SP4501EN データシートの表示（PDF） - Signal Processing Technologies
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SP4501EN Datasheet PDF : 13 Pages
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A logic HIGH on pin PZEN will enable the piezo
driver and apply a waveform to the
piezotransducer until PZEN is released. This
waveform will produce a tone that is 1/16 the
frequency of the internal oscillator. Alternately,
an external clock applied to PZCK or PZCK pins
will enable the piezo driver and generate a tone
at the applied clock frequency.
The external applied clock frequency should be
greater than f /64. To put the circuit in an
osc
inactive state it is required that PZCK remain at
logic LOW and PZCK remain at logic HIGH.
The piezo driver and the EL driver may be
operated simultaneously but with decreased light
output from the EL panel.
DESIGN CONSIDERATIONS
Inductor Selection
If limiting peak current draw from the power
supply is important, small coil values (<1mH)
may need a higher oscillator frequency. Inductor
current ramps faster in a lower inductance coil
than a higher inductance coil for a given coil
switch on time period, resulting in higher peak
coil currents.
It is important to observe the saturation current
rating of a coil. When this current is exceeded,
the coil is incapable of storing any more energy
and then ceases to act as an inductor. Instead, the
coil behaves according to its series DC resis-
tance. Since small coils (<1mH) have inherently
low series DC resistance, the current can peak
dramatically through a small coil during satura-
tion. This situation results in wasted energy not
stored in the magnetics of the coil but expressed
as heating which could lead to failure of the coil.
Generally, selecting a coil with lower series DC
resistance will result in a system with higher
efficiency and lamp brightness.
Lamp Effects
EL lamp parameters vary between manufactur-
ers. Series DC resistance, lighting efficiency
and lamp capacitance per area differ the most
overall.
Larger lamps require more energy to illuminate.
Lowering the oscillator frequency allows more
energy to be stored in the coil during each coil
switch cycle and increases lamp brightness. The
oscillator frequency can be lowered to a point
where the lamp brightness then begins to drop
because the lamp frequency must be above a
critical frequency (approx. 100Hz) to light. Lamp
color is affected by the switching frequency of
the EL driver. Green EL lamps will emit a more
blue light as EL lamp frequency increases.
Noise Decoupling on Logic Inputs
If ELEN, PZEN, PZCK or PZCK are connected
to traces susceptible to noise, it may be necessary
to connect bypass capacitors of approximately
10nF between ELEN and VSS, PZEN and VSS,
PZCK and VSS, and PZCK and VDD. If these
inputs are driven by a microprocessor which
provides a low impedance HIGH and LOW
signal, then noise bypassing may not be neces-
sary. If some inputs are unused (as PZCK and
PZCK may be) then these inputs should be tied
to the power supply that sets the input to an
inactive state.
Increasing Light Output
EL lamp light output can be improved by con-
necting a fast recovery diode from the COIL pin
to the CAP pin. The internal diode is bypassed
resulting in an increase in light output at the EL
lamp. We suggest a fast recovery diode such as
the industry standard 1N4148.
The optimal value of CINT will vary depending on
the lamp parameters and coil value. Lower CINT
values can decrease average supply current but
higher CINT values can increase lamp brightness.
This is best determined by experimentation. A
rule of thumb is larger coils (1mH) are paired
with a smaller CINT (680pF) and smaller coils
(470µH) are paired with a larger CINT (1800pF).
Changing the EL lamp Output Voltage
Waveform
Designers can alter the sawtooth output voltage
waveform to the EL lamp. Increasing the capaci-
tance of the integration capacitor, CINT, will inte-
grate the sawtooth waveform making it
11-14-00
SP4501 EL Plus Piezo Driver
4
© Copyright 2000 Sipex Corporation
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