SP4424CX datasheet(4/12 Pages) SIPEX

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SP4424DS/16

SP4424 Electroluminescent Lamp Driver

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 majority of the

current goes through the coil and typically less

than 3 mA is required for V

DD of the SP4424. VDD

can range from 2.2V to 5V; it is not necessary that

DD=VBATTERY. For example, an unregulated voltage

source (3.3V) can be directly connected to the coil,

while a regulated voltage source (2.85V) can be

connected to the IC V

DD pin.

Coil performance is also a function of the core

material and wire used -- performance variances

may be noticeable from different coil suppliers.

The Sipex SP4424 is tested using a 5mH/18

Ω coil

from Hitachi Metals. For suggested coil sources

see page 9.

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 75% duty cycle signal.

During the time when the f

COIL signal is high, the

coil is connected from V

BATTERY to ground and a

magnetic field is generated in the coil. During

the low part of f

COIL , the ground connection is

switched open, the field collapses and the voltage

generated in the inductor is directed to the high

voltage H-bridge switches. f

COIL will send as many

charge pulses as possible in 1 Lamp Cycle.{Number of

Coil pulses in 1 lamp cycle =}

(see figure 2 on page 6). 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 6).

The H-bridge consists of two SCR structures that

act as high voltage switches. These two switches

control the polarity of the lamp (capacitor) as it is

charged. The SCR switches are controlled by the

LAMP signal which is the oscillator frequency

divided by 2.

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 number of

voltage steps from ground to 65V (typical) on pins

4 and 5 which are 180 degrees out of phase (see

figure 3 on page 6). A differential view of the

outputs is shown in figure 4 on page 6.

SP4424 Test Circuit

HON=VDD=ON

HON=0V=OFF

0.1

µF Low ESR

Decoupling

Capacitor

CLAMP

HON

VDD

EL2

CCOIL

VSS

Coil

EL1

220pF

SP4424

VIN=3V

–

1500pF

55nF

100

Ω

Lamp Freq

1 x

Coil Freq. }

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