11-14-00

SP4501 EL Plus Piezo Driver

© Copyright 2000 Sipex Corporation

4

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

To put the circuit in an

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 C

the lamp parameters and coil value. Lower C

INT

values can decrease average supply current but

higher C

This is best determined by experimentation. A

rule of thumb is larger coils (1mH) are paired

with a smaller C

(470

µH) are paired with a larger C

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, C

grate the sawtooth waveform making it