF unless otherwise noted)
MAX.
3.3
40
3.3
UNITS
V
mA
V
V
HON
=V
DD
=3V
CONDITIONS
0.25V
V
DD
+0.25
20
1.0
V
µΑ
µA
internal pulldown, V
HON
=V
DD
=3V
V
HON
=0V
kHz
%
90
mA
Guaranteed by design.
500
775
Hz
T
AMB
=+25
O
C, V
DD
=3.0V
T
AMB
=-40
O
C to +85
O
C, V
DD
=3.0V
T
AMB
=+25
O
C, V
DD
=2.2V
T
AMB
=+25
O
C, V
DD
=3.0V
T
AMB
=-40
O
C to +85
O
C, V
DD
=3.0V
Peak to Peak Output Voltage
120
160
V
PP
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:
HON
V
DD
EL1
PAD
V
DD
EL1
EL2
D1
COIL
V
SS
C
OSC
X
261.0
813.0
813.0
813.0
767.0
143.5
-790.0
-785.5
Y
427.0
429.0
28.0
-172.0
-381.0
-412.0
-157.5
402.0
C
OSC
EL2
D1
Coil
V
SS
SP4425QDS/12
HON
NOTES:
1. Dimensions are in Microns unless otherwise noted.
2. Bonding pads are 125x125 typ.
3. Outside dimensions are maximum, including scribe area.
4. Die thickness is 380 +/- 25 microns (15 mils +/- 1).
5. Pad center coordinates are relative to die center.
6. Die size 74 x 44 mils.
© Copyright 1998 Sipex Corporation
SP4425Q Electroluminescent Lamp Driver
2
PIN DESCRIPTION
THEORY OF OPERATION
The
SP4425Q
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
Vss 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 provides
maximum light output.
The suggested oscillator frequency is 28.8kHz
(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 28.8kHz signal will be divided into 6
frequency levels: 14.4kHz, 7.2kHz, 3.6kHz,
1.8kHz, 900kHz, and 450Hz. The oscillator output
(28.8kHz) is used to drive the coil (see
figure 2
on
page 6)
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
SP4425Q.
V
BATTERY
= 3.0
VDC with a 2mH/44Ω coil are typical conditions.
Energy is stored in the coil according to the equation
1
2
3
4
8
7
6
5
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.
Low ESR
decoupling
capacitor
.1µF
V
DD
3
Coil
7
V
DD
1MΩ
1
C
OSC
OSC
Cap1
SP4425Q
V
BATTERY
2mH/44Ω
IN4148
8
HON
C
INT
= 820pF typical
4
D1
f
COIL
SCR1
SCR2
180pF
f
LAMP
Q
FF1
FF6
Q
f
LAMP
EL2
5
EL1
6
QR2
EL Lamp
QR1
V
SS
2
SP4425Q Schematic
SP4425QDS/12
SP4425Q Electroluminescent Lamp Driver
© Copyright 1998 Sipex Corporation
3
E
L
=1/2LI
2
, where I is the peak current flowing in
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
-Vsat. 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
SP4425Q.
V
DD
can range from 2.2V to 3.3V; 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
SP4425Q
is
final tested at 3.0V using a 2mH/44Ω coil from
Matsushita. For suggested coil sources see
page 10.
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 6)
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 6).
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 28.8kHz oscillator, f
LAMP
=450Hz.
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 6).
A differential view of the
outputs is shown in
figure 4
on
page 6.
Layout Considerations
The
SP4425Q
circuit board layout must observe
careful analog precautions. For applications with
noisy power supply voltages, a 0.1µF low ESR
decoupling capacitor must be connected from V
DD
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
0.1µF. No integrator capacitor or very small values
(500pF) will have a minor effect on the output,
whereas a 0.1µF capacitor will cause the output to
charge more rapidly creating a square wave output.
For most 3V applications an 820 pF integrator
capacitor is suitable.
SP4425QDS/12
SP4425Q Electroluminescent Lamp Driver
© Copyright 1998 Sipex Corporation
4
Waveshaping
The
SP4425Q
allows the user to "tune" the output
waveform for specific application requirements.
External resistors, QR1 and QR2 (see SP4425QCU
schematic
page 3)
can be adjusted to remove any
sharp, high frequency edges present on the EL
output waveform. Typical values range from 5kΩ
to 20kΩ. The waveforms on
page 9
show the effect
that the Q resistors have on the output. As the
sharp discharge edge is filtered, the available
noise from the vibration of the lamp is reduced.
The user must balance the noise performance
with the light output performance to achieve the
desired results.
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.
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 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,
and lastly, the inductor used. Both voltage and
frequency are directly related to light output. In
other words, as the voltage or the frequency of the
EL output is increased, the light output will also
increase. The voltage has a much larger impact on
light output than the frequency does. For example,
an output signal of 168V
PP
with a frequency of
500Hz can yield 15Cd/m
2
. In the same application
a different EL driver could produce 170V
PP
with a
frequency of 450Hz and can also yield 15Cd/m
2
.
Variations in peak-to- peak voltage and variations
in lamp frequency are to be expected, light output
will also vary from device-to-device however
typical light output variations are usually not
visually noticeable.
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 7
and
8).
SP4425QDS/12
SP4425Q Electroluminescent Lamp Driver
© Copyright 1998 Sipex Corporation
5
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