A3953S データシートの表示（PDF） - Allegro MicroSystems

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A3953S

FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems 

3953

FULL-BRIDGE

PWM MOTOR DRIVER

LOAD CURRENT REGULATION

PWM of the PHASE and ENABLE Inputs. The PHASE

WITH INTERNAL PWM

and ENABLE inputs can be pulse-width modulated to

CURRENT-CONTROL CIRCUITRY

regulate load current. Typical propagation delays from

the PHASE and ENABLE inputs to transitions of the

When the device is operating in slow current-decay

power outputs are specified in the electrical characteris-

mode, there is a limit to the lowest level that the PWM

tics table. If the internal PWM current control is used, the

current-control circuitry can regulate load current. The

comparator blanking function is active during phase and

limitation is the minimum duty cycle, which is a function of enable transitions. This eliminates false tripping of the

the user-selected value of tOFF and the minimum on-time

over-current comparator caused by switching transients

pulse tON(min) max that occurs each time the PWM latch is

(see “RC Blanking” above).

reset. If the motor is not rotating (as in the case of a

stepper motor in hold/detent mode, a brush dc motor when Enable PWM. With the MODE input low, toggling the

stalled, or at startup), the worst case value of current

ENABLE input turns on and off the selected source and

regulation can be approximated by:

sink drivers. The corresponding pair of flyback and

ground-clamp diodes conduct after the drivers are

[(VBB – VSAT(source+sink)) x tON(min)max] – (1.05(VSAT(sink) + VF) x tOFF) disabled, resulting in fast current decay. When

IAVE ≈

1.05 x (tON(min)max + tOFF) x RLOAD

the device is enabled the internal current-control

circuitry will be active and can be used to limit the

where tOFF = RT x CT, RLOAD is the series resistance of the

load, VBB is the motor supply voltage and t ON(min)max is

specified in the electrical characteristics table. When the

motor is rotating, the back EMF generated will influence

the above relationship. For brush dc motor applications,

the current regulation is improved. For stepper motor

applications, when the motor is rotating, the effect is more

complex. A discussion of this subject is included in the

load current in a slow current-decay mode.

For applications that PWM the ENABLE input and

desire the internal current-limiting circuit to function in the

fast decay mode, the ENABLE input signal should be

inverted and connected to the MODE input. This prevents

the device from being switched into sleep mode when the

ENABLE input is low.

section on stepper motors below.

Phase PWM. Toggling the PHASE terminal selects which

The following procedure can be used to evaluate the

worst-case slow current-decay internal PWM load current

regulation in the system:

sink/source pair is enabled, producing a load current that

varies with the duty cycle and remains continuous at all

times. This can have added benefits in bidirectional brush

dc servo motor applications as the transfer function

Set VREF to 0 volts. With the load connected and the

PWM current control operating in slow current-decay

between the duty cycle on the PHASE input and the

average voltage applied to the motor is more linear than in

mode, use an oscilloscope to measure the time the output the case of ENABLE PWM control (which produces a

is low (sink on) for the output that is chopping. This is the discontinuous current at low current levels). For more

typical minimum on time (tON(min) typ) for the device. The

CT then should be increased until the measured value of

tON(min) is equal to tON(min) max as specified in the electrical

characteristics table. When the new value of CT has been

set, the value of RT should be decreased so the value for

tOFF = RT x CT (with the artificially increased value of CT) is

equal to the nominal design value. The worst-case load-

current regulation then can be measured in the system

under operating conditions.

information see “DC Motor Applications” below.

Synchronous Fixed-Frequency PWM. The internal

PWM current-control circuitry of multiple A3953S—

devices can be synchronized by using the simple circuit

shown in figure 3. A 555 IC can be used to generate the

reset pulse/blanking signal (t1) for the device and the

period of the PWM cycle (t2). The value of t1 should be a

minimum of 1.5 ms. When used in this configuration, the

RT and CT components should be omitted. The PHASE

and ENABLE inputs should not be PWM with this circuit

configuration due to the absence of a blanking function

synchronous with their transitions.

115 Northeast Cutoff, Box 15036

8

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

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