2557

PROTECTED QUAD DRIVER

WITH FAULT DETECTION

& SLEEP MODE

www.allegromicro.com

7

CIRCUIT DESCRIPTION AND APPLICATION (continued)

NORMAL LOAD

TIME

Dwg. WP-013-1

NOT TO SCALE

CURRENT LIMIT (12 V SUPPLY)

SHORT CIRCUIT

CURRENT LIMIT (24 V SUPPLY)

IN

FAULT

(SHORTED

LOAD)

FAULT

(OPEN

LOAD)

WITH OUTPUT CAPACITOR

Dwg. WP-035

Thermal considerations

Device power dissipation can be calculated as:

reasonable approximation for most purposes.

This can then be compared against the permitted package

power dissipation, using:

28-lead PLCC (part number suffix ‘–EB’) = 36

°C/W

16-pin PDIP (part number suffix ‘–B’) =

43

°C/W

16-lead SOIC (part number suffix ‘–LB’) = 60

°C/W

about 6

°C/W for the three package types, therefore the power

dissipation can be improved by 20% to 30% by adding an area

of printed wiring board copper (typically 6 to 18 square

centimetres) connected to the power-tab GROUND terminals of

the device.

Under some conditions it is possible to get spurious

glitches on the FAULT output at load turn-on and turn-off

transitions:

•

Light load turn-off. Under light loading conditions the

turn-off delay (see characteristics above) of the output stage

increases and may result in a spurious fault output of a few

µs

(the duration being proportional to the turn-off delay). As it is

difficult to define this over all operating conditions, if a

particular application would be sensitive to this type of glitch,

then it is generally recommended to include a small (about

0.01

µF) smoothing/storage capacitor at the FAULT output.

•

Incandescent lamp turn-on. As described above, driving an

incandescent filament results in the driver operating in current

limit for a period after turn-on. During this period a “fault”

condition will be indicated (over current). As discussed above

this period can be 10s of ms. To avoid this indication, the

capacitor on the FAULT output would need to be increased to

provide an appropriate time constant. Alternatively, in a

microcontroller-based system, the code could be written to

ignore the FAULT condition for an appropriate period after

lamp turn on.

Correct FAULT operation cannot be guaranteed with an

unconnected output — unused outputs should not be turned on,

or unused outputs should be pulled high to >2.5 V, and/or

associated inputs tied low.