A4973SB-T Просмотр технического описания (PDF) - Allegro MicroSystems

Номер в каталоге

Компоненты Описание

производитель

A4973SB-T

Full-Bridge PWM Motor Driver

Allegro MicroSystems 

A4973

Full-Bridge PWM Motor Driver

FUNCTIONAL DESCRIPTION

Internal PWM Current Control During Forward

and Reverse Operation. The A4973 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

approximated by:

ITRIP ≈

VREF

2 × RS

The user selects an external resistor (RT) and capacitor (CT) to

determine the time period (tOFF = RT x CT) during which the

drivers remain disabled (see RC Fixed Off-Time section, 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 (figure 2).

Figure 2

Fast and Slow Current-Decay Waveforms

ENABLE

MODE

In forward or reverse mode the current-control circuitry 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, respectively.

In slow current-decay mode, the selected source driver is

disabled and both sinks are turned on. The load inductance

causes the current to recirculate through the sink drivers. In fast

current-decay mode, the selected sink and source driver pair are

disabled, then the opposite pair is turned on. The load inductance

causes the current to flow from ground to the load supply via the

motor winding and the opposite pair of transistors (see figure 1).

Figure 1 — Load-Current Paths

VBB

I TRIP

RC

LOAD

CURRENT

RC

www.DataSheet.co.kr

Dwg. WP-015-1

Brake Operation. During braking, care should be taken

to ensure that the motor’s current does not exceed the 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, or if the back-EMF voltage of the motor is known,

approximated by:

IPEAK BRAKE ML ≈

VBEMF

RLOAD

RC Fixed Off-Time. The internal PWM current-control

circuitry uses a one shot to control the time the driver(s)

remain(s) 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 timing terminal

to ground. The fixed off-time, over a range of values of CT = 470

pF to 1500 pF and RT = 12 kΩ to 100 kΩ, is approximated by:

RS

Drive Current (Normal)

Recirculation (Fast Decay)

Recirculation (Slow Decay)

tOFF ≈ RT x CT

Allegro MicroSystems, Inc.

4

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Share Link: