Zywyn Corporation
ZSP4412A
steps. As the voltage potential approaches its maximum,
the steps become shorter (see Figure 3).
Circuit Description
The ZSP4412A is made up of three basic circuit elements,
a divider chain, a coil, and a switched H-bridge network.
The countdown chain provides the circuit with a clock
source used to control the charge and discharge phases
for the coil and lamp. An external oscillator is required and
is delivered to pin 3 of the SO-8 package or to the “OSC
IN” pad of the bare die. If a clock frequency other than
32kHz is used, the output of the driver as well as the
amount of current used, will be affected.
The H-bridge consists of two proprietary low on-resis-
tance high voltage switches. These two switches control
the polarity of how the lamp is charged. The high voltage
switches are controlled by the f
signal which is the
LAMP
oscillator frequency divided by 128. For a 32kHz oscilla-
tor, f = 250Hz.
LAMP
The direction of current flow is determined by which high
voltage switch is enabled. One full cycle of the H-bridge
will create 16 voltage steps from ground to 80V (typical)
on pins 6 and 7 which are 180 degrees out of phase with
each other (see Figure 5). A differential view of the
outputs is shown in Figure 6.
The suggested oscillator frequency is 32,768Hz. This
clock frequency is internally divided to create two internal
control signals, f
and f
. The output is internally
COIL
LAMP
divided down by 7 flip-flops; therefore, a 32,768Hz signal
will be divided into the following frequencies; 32, 16, 8, 4,
2, 1, 0.5 and 0.25kHz. The second flip flop output (8kHz)
is used to drive the coil (see Figure 4 ) and the seventh flip
flop output (256Hz) is used to drive the lamp. Although the
oscillator frequency can be varied to optimize the lamp
Electroluminescent Technology
What is electroluminescence?
An EL lamp is basically a strip of plastic that is coated with
a phosphorous material which emits light (fluoresces)
when a high voltage (>40V) which was first applied across
it, is removed or reversed. Long periods of DC voltages
applied to the material tend to breakdown the material and
reduce its lifetime. With these considerations in mind, the
ideal signal to drive an EL lamp is a high voltage sine
wave. Traditional approaches to achieving this type of
waveform included discrete circuits incorporating a trans-
former, transistors, and several resistors and capacitors.
This approach is large and bulky, and cannot be imple-
mented in most hand held equipment. Zywyn now offers
low power single chip driver circuits specifically designed
to drive small to medium sized electroluminescent pan-
els. All that is required is an external inductor and an
external clock signal. Electroluminescent backlighting is
ideal when used with LCD displays, keypads, or other
backlit readouts. Its main use is to illuminate displays in
dim to dark conditions for momentary periods of time. EL
lamps typically consume less current than LEDs or incan-
descent bulbs making them ideal for battery powered
products. Also, EL lamps are able to evenly light an area
without creating “hot spots” in the display. The amount of
light emitted is a function of the voltage applied to the
lamp, the frequency at which it is applied, the lamp
material used and its size, and lastly, the inductor used.
There are many variables which can be optimized for
specific applications.
output, the ratio of f
/f
will always equal 32.
COIL LAMP
The external clock should have a 50% duty cycle and
range from V to ground. The maximum external clock
DD
frequency is 128kHz. The coil is an external component
connected from V to pin 5 of the ZSP4412A. Energy
BATT
is stored in the coil according to the equation
2
E = 1/2(LI ) where I , to the first approximation, is the
L
P
P
product I = (t )((V
– V )/L), where t
is the time
P
ON
BATT
CE
ON
it takes for the coil to reach its peak current, V
is the
CE
voltage drop across the internal NPN switch transistor,
and L is the 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 net-
work. This energy recovery is directly related to the
brightness of the EL lamp output. There are many varia-
tions among coils; magnetic material differences, winding
differences and parasitic capacitances. The Zywyn
ZSP4412A is final tested using a 30mH/125Ω coil. For
suggested coil sources see, “Coil Manufacturers.”
The f
signal controls a switch that connects the end of
COIL
the coil at pin 5 to ground or to open circuit. The f
COIL
signal is a 75% duty cycle square wave, switching at 1/4
the oscillator frequency, (for a 32kHz oscillator f is
COIL
8kHz). During the time when the f
signal is high, the
COIL
coil is connected from V
to ground and a charged
BATT
magnetic field is created in the coil. During the low part of
, the ground connection is switched open, the field
f
COIL
collapses, and the energy in the inductor is forced to flow
toward the high voltage H-bridge switches. f will send
COIL
16 of these charge pulses to the lamp, each pulse in-
creases the voltage drop across the lamp in discrete
Zywyn
4
January2005
rev. 03
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