A3953 Ver la hoja de datos (PDF) - Allegro MicroSystems

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componentes Descripción

Fabricante

A3953

FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems 

3953

FULL-BRIDGE

PWM MOTOR DRIVER

FUNCTIONAL DESCRIPTION

Internal PWM Current Control During Forward and

Reverse Operation. The A3953S— contains 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 peak 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 the voltage on the reference input terminal

(REF) resulting in a transconductance function approxi-

mated by:

ITRIP ≈

VREF

RS

– ISO

where ISO is the offset due to base drive current.

In forward or reverse mode the current-control cir-

cuitry limits the load current as follows: when the load

current reaches ITRIP, the comparator resets a latch that

turns off the selected source driver or selected sink and

source driver pair depending on whether the device is

operating in slow or fast current-decay mode, respec-

tively.

In slow current-decay mode, the selected source

driver is disabled; the load inductance causes the current

to recirculate through the sink driver and ground clamp

diode. In fast current-decay mode, the selected sink and

source driver pair are disabled; the load inductance

causes the current to flow from ground to the load supply

via the ground clamp and flyback diodes.

Figure 1 — Load-Current Paths

V

BB

DRIVE CURRENT

RECIRCULATION (SLOW-DECAY MODE)

RECIRCULATION (FAST-DECAY MODE)

RS

Dwg. EP-006-13A

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

tor (CT) to determine the time period (tOFF = RT x CT)

during which the drivers remain disabled (see “RC Fixed

Off-Time” below). At the end of the RC interval, the

drivers are enabled allowing the load current to increase

again. The PWM cycle repeats, maintaining the peak

load current at the desired value (see figure 2).

Figure 2

Fast and Slow Current-Decay Waveforms

ENABLE

MODE

I TRIP

RC

LOAD

CURRENT

RC

Dwg. WP-015-1

INTERNAL PWM CURRENT CONTROL

DURING BRAKE-MODE OPERATION

Brake Operation - MODE Input High. 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 dynamically brakes the motor. If the back-EMF

voltage is large, and there is no PWM current limiting, the

load current can increase to a value that approaches that

of a locked rotor condition. To limit the current, when the

ITRIP level is reached, the PWM circuit disables the

conducting sink drivers. The energy stored in the motor’s

inductance is discharged into the load supply causing the

motor current to decay.

As in the case of forward/reverse operation, the

drivers are enabled after a time given by tOFF = RT x 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 peak load current

to the desired value.

115 Northeast Cutoff, Box 15036

6

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

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