SP4490EU datasheet(6/12 Pages) SIPEX

6 / 12 page

Rev. 9/12/00

SP4490 Two Panel Electroluminescent Lamp Driver

Since small coils (<1mH) have inherently low

series DC resistance, the current can peak dra-

matically through a small coil during saturation.

This situation results in wasted energy not stored

in the magnetics of the coil but expressed as

heating which could lead to failure of the coil.

Generally, selecting a coil with lower series DC

resistance will result in a system with higher

efficiency and lamp brightness.

Lamp Effects

EL lamp parameters vary between manufactur-

ers. Series DC resistance, lighting efficiency

and lamp capacitance per area differ the most

overall. Larger lamps require more energy to

illuminate. Lowering the oscillator frequency

allows more energy to be stored in the coil during

each coil switch cycle and increases lamp bright-

ness. The oscillator frequency can be lowered to

a point where the lamp brightness then begins to

drop because the lamp frequency must be above

a critical frequency (approx. 100Hz) to light.

Lamp color is affected by the switching fre-

quency of the EL driver. Green EL lamps will

emit a more blue light as EL lamp frequency

increases.

Noise Decoupling on Logic Inputs

If EL1/EL2 or ON/OFF are connected to traces

susceptible to noise, it may be necessary to

connect bypass capacitor of approximately 10nF

between EL1/EL2 and VSS, and ON/OFF and

VSS. If these inputs are driven by a micropro-

cessor which provides a low impedance HIGH

and LOW signal, then noise bypassing may not

ber be necessary.

Increasing Light Output

EL lamp light output can be improved by con-

necting a fast recovery diode from the COIL pin

to the CAP pin. The internal diode is bypassed

resulting in an increase in light output at the EL

lamp. We suggest a fast recovery diode such as

the industry standard 1N4148.

The optimal value of C

INT will vary depending on

the lamp parameters and coil value. Lower C

INT

values can decrease average supply current but

H-Bridge. An external resistor from VDD to

ROSC sets the oscillator frequency. Typically a

464k

Ω resistor sets the frequency to 44.0kHz.

The high frequency clock directly controls the

coil switch. This high frequency clock is divided

by 128 to generate a low frequency clock which

controls the EL H-Bridge and sets the EL lamp

frequency. The oscillator has low sensitivity to

temperature and supply voltage variations, in-

creasing the performance of the EL driver over

the operating parameters.

Dual H-Bridge

The H-Bridge consists of two SCR structures

and two NPN transistors that control how the

lamp is charged. Setting ON/OFF to HIGH

activates the H-Bridge that is selected by EL1/

EL2. If the EL1/EL2 is low, EL1 is illuminated.

If EL1/EL2 is HIGH, EL2 is illuminated.

The

EL driver illuminates the lamp by applying the

high voltage supply of the boost converter to the

lamp terminals through the H-Bridge and then

switching the terminal polarity between the high

voltage supply and ground at a constant fre-

quency. This applies an AC voltage to the lamp

that is twice the peak output voltage of the boost

driver. An AC voltage greater than the 40V

across the terminals of the lamp is typically

necessary to adequately illuminate the EL lamp.

Setting ON/OFF to LOW disables the outputs

and places the circuit in a low power state.

DESIGN CONSIDERATIONS

Inductor Selection

If limiting peak current draw from the power

supply is important, small coil values (<1mH)

may need a higher oscillator frequency. Inductor

current ramps faster in a lower inductance coil

than a higher inductance coil for a given coil

switch on time period, resulting in higher peak

coil currents.

It is important to observe the saturation current

rating of a coil. When this current is exceeded,

the coil is incapable of storing any more energy

and then ceases to act as an inductor. Instead, the

coil behaves according to its series DC resis-

tance.

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