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A2557SEB Datasheet(PDF) 6 Page - Allegro MicroSystems |
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A2557SEB Datasheet(HTML) 6 Page - Allegro MicroSystems |
6 / 12 page 2557 PROTECTED QUAD DRIVER WITH FAULT DETECTION & SLEEP MODE 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 6 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 following circuitry requires (within the specification constraints). For a 5 V supply (i.e., Vcc) 150 k Ω 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. 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. NORMAL LAMP IN-RUSH CURRENT TIME THERMAL GRADIENT SENSING CURRENT LIMIT Dwg. WP-008 NOT TO SCALE ITRIP 0 |
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