zsp4425 ywyn Corporation, zsp4425 Datasheet - Page 5

• Manufacturer Part Number: zsp4425
• Description: Electroluminescent Lamp Driver For 1.5v Or 2.2v To 4.5v Applications
• Manufacturer: ywyn Corporation

Circuit Description
The ZSP4425 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 1 and V
frequency. For a given choice of coil inductance there will
be an optimum C
mum light output.
The suggested oscillator frequency is 25.6kHz (C
=180pF). The oscillator output is internally divided to
create the control signal for f
internally divided down by 6 flip-flops, a 25.6kHz signal will
be divided into 6 frequency levels: 12.8kHz, 6.4kHz, 3.2kHz,
1.6kHz, 800Hz, and 400Hz. The oscillator output (25.6kHz)
is used to drive the coil (see Figure 2) and the sixth flip-
flop output (300Hz) is used to drive the lamp. Although the
oscillator frequency can be varied to optimize the lamp
output, the ratio of f
The coil is an external component connected from
V
with a 470μH/4Ω coil are typical conditions. Energy is
stored in the coil according to the equation E
where I is the 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
voltage across the inductor. At the moment the switch
closes, the current in the inductor is zero and the entire
supply voltage (minus the V
inductor. The 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
across the inductor is decreasing, the current ramp-rate
also decreases which reduces the current in the coil at the
end of t
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 param-
eters of the application such as V
cause the current in the coil to increase beyond its rated
I
efficiency will decrease with no additional light output.
The majority of the current goes through the coil and
typically less than 2mA is required for V
V
necessary that V
the core material and winding used — performance vari-
ances may be noticeable from different coil suppliers. The
Zywyn ZSP4425 is final tested at 1.5V using a 470μH/4Ω
coil from Toko, and a 2mH/44Ω coil from Matsushita at +3V.
For suggested coil sources, see “Coil Manufacturers.”
SAT
Zywyn Corporation
BATTERY
DD
, excessive heat will be generated and the power
can range from +1.5V, or +2.2V to +4.5V; it is not
Zywyn
ON
L
to pin 3 of the ZSP4425. V
the energy stored in the inductor per coil cycle
= V
SS
BATTERY
DD
OSC
allows the user to vary the oscillator
= V
COIL
capacitor value that gives the maxi-
BATTERY
– IR
/f
LAMP
SAT
L
LAMP
– V
. Coils are also a function of
will always equal 64.
of the switch) is across the
SAT
. The oscillator output is
L
BATTERY
/L)t
. Since the voltage
BATTERY
ON
DD
, where V
of the ZSP4425.
, L, R
L
=+1.5VDC
=1/2(LI)
L
, or t
L
is the
OSC
ON
2
,
5
The f
the coil at pin 3 to ground or to open circuit. The f
signal is a 90% duty cycle signal switching at the oscillator
frequency. During the time when the f
the coil is connected from V
charged magnetic field is created in the coil. During the
low part of f
the field collapses and the energy in the inductor is forced
to flow toward the lamp. f
pulses (see Figure 6) 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 5).
The H-bridge consists of two proprietary low on-resistance
high voltage switches. These two switches control the
polarity of how the lamp is charged. The high voltage
switches are controlled by the f
oscillator frequency divided by 64. For a 25.6kHz oscilla-
tor, f
released, a high voltage spike is created triggering the
high voltage switches. The direction of current flow is
determined by which high voltage is enabled. One full
cycle of the H-bridge will create a voltage step from ground
to 80V (typical) on pins 5 and 6 which are 180 degrees out
of phase with each other (see Figure 7). A differential view
of the outputs is shown in Figure 8.
Layout Considerations
The ZSP4425 circuit board layout must observe careful
analog precautions. For applications with noisy power
supply voltages, a 0.1μF low ESR decoupling capacitor
must be connected from V
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.
Integrator Capacitor
An integrating capacitor must be placed from pin 4 (D1) to
ground in order to minimize glitches associated with switch-
ing the coil. A capacitor at this point will collect the high
voltage spikes and will maximize the peak-to-peak voltage
output. High resistance EL lamps will produce more pro-
nounced spiking on the EL output waveform; adding the
C
voltage output at each coil step. The value of the integrator
capacitor is application specific typical values can range
from 500pF to 0.1μF. No integrator capacitor or very small
values (500pF) will have a minor effect on the output,
whereas a 0.1μF capacitor will cause the output to charge
and discharge rapidly creating a square wave output. For
most applications an 1800pF integrator capacitor is suit-
able.
INT
COIL
LAMP
capacitor will minimize the peaking and increase the
signal controls a switch that connects the end of
COIL
= 400Hz. When the energy from the coil is
, the ground connection is switched open,
COIL
DD
will send 32 of these charge
to ground. Any high voltage
BATTERY
LAMP
signal which is the
COIL
to ground and a
signal is high,
October 2005
ZSP4425
rev. 04
COIL