2557

PROTECTED QUAD DRIVER

WITH FAULT DETECTION

& SLEEP MODE

www.allegromicro.com

5

The A2557 low-current quad power drivers provide the

same protected output driver function as (and are pin compat-

ible with) the UDx2543/49/59 devices, combined with a fault

diagnostic scheme similar to the UDx2547, plus an automatic

low-current Sleep-Mode function. These devices monitor their

outputs for fault (open or shorted) conditions. For each channel

the input and output levels are compared. If these are different

from the expected levels then a fault condition is flagged by

pulling the common FAULT output low.

Status

ENABLE

FAULT

Normal Load

HH

L

H

LH

H

H

Sleep Mode

XL

H

H

All L

X

H

H

Over-Current or

Short to Supply

HH

R

L

Open Load or

Short to Ground

LH

L

L

Thermal Fault

H

H

H

L

R = Linear drive, current limited.

The FAULT output is operational only if ENABLE is high.

using a comparator whose threshold is typically 2.5 V. In order

to detect open-circuit outputs, a 30

µA current sink pulls the

output below the comparator threshold. To ensure correct fault

operation, a minimum load of approximately 1 mA is required.

The fault function is disabled when in ‘sleep’ mode, i.e.,

FAULT goes high and the 30

µA output sinks are turned off.

The FAULT output is a switched current sink of typically

60

µA.

Each channel consists of a TTL/CMOS-compatible logic

input gated with a common ENABLE input. A logic high at the

input will provide drive to turn on the output npn switch. Each

output has a current-limit circuit that limits the output current

by detecting the voltage drop across a low-value internal

resistor in the emitter of the output switch. If this drop reaches

a threshold, then the base drive to the output switch is reduced

to maintain constant current in the output.

To keep the device within its safe operating area (SOA) this

output current limit is further reduced

CIRCUIT DESCRIPTION AND APPLICATION

•

if the power dissipation in the output device increases the

local junction temperature above 165

°C (nominal), so as to limit

the power dissipation (and hence the local junction tempera-

ture). As each channel has its own thermal limit circuitry this

provides some independence between the output channels, i.e.,

one channel can be operating in thermally reduced current limit,

while the others can provide full drive capability.

•

as a function of the output voltage. Full current limit of

32 V. This helps to improve SOA by immediately reducing the

A logic low at the ENABLE input causes all outputs to be

switched off regardless of the state of the IN terminals. In

addition, the device is put into a low quiescent current ‘sleep’

µA. If ENABLE is taken high

and any of the inputs go high, the circuit will ‘auto-wake-up’.

However, if the device is enabled, but all inputs stay low, then

the circuit remains in ‘sleep’ mode.

All outputs have internal flyback diodes, with a common-

cathode connection at the K terminal.

Incandescent lamp driver

High incandescent lamp turn-on (in-rush currents) can

contribute to poor lamp reliability and destroy semiconductor

lamp drivers. When an incandescent lamp is initially turned on,

the cold filament is at minimum resistance and would normally

allow a 10x to 12x in-rush current.

Warming (parallel) or current-limiting (series) resistors

protect both driver and lamp but use significant power either

when the lamp is off or when the lamp is on, respectively.

Lamps with steady-state current ratings up to 300 mA can be

driven without the need for warming or current-limiting

resistors, if lamp turn-on time is not a concern (10s of ms).

With these drivers, during turn-on, the high in-rush current

is sensed by the internal sense resistor, drive current to the

output stage is reduced, and the output operates in a linear mode

with the load current limited to approximately 500 mA. During

lamp warmup, the filament resistance increases to its maximum

value, the output driver goes into saturation and applies maxi-

mum rated voltage to the lamp.