SP4425 Datasheet, PDF (5/14 Pages) Sipex Corporation – Electroluminescent Lamp Driver Low Voltage Applications

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 =V -
L BATTERY
IR -V . Since the voltage across the inductor is
L SAT
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 of
DD
the SP4425. V can range from 1.5V to 3.0V; it is
DD
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 470µH/4Ω coil from Toko,
and a 2mH/44Ω 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
to ground and a charged magnetic field is
BATTERY
created in the coil. During the low part of f , the
COIL
ground connection is switched open, the field
collapses and the energy in the inductor is forced
to flow toward the lamp. f will send 32 of these
COIL
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 0.1µF 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 capacitor will minimize the peaking
INT
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 and discharge rapidly creating a square
wave output. For most applications an 1800pF
integrator capacitor is suitable.
SP4425DS/20
SP4425 Electroluminescent Lamp Driver
5
© Copyright 2000 Sipex Corporation