3953

FULL-BRIDGE

PWM MOTOR DRIVER

115 Northeast Cutoff, Box 15036

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

8

LOAD CURRENT REGULATION

WITH INTERNAL PWM

CURRENT-CONTROL CIRCUITRY

When the device is operating in slow current-decay

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

current-control circuitry can regulate load current. The

limitation is the minimum duty cycle, which is a function of

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

stepper motor in hold/detent mode, a brush dc motor when

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

regulation can be approximated by:

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

section on stepper motors below.

The following procedure can be used to evaluate the

worst-case slow current-decay internal PWM load current

regulation in the system:

PWM current control operating in slow current-decay

mode, use an oscilloscope to measure the time the output

is low (sink on) for the output that is chopping. This is the

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

current regulation then can be measured in the system

under operating conditions.

PWM of the PHASE and ENABLE Inputs. The PHASE

and 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 characteris-

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

comparator blanking function is active during phase and

enable transitions. This eliminates false tripping of the

over-current comparator caused by switching transients

(see “RC Blanking” above).

Enable PWM. With the MODE input low, toggling the

ENABLE input turns on and off the selected source and

sink drivers. The corresponding pair of flyback and

ground-clamp diodes conduct after the drivers are

disabled, resulting in fast current decay. When

the device is enabled the internal current-control

circuitry will be active and can be used to limit 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.

Phase PWM. Toggling the PHASE terminal selects which

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

between the duty cycle on the PHASE input and the

average voltage applied to the motor is more linear than in

the case of ENABLE PWM control (which produces a

discontinuous current at low current levels). For more

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

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

and ENABLE inputs should not be PWM with this circuit

configuration due to the absence of a blanking function

synchronous with their transitions.

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