MC33441

http://onsemi.com

5

will be turned off and the energy will release to the EL lamp.

And, those switches will remain off until the next clock cycle.

H–Bridge Network

To achieve the 140V peak–to–peak voltage, H–bridge

network is used to charge and discharge the EL lamp. The

switching frequency of the bridge network is controlled by a

clock signal from the divider with its frequency much lower than

the one to the coil–driver. Moreover, to reduce the current

consumption, the biasing current to the two low–side switches

of the H–bridge is not activated until the coil–driver circuit

needed to release the energy to the EL lamp. Then, the biasing

circuit will be on and be ready before the main switch in the

coil–driver really starts to turn off.

External Components

System designer will base on the application to decide the size

and the type of the EL lamp to be used. The external resistance

(REXT)atRT1pindeterminestheexcitationfrequency(FEL)for

the lamp. The relationship between REXT and the frequency is:

FEL = FOSC B 384

By substitute the equation of FOSC from Oscillator &

Frequency Divider.

F

EL +

4.341

R

EXT

Hz

so

R

EXT +

4.341

F

EL

W

Moreover, if a low TC resistor is used, the oscillator

frequency’s TC can be kept low. The filter capacitor is to provide

a smooth and more stable output waveform for the EL lamp.

The value of this capacitor depends on the input voltage and the

coil’s inductance value. Equations below can be used to estimate

filter capacitor’s value at different input voltage.

Best Case Approximation for the Filter Capacitor:

C

FILTER +

0.026

(V

in *

V

SW

F

OSC

Worst Case Approximation for the Filter Capacitor:

C

FILTER +

0.085

(V

in *

V

SW

F

OSC

where VIN is the input voltage, VSW is voltage across the

switch when it is on, L is the coil’s value and FOSC is the

clock frequency.

Measurement below is recorded with the condition: coil

= 1mH, EL lamp = 2.2nF and at room temperature.

Table 1: Reference for CFILTER

VDD

REXT

CFILTER

1.8V

100K–130K

W

5n–10nF

2.0V

100K–130K

W

10n–22nF

2.5V

100K–130K

W

10n–22nF

3.0V

100K–130K

W

22nF–33nF