SP4438 datasheet(7/19 Pages) SIPEX

7 / 19 page

Rev.11/13/00

SP4438 Ultra-Quiet Electroluminescent Lamp Driver

DESCRIPTION

The SP4438 Electroluminescent Lamp Driver is

a low-cost low voltage device ideal for the

replacement of LED backlighting designs in

cell phones, PDAs and other portable designs

desiring low acoustic noise.

The SP4438

contains a DC-AC inverter that can produce an

AC output of 145V

P-P (typical) from a +2.7V to

+3.3V input voltage. An internal block diagram

of the SP4438 can be found in Figure 2.

The SP4438 is built on Sipex's dielectrically

isolated BiCMOS process that provides the

isolation required to separate the high voltage

AC signal used to drive the EL lamp from the

low voltage logic and signal processing circuitry.

This ensures latch-up free operation in the

interface between the low voltage CMOS

circuitry and the high voltage bipolar circuitry.

The SP4438 is ideal for applications driving EL

lamps to backlight LCD displays and keypads,

used in cellular radios.

A total of only six external components are

required for the standard operation of the SP4438:

an inductor, a fast recovery diode, three

capacitors and a resistor. A diagram of the

SP4438 in a typical application can be found in

Figure 1.

Electroluminescent Technology

An EL lamp is a strip of plastic that is coated

with a phosphorous material which emits light

(fluoresces) when a (>40V) AC signal is applied

across it. Long periods of DC voltages applied

to the lamp tends 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 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 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 SP4438 is a DC-AC inverter made up of:

1. The Oscillator/Frequency Divider, 2. The

Coil/Boost Converter, 3.

The Switched H-

bridge Network, and 4. The Precision Bridge

Control Logic. Further details of each element

follow.

The Oscillator/Frequency Divider

The oscillator provides the SP4438 with an on-

chip clock used to control the coil switch (f

COIL)

and the H-bridge network (f

LAMP ). Although the

oscillator frequency can be varied to optimize

the lamp output, the ratio of f

COIL/fLAMP will

always equal 128.

Figure 2 shows the oscillator output driving the

coil and through 7 flip flops, driving the lamp.

The suggested oscillator frequency is 32kHz for

COIL. The oscillator output is internally divided

down by 7 flip flops to create a second internal

control signal at 250Hz for f

LAMP.

The Coil/Boost Converter

The supply V

COIL can range from +2.7V to +9V.

See figure 4 on page 6. V

COILshould 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 inductor is an external component connected

from V

COIL to the COIL pin of the SP4438.

Energy is stored in the coil according to the

equation

L = 1/2 x L x IP

where I

P, to the first approximation, is the product

P = (tON) x ((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 transistor and L is the

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