SP4414 Datasheet, PDF (6/14 Pages) Sipex Corporation – Low Cost Electroluminescent Lamp Driver

The f signal controls a switch that connects
COIL
the end of the coil at pin 1 to ground or to open
circuit. The f_COIL signal is a 87% duty cycle
signal switching at the oscillator frequency,
8kHz. During the time when the f_COIL signal is
HIGH, the coil is connected from V_BATTERY 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. f will send an array of charge pulses
COIL
(see Figure 4) to the lamp. Each pulse increases
the voltage drop across the lamp in discrete
steps. As the voltage potential approaches its
maximum, the steps become smaller (see Figure 3).
The Switched H-Bridge Network
The H-bridge consists of two SCR structures
that act as high voltage switches. These two
switches control the polarity of how the lamp is
charged. The SCR switches are controlled by
the f signal which is the external oscillator
LAMP
frequency divided by 128. For a 32kHz oscillator,
the 7 flip flops will drive f_LAMP = 250Hz.
When the energy from the coil is released, a high
voltage spike is created triggering the SCR
switches. The direction of current flow is
determined by which SCR is enabled. One full
cycle of the H-bridge will create 32 voltage
steps from ground to 80V (typical) on EL1
and EL2 (pins 6 and 3, respectively) which
are 180 degrees out of phase (see Figure 5).
A differential representation of the output is
shown in Figure 6.
Fine Tuning Performance
Circuit performance of the SP4414 can be
improved with some of the following
suggestions:
Increase EL Lamp Light Output: By
connecting a fast recovery diode from COIL
(pin 4) to C_INT (pin 5), 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.
Printed Circuit Board Layout Suggestions:
The SP4414's high-frequency operation makes
PC layout important for minimizing ground
bounce and noise. Keep the IC's GND pin and
the ground leads of C1 and C_INT in Figure 2 less
than 0.2in (5mm) apart. Also keep the
connections to COIL (pin 4) as short as possible.
To maximize output power and efficiency and
minimize output ripple voltage, use a ground
plane and solder the IC's V (pin 2) directly to
SS
the ground plane.
EL Lamp Driver Design Challenges
There are many variables which can be optimized
for specific applications. The amount of light
emitted is a function of the voltage applied to the
lamp by the inductor, the frequency at which it
is applied, the lamp material, and the lamp size.
Sipex supplies characterization charts to aid the
designer in selecting the optimum circuit
configuration (see Figures 7 to 9).
Sipex will perform customer application
evaluations, using the customer's actual EL
lamp to determine the optimum operating
conditions for specific applications. For
customers considering an EL backlighting
solution for the first time, Sipex is able to offer
retrofitted solutions to the customer's existing
LED or non-backlit product for a thorough
electrical and cosmetic evaluation. Please
contact your local Sales Representative for
Sipex or the Sipex factory directly to initiate
this valued service.
SP4414DS/17
SP4414 Electroluminescent Lamp Driver
6
© Copyright 2000 Sipex Corporation