SP4403 datasheet(5/14 Pages) SIPEX

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SP4403DS/05

SP4403 Electroluminescent Lamp Driver

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 for the EL driver circuitry is

an external inductor and an external capacitor.

Market Applications

Electroluminescent backlighting is ideal when

used with LCD displays, keypads, or other backlit

readouts. Its main use is to illuminate displays in

dim to dark conditions for momentary periods

of time. EL lamps typically consume less power

than LEDs or incandescent bulbs making them

ideal for battery powered products. Also, EL

lamps are able to evenly light an area without

creating any undesirable "hot spots" in the

display.

THEORY OF OPERATION

The SP4403 is a DC-AC inverter made up of:

1. The Oscillator/Frequency Divider, 2. The

Coil, and 3. The Switched H-bridge Network.

Further details of each element follow.

The Oscillator/Frequency Divider

The oscillator provides the SP4403 with an

on-chip clock used to control the coil switch

COIL) and the H-bridge network (fLAMP and fLAMP).

Although the oscillator frequency can be varied

to optimize the lamp output, the ratio of f

COIL/

LAMP will always equal 128.

Figure 1 shows the oscillator output driving the

coil and the output of the oscillator with 7 flip

flops driving the lamp. The suggested oscillator

frequency is 50kHz (R

OSC = 450kΩ) for fCOIL.

The oscillator output is internally divided down

by 7 flip flops to create a second internal control

signal at 390Hz for f

LAMP.

The Coil

The supply V

BATT can range from +2.2V to

+4.5V. V

BATT should not exceed the maximum

coil current specification. The majority of the

current goes through the coil and is typically

much greater than I

DD.

The coil is an external component connected

from V

BATT to pin 3 of the SP4403. Energy is

stored in the coil according to the equation

L=1/2LIP

2 where I

P, to the first approximation,

is the product I

P = (tON)(VBATT - VCE)/L),

where t

ON is the time it takes for the coil to reach

its peak current, V

CE is the voltage drop across

the internal NPN switch transistor, and L is the

inductance of the coil. When the NPN transistor

switch is off, the energy is forced through an

internal diode which drives the switched H-

bridge network. This energy recovery is directly

related to the brightness of the EL lamp output.

There are many variations among coils; magnetic

material differences, winding differences and

parasitic capacitances. For suggested coil

suppliers, refer to Page 7.

The f

COIL signal controls a switch that connects

the end of the coil at pin 3 to ground or to open

circuit. The f

COIL signal is a 90% duty cycle

signal switching at the oscillator frequency,

50kHz. During the time when the f

COIL signal is

HIGH, the coil is connected from V

BATT to ground

and a charged magnetic field is created in the

coil. When the f

COIL signal is LOW, 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.

COIL

will send an array of charge pulses (see

Figure 4) 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 smaller (see Figure 3).

Figure 2. Typical Application Circuit for the SP4403, Set

for a Square Wave Output with C

INT = 0.1µF

ROSC

VSS

COIL

CINT

SP4403

VBATT

0.1

µF

ROSC

450k

Ω

470

µH

ELEN

1N4148

CINT

1800pF

optional device

VDD

EL2

EL1

EL Lamp

10nF

SP4403 Datasheet(PDF) 5 Page - Sipex Corporation

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