SP4438 Datasheet, PDF (8/19 Pages) Sipex Corporation – Ultra-Quiet Electroluminescent Lamp Driver For Cellular Phone Applications

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 signal controls a switch that connects
COIL
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 signal is LOW, the ground
COIL
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: By
connecting a fast recovery diode from COIL
(pin 5) to C (pin 6), the internal diode of the
INT
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 . This power supply can be used to power
CC
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.
Rev. 11/13/00
SP4438 Ultra-Quiet Electroluminescent Lamp Driver
8
© Copyright 2000 Sipex Corporation