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

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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)

NORMAL LAMP IN-RUSH CURRENT

NOT TO SCALE

THERMAL GRADIENT SENSING

CURRENT LIMIT

ITRIP

0

TIME

Dwg. WP-008

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 voltage (VSUPPLY + VZ + VF < VO(SUS)).

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.

Fault diagnostics

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

FAULT output. This can be connected to whatever supply level

the following 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 following 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’ channels while disabling signals to the faulty

channel.

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

controller 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.

115 Northeast Cutoff, Box 15036

7

Worcester, Massachusetts 01615-0036 (508) 853-5000

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