SP4414 datasheet(5/14 Pages) SIPEX

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SP4414DS/17

SP4414 Electroluminescent Lamp Driver

Electroluminescent Technology

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 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 SP4414 is a DC-AC inverter made up of:

1. The Frequency Divider, 2. The Coil, and

3. The Switched H-bridge Network. Further

details of each element follow.

The Oscillator/Frequency Divider

The external oscillator provides the SP4414

with a clock signal used to control the coil

switch (f

COIL) and the H-bridge network (fLAMP

and f

LAMP).

The ratio of f

COIL/fLAMP will always

equal 32 for the SP4414.

The external clock frequency is 32kHz. Figure 1

shows the external clock is internally divided

down by a flip flop to drive the coil at 8kHz.

This signal is divided down by 5 more flip flops

to drive the lamp at 250Hz.

The Coil

The supply V

BATTERY can range from +2.2V to

+4.5V. V

BATTERY and the coil should be chosen

such that I

COIL 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 4 of the SP4414. 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

the inductance of the coil.

When the NPN

transistor switch is off, the energy is forced

through a 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.

Figure 2. Typical Application Circuit for the SP4414

VSS

COIL

CINT

SP4414

VBATTERY

10nF

2.7mH

ELEN

*D1

1N4148

CINT

100pF

*optional device

VDD

EL2

EL1

EL Lamp

CLK

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