A2557 Ver la hoja de datos (PDF) - Allegro MicroSystems

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componentes Descripción

Fabricante

A2557

PROTECTED QUAD LOW-SIDE DRIVER WITH FAULT DETECTION and SLEEP MODE

Allegro MicroSystems 

2557

PROTECTED QUAD DRIVER

WITH FAULT DETECTION

& SLEEP MODE

CIRCUIT DESCRIPTION AND APPLICATION (continued)

IN

FAULT

(SHORTED

LOAD)

FAULT

(OPEN

LOAD)

WITH OUTPUT CAPACITOR

Dwg. WP-035

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.

NOT TO SCALE

SHORT CIRCUIT

NORMAL LOAD

CURRENT LIMIT (12 V SUPPLY)

CURRENT LIMIT (24 V SUPPLY)

TIME

Thermal considerations

Dwg. WP-013-1

Device power dissipation can be calculated as:

PD = (VO1 x IO1 x duty cycle1) + … + (VO4 x IO4 x duty cycle4)

+ (VCC x ICC)

Note - ICC is also modulated by the duty cycle, but this is a

reasonable approximation for most purposes.

This can then be compared against the permitted package

power dissipation, using:

Permitted PD = (150 – TA)/RθJA

where RθJA is given as:

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’) = 90°C/W

RθJA is measured on typical two-sided PCB. Additional

information is available on the Allegro web site.

www.allegromicro.com

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