SP4438 datasheet(8/19 Pages) SIPEX

8 / 19 page

Rev. 11/13/00

SP4438 Ultra-Quiet Electroluminescent Lamp Driver

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.

The f

COIL signal controls a switch that connects

the coil at the COIL pin to ground or to open

circuit. The f

COIL signal is a 90% duty cycle

signal switching at the oscillator frequency,

32kHz. During the time when the f

COIL signal is

HIGH, the coil is connected from V

COIL 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.

The Switched H-Bridge Network

Current sources and precision controlled timing

of the SP4438 switched H-bridge network are

designed to reduce EMI emissions, extend EL

lamp life, and reduce the overall power

dissipation of the device.

Current sources were added to the high and low

side of the H-bridge network to ensure control of

the charge and discharge of the EL lamp. The

precision MOSFET timing of the SP4438 allows

for controlled charging and discharging of the

EL lamp to minimize EMI and audible noise.

Refer to Figure 7 for the single ended and

differential output waveforms to the EL lamp.

The Precision Bridge Control Circuitry

This circuitry is driven by the internal oscillator

to control the timing of the charge and discharge

of the EL lamp to eliminate EMI and noise

concerns. This control circuitry drives the H-

bridge timing. Refer to Figure 2 for the internal

block diagram of the SP4438.

Fine Tuning Performance

Circuit performance of the SP4438 can be

improved with some of the following

suggestions:

Increase EL Lamp Light Output:

connecting a fast recovery diode from COIL

(pin 5) to C

INT (pin 6), the internal diode of the

switched H-bridge network is bypassed resulting

in an increase in light output at the EL lamp. We

suggest a fast recovery diode, such as the industry

standard 1N4148, be used for D1. This circuit

connection can be found in Figure 2.

Split Power Supplies: Instead of driving

both the logic and power circuitry of the SP4438

from a single battery supply, two supplies can be

implemented, one to drive the IC and the other

to drive the inductor.

The +2.7V to +3.3V power supply is connected

to V

CC. This power supply can be used to power

the oscillator and control logic. The current

draw on this power supply will be minimal,

typically <1mA for SP4438.

When selecting a separate +2.7V to +9.0V power

supply connected to the inductor to drive the IC

power circuitry, it is essential that the proper

inductor is selected. The peak current draw of

the inductor should not exceed maximum

conditions over the entire input voltage operating

range.

If the inductor selected is too large, the power

current may rise too slowly.

At low input

voltages, this will not allow the peak current to

rise to the set peak current trip point. At high

input voltages, the power current might rise too

quickly and overshoot the set peak current trip

point. These conditions will prevent optimal

light output at the EL lamp. Excessive currents

may permanently affect the normal operation of

the SP4438.

Changing the EL Lamp Output Voltage

Waveform: Designers can alter the trapezoidal

output voltage waveform to the EL Lamp.

Changing the capacitance of the integrating

capacitor, C

INT, will ideally integrate the output

waveform making it appear more sinusoidal.

This will minimize any noise inherent to the

application.

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