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SP4501 EL Plus Piezo Driver

© Copyright 2000 Sipex Corporation

3

Electroluminescent Technology

An EL lamp consists of a thin layer of phospho-

rous material sandwiched between two strips of

plastic which emits light (flouresces) when a

high voltage AC signal is applied across it. It

behaves primarily as a capacitive load. Long

periods of DC voltage applied to the material

tend to reduce its lifetime. With these conditions

in mind, the ideal signal to drive an EL lamp is a

high voltage sine wave. Traditional approaches

to achieve this type of waveform include discrete

circuits incorporating a transformer, transistors

and several resistors and capacitors. This ap-

proach is large and bulky and cannot be imple-

mented in most handheld equipment.

Sipex

offers low power single chip driver circuits spe-

cifically designed to drive small to medium sized

electroluminescent panels. Sipex EL drivers

provide a differential AC voltage without a DC

offset to maximize EL lamp lifetime. The only

additional components required for the EL driver

circuitry are an inductor, resistor and capacitor.

Electroluminescent backlighting is ideal when

used with LCD displays, keypads or other back-

lit readouts. EL lamps uniformly light an area

without creating any undesirable "hot spots" in

the display. Also, an EL lamp typically con-

sumes less power that LED's or incandescent

bulbs in similar lighting situations. These fea-

tures make EL ideal for attractive, battery pow-

ered products.

THEORY OF OPERATION

Coil Switch

The SP4501 has an inductor-based boost con-

verter to generate the high voltage used to drive

the EL lamp. Energy is stored in the inductor

according to the equation E

I

tor controls the coil switch. During the time the

coil switch is on, the coil is connected between

V

and a magnetic field develops in the coil. When

the coil switch turns off, the switch opens, the

magnetic field collapses and the voltage across

the coil rises.

The internal diode forward biases when the coil

voltage rises above the H-Bridge voltage and the

energy enters the EL lamp. Each pulse increases

the voltage across the lamp in discrete steps.

As the voltage approaches its maximum, the

steps become smaller. (see figure 4).

The brightness of the EL lamp output is directly

related to energy recovery in the boost converter.

There are many variations among coils such as

magnetic core differences, winding differences

and parasitic capacitances. For suggested coil

suppliers refer to page 10.

Oscillator

The internal oscillator generates a high frequency

clock used by the boost converter and H-Bridge.

An external resistor from VDD to ROSC sets the

oscillator frequency. Typically a 500k

Ω resistor

sets the frequency to 45.7kHz. The high fre-

quency clock directly controls the coil switch.

This high frequency clock is divided by 128 to

generate a low frequency clock which controls

the EL H-Bridge and sets the EL lamp fre-

quency. The high frequency clock is divided by

16 to create a medium frequency clock to drive

the piezo H-Bridge.

The oscillator has low

sensitivity to temperature and supply voltage

variations, increasing the performance of the EL

driver over the operating parameters.

Dual H-Bridge

The H-Bridge consists of two SCR structures

and two NPN transistors that control how the

lamp is charged. Setting ELEN to HIGH acti-

vates the EL H-Bridge. The EL driver illumi-

nates the lamp by applying the high voltage

supply of the boost converter to the lamp termi-

nals through the H-Bridge and then switching

the terminal polarity between the high voltage

supply and ground at a constant frequency. This

applies an AC voltage to the lamp that is twice

the peak output voltage of the boost driver. An

AC voltage greater than the 40V across the

terminals of the lamp is necessary to adequately

illuminate the EL lamp. The piezo driver output

applies an AC voltage to the piezotransducer in

a similar manner. The piezo driver operates in

two modes.