SP4423DS/14

SP4423 Electroluminescent Lamp Driver

© Copyright 2000 Sipex Corporation

4

The on-chip oscillator of the SP4423 can be

overdriven with an external clock source by

removing the C

clock source to pin 8 (Cap 2). The clock should

have a 50% duty cycle and range fromV

ground. An external clock signal may be

desirable in order to synchronize any parasitic

switching noise with the system clock. The

maximum external clock frequencies that can

be supplied is 400kHz.

The coil is an external component connected from

V

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

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

IR

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 parameters of the application such

as V

coil to increase beyond its rated I

heat will be generated and the power efficiency

will decrease with no additional light output. The

Sipex SP4423 is final tested using a 20mH/70

Ω

coil from CTC. For suggested coil sources see

page 10.

The supply V

necessary that Vdd = V

exceed max coil current specification. The majority

of the current goes through the coil and is typically

much greater than I

The f

the end of the coil at pin 3 to ground or to open

circuit. The f

signal, switching at 1/8 the oscillator frequency.

For a 64kHz oscillator f

time when the f

connected from V

magnetic field is created in the coil. During the

low part of f

switched open, the field collapses and the

energy in the inductor is forced to flow toward

the high voltage H-bridge switches. f

send 16 of these charge pulses 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

shorter (see Figure 1 on page 9).

The H-bridge consists of two SCR structures

that act as high voltage switches. These two

switches control the polarity of how the lamp is

charged. The SCR switches are controlled by

the f

divided by 256. For a 64kHz oscillator,

f

When the energy from the coil is released, a high

voltage spike is created triggering the SCR

switches. The direction of current flow is

determined by which SCR 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 with each

other (see Figure 3 on page 9). A differential

representation of the outputs is shown in Figure

4 on page 9. To minimize AC interference it is

advisable to use a decoupling filter capacitor

between V

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 transformer,

transistors, and several resistors and capacitors.

This approach is large and bulky, and cannot be

implemented in most hand held equipment. Sipex

now offers low power single chip driver circuits

specifically designed to drive small to medium

sized electroluminescent panels. All that is

required is one external inductor and capacitor.