SP4405 datasheet(5/20 Pages) SIPEX

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SP4405DS/18

SP4405 Low Voltage Electroluminescent Lamp Driver

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 can be

difficult to implement in some smaller hand

held equipment. Sipex now offers low power

single chip driver circuits specifically

designed to drive small to medium sized

electroluminescent panels.

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 SP4405 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 SP4405 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 48.6kHz (R

OSC = 450kΩ) for fCOIL.

The oscillator output is internally divided down

by 7 flip flops to create a second internal control

signal at 380Hz for f

LAMP.

The Coil

The supply V

BATTERY can range from +2.2V to

+4.5V. V

BATTERY should be chosen such that ICOIL

does 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

BATTERY to pin 3 of the SP4405. 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) (VBATTERY - 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 transistor for f

COIL, 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,

Figure 2. Typical Application Circuit for the SP4405, Set

for a Square Wave Output with C

INT = 0.1µF

ROSC

VSS

COIL

CINT

SP4405

VBATTERY

0.1

µF

ROSC

450k

Ω

470

µH

ELEN

*D1

1N4148

CINT

0.1

µF

*optional device

VDD

EL2

EL1

EL Lamp

1nF

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