SP4422ACX datasheet(4/13 Pages) SIPEX

4 / 13 page

SP4422ADS/15

SP4422A Electroluminescent Lamp Driver

clock source to pin 8. The clock should have a 50%

duty cycle and range fromV

DD-1V to ground. An

external clock signal may be desirable 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

BATTERY to pin 3 of the SP4422A. Energy is stored

in the coil according to the equation E

L=1/2LI

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

L)t

ON, where VL is the 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

SAT of the switch) is across the 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

L=VBATTERY-IRL-VSAT. Since the

voltage across the inductor is decreasing, the current

ramp-rate also decreases which reduces the current

in the coil at the end of t

ON the energy stored in the

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 V

BATTERY, L,

RL or ton cause the current in the coil to increase

beyond its rated I

SAT, excessive heat will be

generated and the power efficiency will decrease

with no additional light output. The Sipex SP4422A

is final tested using a 5mH/18

Ω coil from Hitachi

Metals. For suggested coil sources see page 10.

Typical SP4422A Application Circuit

The supply V

DD can range from 2.2 to 5.0V. It is not

necessary that V

DD = VBATTERY. VBATTERY should not

exceed max coil current specification. The majority

of the current goes through the coil and is typically

much greater than I

DD.

The f

COIL signal controls a switch that connects the

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

The f

COIL signal is a 94% duty cycle signal switching

at 1/8 the oscillator frequency. For a 64kHz

oscillator f

COIL is 8kHz. During the time when the

COIL signal is high, the coil is connected from

BATTERY to ground and a charged magnetic field is

created in the coil. During the low part of f

COIL , the

ground connection is switched open, the field

collapses and the energy in the inductor is forced

to flow toward the high voltage H-bridge

switches. f

COIL will send 16 of these charge pulses

(see figure 2 on page 9) 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

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

LAMP

signal which is the oscillator frequency divided by

256. For a 64kHz oscillator, f

LAMP=256Hz.

0.1

µF Low ESR

Decoupling

Capacitor

EL Lamp

NOTE:

Keep high voltage traces

short and away from VDD

and clock lines

Cap2

Cap1

VDD

EL1

HON

VSS

Coil

EL2

HON=VDD=ON

HON=0V=OFF

SP4422A

VIN=3V

NOTE:

Keep coil as close to the

SP4422A as possible

–

COSC =100pF

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