SP4480DS/06

SP4480 Dual Electroluminescent Lamp Driver

© Copyright 2000 Sipex Corporation

5

lighting situations. These features make EL

ideal for attractive, battery powered products.

THEORY OF OPERATION

Coil Switch

The SP4480 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

lator controls the coil switch. During the time

the coil switch is on, the coil is connected

between V

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 volt-

age 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 con-

verter. There are many variations among coils

such as magnetic core differences, winding dif-

ferences and parasitic capacitances. For sug-

gested coil suppliers refer to page 10.

Oscillator

The internal oscillator generates a high fre-

quency clock used by the boost converter and H-

Bridge.

An external resistor from VDD to

ROSC sets the oscillator frequency. Typically

a 402k

Ω resistor sets the frequency to 57kHz.

The high frequency clock directly controls the

coil switch. This high frequency clock is di-

vided by 128 to generate a low frequency clock

which controls the EL H-Bridge and sets the EL

lamp frequency. The oscillator has low sensi-

tivity to temperature and supply voltage varia-

tions, increasing the performance of the EL

Figure 2: Typical Application Circuit of the SP4480

COIL

CAP

SP4480

C1

0.1

µF

402k

Ω

L1

470

µH

8

14

3

1

9

7

ELEN1

D1

1N4148

1800pF

EL2

EL1

EL Lamp

10

12

1nF

2

ELEN2

* optional device

EL4

EL3

13

11

EL Lamp