SP4490EU datasheet(5/12 Pages) SIPEX

5 / 12 page

Rev. 9/12/00

SP4490 Two Panel Electroluminescent Lamp Driver

Figure 2: Typical Application Circuit of SP4490

ROSC

VSS

COIL

CAP

SP4490

VBATT

0.1

µF

ROSC

464k

Ω

470

µH

EL1/EL2

1N4148

CINT

1800pF

VDD

ELcommon

EL1

EL Lamp

C2*

10nF

ON/OFF

* optional device

EL2

EL Lamp

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 SP4490 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 is on, the coil is connected between

DD and coil switch and a magnetic field devel-

ops 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 en-

ters the EL lamp.

Each pulse increases the

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-

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