SP4425DS/20

SP4425 Electroluminescent Lamp Driver

© Copyright 2000 Sipex Corporation

5

the inductor. The current in the inductor is time

dependent and is set by the "ON" time of the coil

switch: I=(V

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

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

to increase beyond its rated I

will be generated and the power 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

the SP4425. V

not necessary that V

function of the core material and winding used --

performance variances may be noticeable from

different coil suppliers. The Sipex SP4425 is final

tested at 1.5V using a 470

µH/4Ω coil from Toko,

and a 2mH/44

Ω coil from Matsushita at 3V.

For suggested coil sources see page 12.

The f

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

The f

at the oscillator frequency. During the time when

the f

V

created in the coil. During the low part of f

ground connection is switched open, the field

collapses and the energy in the inductor is forced

to flow toward the lamp. f

charge pulses (see figure 2 on page 11) 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

1 on page 11).

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

LAMP

signal which is the oscillator frequency divided

by 64. For a 25.6kHz 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 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 3 on page 11). A differential view of

the outputs is shown in figure 4 on page 11.

Layout Considerations

The SP4425 circuit board layout must observe

careful analog precautions. For applications with

noisy power supply voltages, a 0.1

µF low ESR

decoupling capacitor must be connected from Vdd

to ground. Any high voltage 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 switching 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

pronounced spiking on the EL output waveform;

adding the C

and increase the voltage output at each coil step.

The value of the integrator capacitor is application

specific typical values can range from 500pF to

0.1

µF.Nointegratorcapacitororverysmallvalues

(500pF) will have a minor effect on the output,

whereas a 0.1

µF capacitor will cause the output to

charge and discharge rapidly creating a square

wave output. For most applications an 1800pF

integrator capacitor is suitable.