Automotive DMOS Microstepping Driver

with Translator

A3980

12

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

mixed decay mode, as shown in figures 5 through 8. As the

trip point is reached, the A3980 goes into fast decay mode

until the voltage on the RC pin decays to the same level as the

voltage applied to the PFD pin. The duration of time that the

After this fast decay period, the A3980 switches to slow

decay mode for the remainder of the fixed off-time period.

is triggered by an internal fixed-off-time cycle, load current

recirculates according to the decay mode selected by the

control logic. The synchronous rectification feature turns on

the appropriate FETs during current decay, and effectively

reduces power dissipation significantly, and eliminates the

need for external Schottky diodes. Synchronous rectification

has two modes: Active mode and Disabled mode (described

below).

at logic low, Active mode is enabled. This mode allows

synchronous rectification to occur, but when a zero current

level is detected, it also prevents reversal of the load current

by turning off synchronous rectification. This prevents the

motor winding from conducting in the reverse direction.

at logic high, Disabled mode takes effect. This mode disables

synchronous rectification. This mode is typically used when

external diodes are required to transfer power dissipation

from the A3980 package to the external diodes.

an undervoltage fault on VREG, the DMOS outputs of the

A3980 are disabled until the fault condition is removed. In

the case of an overvoltage fault, the sink DMOS FETs are

switched on, and the source FETs off. At power-up, and in

above the minimum level, the translator resets to the Home

state and the DMOS outputs are re-enabled.

junction temperature reaches the thermal shutdown value,

typically 170

°C. This is intended only to protect the A3980

from failures due to excessive junction temperatures. Ther-

mal protection will not protect the A3980 from continuous

short circuits, and additional fault diagnostics are integrated for

this purpose. Thermal shutdown has a hysteresis of approxi-

mately 15

°C.

circuits that can detect shorts to VBB, shorts to ground, and

shorted or open circuit load. Short circuits are detected by

monitoring the voltage across the driving DMOS FETs and

the open load is detected by monitoring the phase current

when the motor is in the Home microstep position. All fault

detection takes place following a delay after the blank time.

Short to VBB. A short from any of the motor connections

to the battery or VBB connection is detected by monitoring

the voltage across the bottom FETs in each full-bridge. When

the FET is on, the voltage should be no greater than the

Short to Ground. A short from any of the motor connec-

tions to ground is detected by monitoring the voltage across

the top FETs in each full-bridge. When the FET is turned

defined in the Electrical Characteristics table.

Shorted Load. A short across the load is detected by

monitoring 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

circuit has been detected, all outputs for the faulty phase are

disabled until the next step command. At the next step com-

mand, the outputs are re-enabled and the voltage across the

FET is resampled.