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A3953S

FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems

A3953S Datasheet PDF : 14 Pages

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3953

FULL-BRIDGE

PWM MOTOR DRIVER

mended) placed as close to the device as is physically

practical. To minimize the effect of system ground I x R

drops on the logic and reference input signals, the system

ground should have a low-resistance return to the motor

supply voltage.

See also “Current Sensing” and “Thermal Consider-

ations” above.

Fixed Off-Time Selection. With increasing values of tOFF,

switching losses will decrease, low-level load-current

regulation will improve, EMI will be reduced, the PWM

frequency will decrease, and ripple current will increase.

The value of tOFF can be chosen for optimization of these

parameters. For applications where audible noise is a

concern, typical values of tOFF are chosen to be in the

range of 15 ms to 35 ms.

Stepper Motor Applications. The MODE terminal can

be used to optimize the performance of the device in

microstepping/sinusoidal stepper-motor drive applications.

When the load current is increasing, slow decay mode is

used to limit the switching losses in the device and iron

losses in the motor. This also improves the maximum rate

at which the load current can increase (as compared to

fast decay) due to the slow rate of decay during tOFF.

When the load current is decreasing, fast-decay mode is

used to regulate the load current to the desired level. This

prevents tailing of the current profile caused by the back-

EMF voltage of the stepper motor.

In stepper-motor applications applying a constant

current to the load, slow-decay mode PWM is typically

used to limit the switching losses in the device and iron

losses in the motor.

DC Motor Applications. In closed-loop systems, the

speed of a dc motor can be controlled by PWM of the

PHASE or ENABLE inputs, or by varying the reference

input voltage (REF). In digital systems (microprocessor

controlled), PWM of the PHASE or ENABLE input is used

typically thus avoiding the need to generate a variable

analog voltage reference. In this case, a dc voltage on the

REF input is used typically to limit the maximum load

current.

In dc servo applications, which require accurate

positioning at low or zero speed, PWM of the PHASE

input is selected typically. This simplifies the servo control

loop because the transfer function between the duty cycle

on the PHASE input and the average voltage applied to

the motor is more linear than in the case of ENABLE

PWM control (which produces a discontinuous current at

low current levels).

With bidirectional dc servo motors, the PHASE

terminal can be used for mechanical direction control.

Similar to when braking the motor dynamically, abrupt

changes in the direction of a rotating motor produces a

current generated by the back-EMF. The current gener-

ated will depend on the mode of operation. If the internal

current control circuitry is not being used, then the maxi-

mum load current generated can be approximated by

ILOAD = (VBEMF + VBB)/RLOAD where VBEMF is proportional to

the motor’s speed. If the internal slow current-decay

control circuitry is used, then the maximum load current

generated can be approximated by ILOAD = VBEMF/RLOAD.

For both cases care must be taken to ensure that the

maximum ratings of the device are not exceeded. If the

internal fast current-decay control circuitry is used, then

the load current will regulate to a value given by:

ILOAD = VREF/RS.

CAUTION: In fast current-decay mode, when the direction

of the motor is changed abruptly, the kinetic energy stored

in the motor and load inertia will be converted into current

that charges the VBB supply bulk capacitance (power

supply output and decoupling capacitance). Care must be

taken to ensure that the capacitance is sufficient to absorb

the energy without exceeding the voltage rating of any

devices connected to the motor supply.