Protected Quad Driver
with Fault Detection and Sleep Mode
A2557
CIRCUIT DESCRIPTION AND APPLICATION (continued)
NORMAL LAMP IN-RUSH CURRENT
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:
NOT TO SCALE
THERMAL GRADIENT SENSING
CURRENT LIMIT
ITRIP
•
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.
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)).
•
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).
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.
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.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
8
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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