A3951 데이터 시트보기 (PDF) - Allegro MicroSystems

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A3951

FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems 

3951

FULL-BRIDGE

PWM MOTOR DRIVER

FUNCTIONAL DESCRIPTION

INTERNAL PWM CURRENT CONTROL DURING

FORWARD AND REVERSE OPERATION

The A3951SB/SW contain a fixed off-time pulse-width

modulated (PWM) current-control circuit that can be used

to limit the load current to a desired value. The value of

the current limiting (ITRIP) is set by the selection of an

external current sensing resistor (RS) and reference input

voltage (VREF). The internal circuitry compares the

voltage across the external sense resistor to one tenth the

voltage on the REF input terminal, resulting in a function

approximated by

ITRIP = VREF/(10•RS).

In forward or reverse mode the current-control cir-

cuitry limits the load current. When the load current

reaches ITRIP, the comparator resets a latch to turn off the

selected sink driver. The load inductance causes the

current to recirculate through the source driver and

flyback diode (two-quadrant operation or slow decay).

See figure 1.

VBB

RS

DRIVE CURRENT

RECIRCULATION

Dwg. EP-006-9

Figure 1 — Load-Current Paths

The user selects an external resistor (RT) and capaci-

tor (CT) to determine the time period (toff = RT•CT) during

which the drivers remain disabled (see “RC Fixed OFF

Time” below). At the end of the RTCT interval, the drivers

are re-enabled allowing the load current to increase again.

The PWM cycle repeats, maintaining the load current at

the desired value (see figure 2).

ENABLE

I TRIP

RC

LOAD

CURRENT

Dwg. WP-015-3

Figure 2 — Load-Current Waveform

INTERNAL PWM CURRENT CONTROL DURING

BRAKE MODE OPERATION

The brake circuit turns off both source drivers and

turns on both sink drivers. For dc motor applications, this

has the effect of shorting the motor’s back-EMF voltage,

resulting in current flow that brakes the motor dynamically.

However, if the back-EMF voltage is large and there is no

PWM current limiting, then the load current can increase to

a value that approaches a locked rotor condition. To limit

the current, when the ITRIP level is reached, the PWM

circuit disables the conducting sink driver. The energy

stored in the motor’s inductance is then discharged into

the load supply causing the motor current to decay.

As in the case of forward/reverse operation, the drivers

are re-enabled after a time given by toff = RT•CT (see “RC

Fixed Off Time” below). Depending on the back-EMF

voltage (proportional to the motor’s decreasing speed), the

load current again may increase to ITRIP. If so, the PWM

cycle will repeat, limiting the load current to the desired

value.

Brake Operation

During braking, the peak current limit defaults inter-

nally to a value approximated by

ITRIP = 1.5 V/RS.

In this mode, the value of RS determines the ITRIP value

independent of VREF. This is useful in applications with

differing run and brake currents and no practical method of

varying VREF.

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