SP4425Q Ver la hoja de datos (PDF) - Signal Processing Technologies
Número de pieza
componentes Descripción
Fabricante
SP4425Q Datasheet PDF : 12 Pages
| |||
EL=1/2LI2, 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=(VL/L)tON, where VL 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 of the switch) is across the
SAT
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
IRL-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
tON, 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 VBATTERY, L, RL or tON cause the current in the coil
to increase beyond its rated I , excessive heat
SAT
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 SP4425Q. VDD can range from 2.2V to 3.3V; it
is not necessary that VDD=VBATTERY. 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 fCOIL signal controls a switch that connects the
end of the coil at pin 3 to ground or to open circuit.
The fCOIL signal is a 90% duty cycle signal switching
at the oscillator frequency. During the time when
the fCOIL 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 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, fLAMP=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 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 more rapidly creating a square wave output.
For most 3V applications an 820 pF integrator
capacitor is suitable.
SP4425QDS/12
SP4425Q Electroluminescent Lamp Driver
4
© Copyright 1998 Sipex Corporation
Share Link: 