Zywyn Corporation
ZSP4422A
The supply V
necessary that V
exceed max coil current specification. The majority of the
current goes through the coil and is typically much greater
can range from +2.2V to +5.0V. It is not
Circuit Description
DD
= V
. V
should not
DD
BATTERY
BATTERY
The ZSP4422A is made up of three basic circuit elements,
an oscillator, coil,and switched H-bridge network. The
oscillator provides the device with an on-chip clock source
used to control the charge and discharge phases for the
coil and lamp. An external capacitor connected between
pins 7 and 8 allows the user to vary the oscillator frequency
than I
.
DD
The f
signal controls a switch that connects the end of
COIL
the coil at pin 3 to ground or to open circuit. The f
signal is a 94% duty cycle signal switching at 1/8 the
oscillator frequency. For a 64kHz oscillator f is 8kHz.
During the time when the f
connected from V
netic field is created in the coil. During the low part of f
the ground connection is switched open, the field col-
lapses and the energy in the inductor is forced to flow
COIL
from 32kHz to 400kHz. In general, increasing the C
capacitor will increase the lamp output.
OSC
COIL
signal is high, the coil is
COIL
The suggested oscillator frequency is 90kHz (C
=100pF). The oscillator output is internally divided to
create two internal control signals, f
oscillator output is internally divided down by 8 flip-flops,
a 90kHz signal will be divided into 8 frequencies; 45kHz,
22.5kHz, 11.2kHz, 5.6kHz, 2.8kHz, 1.4kHz, 703Hz, and
352Hz. The third flip-flop output (8kHz) is used to drive the
coil (see Figure 1) and the eighth flip-flop output (250Hz)
is used to drive the lamp. Although the oscillator fre-
quency can be varied to optimize the lamp output, the
OSC
to ground and a charged mag-
BATTERY
,
COIL
and f
. The
COIL
LAMP
toward the high voltage H-bridge switches. f
will send
COIL
16 of these charge pulses (see Figure 5) 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 4).
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
ratio of f
/f
will always equal 32.
COIL LAMP
The on-chip oscillator of the ZSP4422A can be overdriven
with an external clock source by removing the C
capacitor and connecting a clock source to pin 8. The
clock should have a 50% duty cycle and range from
OSC
switches are controlled by the f
signal which is the
LAMP
oscillator frequency divided by 256. For a 64kHz oscillator,
= 256Hz. The direction of current flow is determined
f
LAMP
V
to ground. An external clock signal may be desirable
DD
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 4 and 5 which are 180 degrees out of
phase from each other (see Figure 6). A differential repre-
sentation of the outputs is shown in Figure 7.
in order to synchronize any parasitic switching noise with
the system clock. The maximum external clock frequency
that can be supplied is 400kHz.
The coil is an external component connected from
V
to pin 3 of the ZSP4422A. Energy is stored in the
BATTERY
2
Layout Considerations
coil according to the equation E =1/2LI , where I is the
L
peak current flowing in the inductor. The current in the
inductor is time dependent and is set by the “ON” time of
the coil switch: I = (V /L)t , where V is the voltage
The ZSP4422A circuit board layout must observe careful
analog precautions. For applications with noisy voltage
power supplies a 0.1µF low ESR decoupling capacitor
L
ON
L
across the inductor. At the moment the switch closes, the
current in the inductor is zero and the entire supply voltage
must be connected from V to ground. Any high voltage
DD
traces should be isolated from any digital clock traces or
enable lines. A solid ground plane connection is strongly
recommended. All traces to the coil or to the high voltage
outputs should be kept as short as possible to minimize
capacitive coupling to digital clock lines and to reduce EMI
emissions.
(minus the V
of the switch) is across the inductor. The
SAT
current in the inductor will then ramp up at a linear rate. As
the current in the inductor builds up, the voltage across the
inductor will decrease due to the resistance of the coil and
the “ON” resistance of the switch: V = V
– IR –
L
BATTERY
L
V
. Since the voltage across the inductor is decreasing,
SAT
Electroluminescent Technology
the current ramp-rate also decreases which reduces the
What is Electroluminescence?
current in the coil at the end of t the energy stored in the
ON
inductor per coil cycle and therefore the light output. The
other important issue is that maximum current (saturation
current) in the coil is set by the design and manufacturer
of the coil. If the parameters of the application such as
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.
V
, L, RL or t
cause the current in the coil to
BATTERY
ON
increase beyond its rated I
, excessive heat will be
SAT
generated and the power efficiency will decrease with no
additional light output. The Zywyn ZSP4422A is final
tested using a 5mH/18Ω coil from Hitachi Metals. For
suggested coil sources see, “Coil Manfacturers.”
Zywyn
4
March 2004
rev. 01
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