A3952SW datasheet(6/16 Pages) ALLEGRO

6 / 16 page

3952

FULL-BRIDGE

PWM MOTOR DRIVER

115 Northeast Cutoff, Box 15036

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

Brake Operation - MODE Input Low

During braking, with the MODE input low, the peak

current limit defaults internally to a value approximated by

TRIP = 1.5 V/RS.

In this mode, the value of R

S determines the ITRIP value

independent of V

REF. This is useful in applications with

differing run and brake currents and no practical method of

varying V

REF.

Choosing a small value for R

S essentially disables the

current limiting during braking. Therefore, care should be

taken to ensure that the motor’s current does not exceed

the absolute maximum 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.

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, t

off (fixed OFF time), is determined by the

selection of an external resistor (R

T) and capacitor (CT)

connected in parallel from the RC terminal to ground. The

fixed OFF time, over a range of values of C

T = 820 pF to

1500 pF and R

T = 12 kΩ to 100 kΩ, is approximated by

off = RTCT.

When the PWM latch is reset by the current compara-

tor, the voltage on the RC terminal will begin to decay from

approximately 3 volts. When the voltage on the RC

terminal reaches approximately 1.1 volt, the PWM latch is

set, thereby re-enabling the driver(s).

RC Blanking

In addition to determining the fixed OFF-time of the

PWM control circuit, the C

T component sets the compara-

tor blanking time. This function blanks the output of the

comparator when the outputs are switched by the internal

current control circuitry (or by the PHASE, BRAKE, or

ENABLE inputs). The comparator output is blanked to

prevent false over-current detections due to reverse

recovery currents of the clamp diodes, and/or switching

transients related to distributed capacitance in the load.

During internal PWM operation, at the end of the t

off

time, the comparator’s output is blanked and C

T begins to

be charged from approximately 1.1 V by an internal current

source of approximately 1 mA. The comparator output

remains blanked until the voltage on C

T reaches approxi-

mately 3.0 volts.

Similarly, when a transition of the PHASE input occurs,

T is discharged to near ground during the crossover delay

time (the crossover delay time is present to prevent

simultaneous conduction of the source and sink drivers).

After the crossover delay, C

T is charged by an internal

current source of approximately 1 mA. The comparator

output remains blanked until the voltage on C

T reaches

approximately 3.0 volts.

Similarly, when the device is disabled via the ENABLE

input, C

T is discharged to near ground. When the device is

re-enabled, C

T is charged by the internal current source.

The comparator output remains blanked until the voltage

on C

T reaches approximately 3.0 V.

For applications that use the internal fast-decay mode

PWM operation, the minimum recommended value is C

T =

1200 pF

±5 %. For all other applications, the minimum

recommended value is C

T = 820 pF ±5 %. These values

ensure that the blanking time is sufficient to avoid false

trips of the comparator under normal operating conditions.

For optimal regulation of the load current, the above

values for C

T are recommended and the value of RT can

be sized to determine t

off. For more information regarding

load current regulation, see below.

LOAD CURRENT REGULATION WITH THE INTERNAL

PWM CURRENT-CONTROL CIRCUITRY

When the device is operating in slow-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 the user-

selected value of t

off and the maxuimum value of the

minimum ON-time pulse, t

on(min), that occurs each time the

PWM latch is reset. If the motor is not rotating, as in the

case of a stepper motor in hold/detent mode, or a brush dc

motor when stalled or at startup, the worst-case value of

current regulation can be approximated by

(AV) ≈

[(V

BB – VSAT(source+sink)) • ton(min)max] – [1.05 (VSAT(sink) + VD) • toff]

1.05 (t

on(min)max + toff) • RLOAD

where t

off = RTCT, RLOAD is the series resistance of the

load, V

BB is the load/motor supply voltage, and ton(min)max

is specified in the electrical characteristics table. When

the motor is rotating, the back EMF generated will influ-

ence the above relationship. For brush dc motor applica-

tions, 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 under “Applications”.

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