Protected Quad Driver

with Fault Detection and Sleep Mode

A2557

8

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

CIRCUIT DESCRIPTION AND APPLICATION (continued)

Fault diagnostics

A pull-up resistor or current source is required on the FAULT

output. This can be connected to whatever supply level the fol-

lowing circuitry requires (within the specification constraints).

For a 5 V supply (i.e., Vcc) 150 kΩ or greater should be used.

As the fault diagnostic function is to indicate when the output

state is different from the input state for any channel, the FAULT

output waveform will obviously produce a pulse waveform fol-

lowing the combined duty-cycle of all channels showing a fault

condition. There are therefore two basic approaches to using the

function in an application:

•

As an interrupt in a controller-based system. If the system

has a microcontroller then a FAULT low causes an interrupt,

which then initiates a diagnostic sequence to find the culprit

channel. This sequence usually consists of cycling through each

channel one at a time, while monitoring the FAULT output. It

is then easy to determine which channel has the faulty output

and how it is failing (i.e., short to supply, open-circuit or short to

ground). The system may then take whatever action is required,

but could continue with operation of the remaining ‘good’ chan-

nels while disabling signals to the faulty channel.

•

As a simple ‘common’ fault indication. If there is no con-

troller in the system then the FAULT output can be set to give an

indication (via a lamp or LED, etc.) of a fault condition which

might be anywhere on the four channels. Because the FAULT

output is dependent on the states of the input and output (four

possibilities) but will only indicate on two of them, the duty

cycle at the FAULT output will reflect the duty cycle at the faulty

channel’s input (or its inverse, depending upon fault type).

In typical applications (50% duty cycles) a simple solution is to

make the pull-up current on the FAULT output much less than

the pull-down current (60 μA), and add a capacitor to give a

time constant longer than the period of operation. For typical

values, the device will produce a continuous dc output level.

Component values will need to be adjusted to cope with different

conditions.

Inductive load driver

Bifilar (unipolar) stepper motors (and other inductive loads)

can be driven directly. The internal diodes prevent damage to

the output transistors by suppressing the high-voltage spikes

that occur when turning off an inductive load. For rapid current

decay (fast turn-off speeds), the use of Zener diodes will raise

the flyback voltage and improve performance. However, the

peak voltage must not exceed the specified minimum sustaining

Over-current conditions

In the event of a shorted load, or stalled motor, the load current

will attempt to increase. As described above, the drive current to

the affected output stage is linearly reduced, causing the output

to go linear (limiting the load current to about 500 mA). As the

junction temperature of the output stage increases, the thermal-

shutdown circuit will shut off the affected output. If the fault

condition is corrected, the output driver will return to its normal

saturated condition.

NORMAL LAMP IN-RUSH CURRENT

TIME

THERMAL GRADIENT SENSING

CURRENT LIMIT

Dwg. WP-008

NOT TO SCALE

0