TK6591x
THEORY OF OPERATION
An Electroluminescent (EL) Lamp is a strip of plastic,
coated with a phosphorous material that emits light when
a high voltage AC signal is applied to the terminals of the
device. EL panels have the ability to light the entire panel
uniformly. Because of this, they are gradually becoming
more popular and widespread than LEDs. The amount of
light emitted from an EL Lamp is typically proportional to
the magnitude of the voltage applied to the lamp.
Furthermore, the color of the light emitted by an EL Lamp
is somewhat dependent upon the frequency of the applied
drivesignal. Formostapplications, apeak-to-peakvoltage
of 100 to 170 V, with a drive frequency of 175 to 400 Hz,
provides optimal trade-off between lamp intensity and
power consumption.
clock is generated by dividing the high frequency clock by
128; this lower frequency clock corresponds to the drive
frequency of the EL Lamp. The laser-trimmed oscillators
are relatively insensitive to variations in temperature and
supply voltage. Therefore, they provide good control of the
lamp color emitted by the panel.
The circuit below illustrates a typical application where the
TK6591x is driving a 1-square-inch EL Lamp with a
capacitance of approximately 5 nF.
+
V
EL
CC
V
IN
HV
-
GND
IND
C
EL
5 nF
The capacitance of the EL Panel is typically proportional to
the size of the lamp (a 1 square inch EL Panel typically
exhibits approximately 5 nF of capacitance load). The
TK6591x series of devices has been optimized to drive EL
panels, whichareapproximately1-2squareinchesinsize.
EL
L
1
D
1
C
1
22 nF
The Boost section of the TK6591x consists of a controller
for stepping up a relatively low voltage (2.7 to 6 V) to a
much higher voltage (50 to 90 V) needed to drive the EL
Lamp.TheboostsectionoftheTK6591xusesaproprietary
architecture which provides a relatively constant output
power, independent of the input supply, without the need
for sensing the high voltage output of the boost converter.
By controlling the peak current through the switching
element of the boost converter, the boost section provides
a constant output power independent of the input supply.
FIGURE 1: TYPICAL APPLICATION
By keeping the ratio of the boost frequency and the H-
Bridgefrequencyconstant,thepeak-to-peakoutputvoltage
from the TK6591x becomes primarily dependent upon the
capacitance of the EL Lamp, the peak current threshold of
the boost converter, and the value of the inductive element
used in the boost converter. For the TK6591x, the peak
currentthresholdislaser-trimmedto32mA.Thecapacitive
load of the EL Lamp is a function of panel size and is
typically fixed. Therefore, the high voltage output of the
boostconvertercanbesettoadesiredvoltagebyselecting
the appropriate value of the inductive element used in the
boost converter.
The H-Bridge section of the TK6591x switches the high
voltage output of the boost converter to the two terminals
of the EL Lamp. By alternately switching the terminals of
the lamp between the high voltage supply and ground, the
peak-to-peak voltage developed across the lamp is
effectively twice the high voltage generated by boost
converter. Furthermore, the TK6591x limits the magnitude
of the drive currents through the H-Bridge switches in
order to minimize the edge rates developed across the EL
Lamp. This approach protects the EL Panel from large
currentspikesandreducesthelikelihoodofhighfrequency
noisecomponentsbeinginjectedintoneighboringcircuitry.
I
PEAK = Boost Peak Current Threshold (32 mA)
CEL = Capacitance of EL Lamp
L = Inductance Value
The Oscillator section of the TK6591x generates a fixed
frequency clock source for the previously described Boost
and H-Bridge sections, without the need for external
components. The high frequency output of the oscillator is
used for driving the boost controller. A lower frequency
VHV = (IPEAK / 2) x (L /CEL) x 128
May 2000 TOKO, Inc.
Page 9
TOKO [ TOKO, INC ]
