SP4422A Datasheet, PDF(4/13 Page) Sipex Corporation

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SP4422A Datasheet, PDF (4/13 Pages) Sipex Corporation – Electroluminescent Lamp Driver

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

duty cycle and range fromVDD-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

VBATTERY to pin 3 of the SP4422A. Energy is stored

in the coil according to the equation E =1/2LI2,

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=(VL/

L)tON, 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 VSAT 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 =V -IR -V . Since the

L BATTERY L SAT

voltage across the inductor is decreasing, the current

ramp-rate also decreases which reduces the current

in the coil at the end of tON 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 VBATTERY, L,

RL or ton cause the current in the coil to increase

beyond its rated ISAT, 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/18â¦ coil from Hitachi

Metals. For suggested coil sources see page 10.

The supply VDD can range from 2.2 to 5.0V. It is not

necessary that VDD = 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 .

The fCOIL signal controls a switch that connects the

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

The f signal is a 94% duty cycle signal switching

COIL

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

oscillator fCOIL is 8kHz. During the time when the

f signal is high, the coil is connected from

COIL

to ground and a charged magnetic field is

BATTERY

created in the coil. During the low part of fCOIL , 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. fCOIL 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, fLAMP=256Hz.

HON=VDD=ON

HON=0V=OFF

VIN=3V +

NOTE:

Keep coil as close to the

SP4422A as possible

HON

VSS

Coil

COSC =100pF

Cap2

Cap1

VDD

0.1ÂµF Low ESR

Decoupling

Capacitor

EL2

EL1

SP4422A

EL Lamp

NOTE:

Keep high voltage traces

short and away from VDD

and clock lines

Typical SP4422A Application Circuit

SP4422ADS/15

SP4422A Electroluminescent Lamp Driver

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