SP4405 Datasheet, PDF (6/20 Pages) Sipex Corporation – Low Voltage Electroluminescent Lamp Driver with Regulated Output Voltage

35kHz. 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
COIL
pulses (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 oscillator frequency
LAMP
divided by 128. For a 48.6kHz oscillator, the
7 flip flops will drive f_LAMP= 380Hz.
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 64 voltage
steps from ground to 80V (typical) on EL1 and
EL2 (pins 4 and 5, respectively) which are 180
degrees out of phase (see Figure 5). A differential
representation of the output is shown in Figure 6.
The feedback loop connects the H-bridge
network to the error comparator through an
internal resistor network. When the error
comparator senses a lamp voltage below
regulation (< 80V_P typical), the error comparator
enables the oscillator to perform step-up
conversions to the lamp output. When the error
comparator senses lamp voltages above
regulation (> 80V_P typical), it disables the
oscillator minimizing power consumption until
the lamp voltage drops below regulation. With
a varying supply voltage, the EL lamp brightness
will appear constant as a direct result of this
regulated output voltage.
Fine Tuning Performance
Circuit performance of the SP4405 can be
improved with some of the following
suggestions:
Increase EL Lamp Light Output: By
connecting a fast recovery diode from COIL
(pin 3) to C_INT (pin 7), 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.
Changing the EL Lamp Output Voltage
Waveform: Designers can alter the sawtooth
output voltage waveform to the EL Lamp.
Increasing the capacitance of the integration
capacitor C_INT will integrate the sawtooth
waveform making it appear more like a square
wave.
Printed Circuit Board Layout Suggestions:
The SP4405'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 in Figure 2 less
INT
than 0.2in (5mm) apart. Also keep the
connections to COIL (pin 3) 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_SS (pin 2) directly to
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 25).
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.
SP4405DS/18
SP4405 Low Voltage Electroluminescent Lamp Driver
6
© Copyright 2000 Sipex Corporation