DMOS Dual Full-Bridge PWM Motor Driver

with Overcurrent Protection

A4986

6

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

Functional Description

stepper motor in full, half, or quarter step mode. The currents in

each of the output full-bridges, all N-channel DMOS, are regu-

lated with fixed off-time pulse width modulated (PWM) control

circuitry. Each full-bridge peak current is set by the value of

Percentages of the peak current are set using a 2-bit nonlinear

DAC that programs 33%, 66%, or 100% of the peak current, or

disables the outputs.

trolled by a fixed off-time PWM current control circuit that limits

of source and sink FET outputs are enabled and current flows

current sense comparator resets the PWM latch. The latch then

turns off the sink and source FETs.

The maximum value of current limiting is set by the selection of

tion is approximated by the maximum value of current limiting,

the input voltage on the REF pin (V).

sense comparator in precise steps, such that

It is critical that the maximum rating (0.5 V) on the SENSE1 and

SENSE2 pins is not exceeded.

uses a one-shot circuit to control the duration of time that the

ROSC terminal. The ROSC terminal has two settings:

▪ ROSC tied to VDD or ground — off-time internally set to

30 μs

▪ ROSC through a resistor to ground — off-time is determined

by the following formula

comparators when the outputs are switched by the internal

current control circuitry. The comparator outputs are blanked

to prevent false overcurrent detection due to reverse recovery

currents of the clamp diodes, and switching transients related

approximately

Shorted-Load and Short-to-Ground Protection.

If the motor leads are shorted together, or if one of the leads is

shorted to ground, the driver will protect itself by sensing the

overcurrent event and disabling the driver that is shorted, protect-

ing the device from damage. In the case of a short-to-ground, the

device will remain disabled (latched) until the S¯¯L¯¯E¯¯E¯¯P¯ input goes

high or VDD power is removed. A short-to-ground overcurrent

event is shown in figure 1.

When the two outputs are shorted together, the current path is

through the sense resistor. After the blanking time (

≈1 μs) expires,

the sense resistor voltage is exceeding its trip value, due to the

overcurrent condition that exists. This causes the driver to go into

a fixed off-time cycle. After the fixed off-time expires the driver

turns on again and the process repeats. In this condition the driver

is completely protected against overcurrent events, but the short

is repetitive with a period equal to the fixed off-time of the driver.

This condition is shown in figure 2.

If the driver is operating in Mixed decay mode, it is normal for

the positive current to spike, due to the bridge going in the for-

ward direction and also in the negative direction, as a result of the

direction change implemented by the Mixed decay feature. This

is shown in figure 3. In both instances the overcurrent circuitry is

protecting the driver and prevents damage to the device.

generate a gate supply greater than that of VBB for driving the

Functional Description