A3951SW 查看數據表（PDF） - Allegro MicroSystems

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A3951SW

FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems 

3951

FULL-BRIDGE

PWM MOTOR DRIVER

Choosing a small value for RS essentially disables the

current limiting during braking. Therefore, care should be

taken to ensure that the motor’s current does not exceed

the absolute maximum ratings of the device. The braking

current can be measured by using an oscilloscope with a

current probe connected to one of the motor’s leads.

CAUTION: Because the kinetic energy stored in the

motor and load inertia is being converted into current,

which charges the VBB supply bulk capacitance (power

supply output and decoupling capacitance), care must be

taken to ensure the capacitance is sufficient to absorb the

energy without exceeding the voltage rating of any devices

connected to the motor supply.

RC Fixed Off Time

The internal PWM current control circuitry uses a one

shot to control the time the driver remains off. The one

shot time, toff (fixed off time), is determined by the selection

of an external resistor (RT) and capacitor (CT) connected in

parallel from the RC terminal to ground. The fixed off time,

over a range of values of CT = 820 pF to 1500 pF and RT =

12 kΩ to 100 kΩ, is approximated by

toff = RT•CT.

When the PWM latch is reset by the current compara-

tor, the voltage on the RC terminal will begin to decay from

approximately 3 volts. When the voltage on the RC

terminal reaches approximately 1.1 volts, the PWM latch is

set, thereby re-enabling the driver.

RC Blanking

In addition to determining the fixed off time of the

PWM control circuit, the CT component sets the compara-

tor blanking time. This function blanks the output of the

comparator when the outputs are switched by the internal

current control circuitry (or by the PHASE, BRAKE, or

ENABLE inputs). The comparator output is blanked to

prevent false over-current detections due to reverse

recovery currents of the clamp diodes, and/or switching

transients related to distributed capacitance in the load.

During internal PWM operation, at the end of the off

time, the comparator’s output is blanked and CT begins to

be charged from approximately 1.1 V by an internal current

source of approximately 1 mA. The comparator output

remains blanked until the voltage on CT reaches approxi-

mately 3.0 volts.

Similarly, when a transition of the PHASE input occurs,

CT is discharged to near ground during the crossover delay

time (the crossover delay time is present to prevent

simultaneous conduction of the source and sink drivers).

After the crossover delay, CT is charged by an internal

current source of approximately 1 mA. The comparator

output remains blanked until the voltage on CT reaches

approximately 3.0 volts.

Similarly, when the device is disabled via the ENABLE

input, CT is discharged to near ground. When the device is

re-enabled, CT is charged by the internal current source.

The comparator output remains blanked until the voltage

on CT reaches approximately 3.0 V.

For most applications, the minimum recommended

value is CT = 820 pF ±5 %. This value ensures that the

blanking time is sufficient to avoid false trips of the com-

parator under normal operating conditions. For optimal

regulation of the load current, the above value for CT is

recommended and the value of RT can be sized to deter-

mine toff. For more information regarding load current

regulation, see below.

LOAD CURRENT REGULATION WITH THE INTERNAL

PWM CURRENT-CONTROL CIRCUITRY

During operation, the A3951S– have a lower limit to

the range of PWM current control. This directly relates to

the limitations imposed by the VREF input (2.0 V, minimum).

Applications requiring a broader or full range (≈0% to

100%) should utilize the A3952S–, which are recom-

mended for the improvements they bring to new designs.

LOAD CURRENT REGULATION WITH EXTERNAL

PWM OF THE PHASE OR ENABLE INPUTS

The PHASE or ENABLE inputs can be pulse-width

modulated to regulate load current. Typical propagation

delays from the PHASE and ENABLE inputs to transitions

of the power outputs are specified in the electrical charac-

teristics table. If the normal PWM current control is used,

then the comparator blanking function is active during

phase and enable transitions. This eliminates false

tripping of the over-current comparator caused by switch-

ing transients (see “RC Blanking” above).

115 Northeast Cutoff, Box 15036

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

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