SP4412ACX datasheet(4/10 Pages) SIPEX

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SP4412ADS/12

SP4412ACN Electroluminescent Lamp Driver

Typical SP4412ACN Application Circuit

The external clock should have a 50% duty

cycle and range from V

DD-1V to ground.

The

maximum external clock frequency is 128 kHz.

The coil is an external component connected

from V

BATTERY to pin 5 of the SP4412A. Energy

is developed in the coil according to the equation

L=1/2LI

2 where the current I is defined as

I=(V

BATTERY-IR-VOL)/RT. In order to maximize

the energy produced by the coil, V

BATTERY should

represent the largest voltage in the system (up to

a maximum of 3.6 v); V

BATTERY= 3.0 VDC with

a 35mH/125

Ω coil is a typical example. It is not

necessary that V

DD=VBATTERY. The coil operation

is a function of the core material and winding

used -- performance variances may be noticeable

from different coil suppliers even though the

values are the same. The Sipex SP4412A is

final tested using a 35 mH/135 ohm coil. For

suggested coil sources see page 8.

The f

COIL signal controls a switch that connects

the end of the coil at pin 5 to ground or to open

circuit. The f

COIL signal is a 94% duty cycle

square wave, switching at 1/4 the oscillator

frequency, (for a 32 kHz oscillator f

COIL

is 8

kHz). During the time when the f

COIL signal is

high, the coil is connected from V

BATTERY to

ground and a charged magnetic field is created

in the coil. During the low part of f

COIL , the

ground connection is switched open, the field

collapses, and the energy in the inductor is forced

to flow toward the high voltage H-bridge

switches. f

COIL

will send 16 of these charge

pulses to the lamp, each pulse increases the

voltage drop across the lamp in discrete steps. As

the voltage potential approaches its maximum,

the steps become shorter (see figure 1 on page 7).

The H-bridge consists of two SCR structures that

act as high voltage switches. These two switches

control the polarity of how the lamp is charged.

The SCR switches are controlled by the f

LAMP

signal which is the oscillator frequency divided

by 128. For a 32kHz oscillator, f

LAMP= 250Hz.

When the energy from the coil is released, a high

voltage spike is created triggering the SCR

switches. The direction of current flow is

determined by which SCR is enabled. One full

cycle of the H-bridge will create 16 voltage steps

from ground to 80V (typical) on pins 6 and 7

which are 180 degrees out of phase with each

other (see figure 3 on page 7). A differential view

of the outputs is shown in figure 4 on page 7.

ELECTROLUMINESCENT TECHNOLOGY

What is electroluminescence?

An EL lamp is basically a strip of plastic that is

coated with a phosphorous material which emits

light (fluoresces) when a high voltage (>40V)

which was first applied across it, is removed or

reversed. Long periods of DC voltages applied to

the material tend to breakdown the material and

reduce its lifetime. With these considerations in

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

high voltage sine wave. Traditional approaches

to achieving this type of waveform included

discrete circuits incorporating a transformer,

transistors, and several resistors and capacitors.

This approach is large and bulky, and cannot be

implemented in most hand held equipment. Sipex

now offers low power single chip driver circuits

specifically designed to drive small to medium

sized electroluminescent panels. All that is

required is an external inductor and an external

clock signal.

LAMP

Coil

32-64 Khz

3 VDC

HON

Clk

VSS

VDD

Lamp

Coil

30 mH 125 ohm Coil

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