MC3392

9

MOTOROLA ANALOG IC DEVICE DATA

Instrument Panel Lamp Dimmer Control

The MC3392 can be used to control the dimming function

associated with instrument panel lamps. The brightness of

incandescent lamps can be varied by pulse width modulating

the input of the MC3392. The modulating signal for the

MC3392 can be obtained directly from a microprocessor or,

as in Figure 16, from an MC1455 timer. The MC1455 timer is

configured as a free–running clock having both frequency

and duty cycle control. The typical timer frequency is

approximately 80 Hz when the frequency potentiometer is

adjusted to 1.0 k. This frequency was chosen so as to avoid

any perceptible lamp flicker. The duty cycle potentiometer

controls the duty cycle over a range of approximately 3.0% to

97%; When at 3.0% duty cycle, the lamps are essentially off;

When at 97% duty cycle, the lamps are essentially full lit. Six

incandescent lamps are shown in this application drawing

720 mA total current. Similar applications can be used to

drive a variety of lamp loads. The total load current is the

primary factor of consideration when driving lamp loads. The

total value of IO must be less than ISC.

Another convenient aspect of this application is the LED.

The LED can be used to denote the existence of a system

fault (overvoltage, current limiting, or thermal shutdown).

Figure 16. Instrument Panel Lamp Dimmer Control

Fault

+

–

5.0V

1.0k

+

–

12V

Gnd

+

–

5.0V

CMOS Input

Fast

Turnoff

Over

Voltage

Detect

Thermal

Shutdown

Current

Limit

Instrument

Panel Lamps

Vout

Vin

Duty Cycle

1.0M

1N4001

Frequency

1.0k

1.0k

0.01

µF

MC1455

1

2

3

45

6

7

8

1N4001