SP4491EU datasheet(6/13 Pages) SIPEX

6 / 13 page

Rev. 12/19/00

SP4490 Two Panel Electroluminescent Lamp Driver

An external resistor from VDD to ROSC sets the

oscillator frequency. Typically a 560k

Ω resistor

sets the frequency to 44.0kHz. 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 oscillator has low sensitivity to

temperature and supply voltage variations, in-

creasing 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 ON/OFF to HIGH

activates the H-Bridge that is selected by EL1/

EL2. If the EL1/EL2 is low, EL1 is illuminated.

If EL1/EL2 is HIGH, EL2 is illuminated.

The

EL driver illuminates the lamp by applying the

high voltage supply of the boost converter to the

lamp terminals through the H-Bridge and then

switching the terminal polarity between the high

voltage supply and ground at a constant fre-

quency. 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 typically

necessary to adequately illuminate the EL lamp.

Setting ON/OFF to LOW disables the outputs

and places the circuit in a low power state.

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.

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 SP4491 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

L = 1/2 (LIpk

2) where

pk = (tON) (VBATT - VCEsat) /L. An internal oscilla-

tor controls the coil switch. During the time the

coil switch is on, the coil is connected between

DD and the saturation voltage of the coil switch.

While the coil is connected between V

DD and coil

switch 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.

And

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