Automotive DMOS Microstepping Driver

with Translator

A3980

13

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Shorted Load. A short across the load is detected by moni-

toring the voltage across both the top and bottom FETs in each

full-bridge.

Short Fault Operation. Because motor capacitance may

cause the measured voltages to show a fault as the full-bridge

switches, voltages are not sampled until after the blank time plus

detected, all outputs for the faulty phase are disabled until the

next step command. At the next step command, the outputs are

re-enabled and the voltage across the FET is resampled.

While the fault persists, the A3980 continues this cycle at each

step command: enabling the outputs for a short period, and then

disabling the outputs. This allows the A3980 to handle a continu-

ous short circuit without damage. If, while stepping rapidly, a

short circuit appears and no action is taken, the repeated short-cir-

cuit current pulses eventually cause the temperature of the A3980

to rise and an overtemperature fault occurs.

Low Load Current Fault Operation. A low load current is

detected by monitoring the measured phase current in each output

while driving the motor in the Home microstep position. At the

Home microstep position, each phase current should reach 70%

microstep position, then a low load current condition is reported

on the next rising edge of the STEP input. If the measured cur-

generated on the next rising edge of the STEP input.

If an open load condition appears while stepping, then it is

detected after the translator cycles through the Home state.

Although the A3980 continues to drive the DMOS outputs during

an open load condition, it does not clear the fault flags until the

next Home state occurs.

There are two conditions that can cause a low load current. The

first is an open circuit on either or both motor phase connections.

In this condition, current can never flow through the phase so a

low load current will always be flagged. The second condition is

where the back EMF of the motor limits the phase current to less

than the low load current trip level. This will happen when the

stepper motor is running close to its limiting speed. To confirm an

open load condition when a low load current is flagged, the step

rate should be reduced to a level below half the maximum step

rate. If the low load current flag remains active at the lower step

rate, after completing the number of steps required to pass the

home condition, then an open circuit condition is confirmed.

To allow immediate detection of an open load condition at power

up or after coming out of sleep mode, the A3980 translator is

reset to the Home microstep position and the low load current

fault flags are set. If no open load condition exists then the fault

flags will be reset on the next rising edge of the STEP input.

tored to ensure that they are within the correct operating range.

flags are set and the A3980 enters a safety mode in which all low-

side DMOS FETs are enabled and all high-side DMOS FETs are

disabled. This allows the A3980 to survive a load dump transient

condition that has up to 50 V on VBATT and a duration of up

fault flags are set, the DMOS outputs are disabled, and the internal

logic is reset to the power-on state (the translator is set to the Home

state).

ditions are reported using the two fault flag outputs (open drain).

These are active-low outputs which are coded as shown in table

2 to discriminate between the fault conditions. When both fault

flags are high, no fault exists.