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4
Theory of Operation
Electroluminescent (EL) lamps are essentially capacitors with one transparent electrode and a special phosphor material
in the dielectric. When a strong AC voltage is applied across the EL lamp electrodes, the phosphor glows. The
required AC voltage is typically not present in most systems and must be generated from a low voltage DC source.
Thus, Durel developed its patented Three-Port (3P) switch-mode driver circuit to convert the available DC supply to
an optimal drive signal for high brightness and low-noise EL lamp applications. The Durel 3P topology offers the
simplicity of a single DC input, single AC output, and a shared common ground that provides an integrated EMI
shielding.
The D371 drives the EL lamp by repeatedly pumping charge through an external inductor with current from a DC
source and discharging into the capacitance of the EL lamp load. With each high frequency (HF) cycle the voltage on
the lamp is increased. At a period specified by the lamp frequency (LF) oscillator, the voltage on the lamp is discharged
to ground and the polarity of the inductive charging is reversed. By this means, an alternating positive and negative
voltage is developed at the single output lead of the device to one of the electrodes of the EL lamp. The other lamp
electrode is commonly connected to a ground plane, which can then be considered as electrical shielding for any
underlying circuitry in the application.
The EL driving system is divided into several parts: on-chip logic and control, on-chip high voltage output circuitry,
discharge logic circuitry, and off-chip components. The on-chip logic controls the lamp operating frequency (LF), as
well as the inductor switching frequency (HF), and HF and LF duty cycles. These signals are combined and buffered
to regulate the high voltage output circuitry. The output circuitry handles the power through the inductor and delivers
the high voltage to the lamp. The integrated discharge logic circuit enables the low-noise functionality of this EL driver
with four levels of discharge slopes on the output waveform. The selection of off-chip components provides a degree
of flexibility to accommodate various lamp sizes, system voltages, and brightness levels. Since a key objective for EL
driver systems is to save space and cost, required off-chip components were kept to a minimum.
Durel provides a D371 Designer’s Kit, which includes a printed circuit evaluation board intended to aid you in developing
an EL lamp driver configuration using the D371 that meets your requirements. A section on designing with the D371 is
included in this datasheet to serve as a guide to help you select the appropriate external components to complete your
D371 EL driver system.
Typical D371 configurations for driving EL lamps in various applications are shown on the following page. The
expected system outputs, such as lamp luminance, lamp output frequency and voltage, and average supply current
draw, for the various sample configurations are also shown with each respective figure.
Block Diagram of the Driver Circuitry
EL Lamp
E
V+
L+
L-
GND
VOUT
Low
Frequency
Oscillator
CLF
High
Frequency
Oscillator
1.0
μ
F
CHF
Discharge
logic
Constant
dcurrent
R
enable
DCH
V
BAT