A3953_技术文档

3953

FULL-BRIDGE PWM MOTOR DRIVER

Designed for bidirectional pulse-width modulated (PWM) current control

of inductive loads, the A3953S— is capable of continuous output currents to

1.3 A and operating voltages to 50 V. Internal fixed off-time PWM current-

control circuitry can be used to regulate the maximum load current to a

desired value. The peak load current limit is set by the user’s selection of an

input reference voltage and external sensing resistor. The fixed off-time pulse

duration is set by a user- selected external RC timing network. Internal circuit

protection includes thermal shutdown with hysteresis, transient-suppression

diodes, and crossover current protection. Special power-up sequencing is not

required.

LOAD

SUPPLY

16

BRAKE

1

2

3

4

V

BB

15 OUT

REF

RC

B

14

13

MODE

GROUND

LOGIC

12

11

5

6

GROUND

SENSE

LOGIC

SUPPLY

V

CC

With the ENABLE input held low, the PHASE input controls load current

polarity by selecting the appropriate source and sink driver pair. The MODE

input determines whether the PWM current-control circuitry operates in a

slow current-decay mode (only the selected source driver switching) or in a

fast current-decay mode (selected source and sink switching). A user-

selectable blanking window prevents false triggering of the PWM current-

control circuitry. With the ENABLE input held high, all output drivers are

disabled. A sleep mode is provided to reduce power consumption.

PHASE

7

8

10 OUT

A

V

LOAD

SUPPLY

BB

9

ENABLE

Dwg. PP-056

Note the A3953SB (DIP) and the A3953SLB

(SOIC) are electrically identical and share a

common terminal number assignment.

When a logic low is applied to the BRAKE input, the braking function is

enabled. This overrides ENABLE and PHASE to turn off both source drivers

and turn on both sink drivers. The brake function can be used to dynamically

brake brush dc motors.

ABSOLUTE MAXIMUM RATINGS

Load Supply Voltage, V_BB. . . . . . . . . . 50 V

Output Current, I_OUT

(Continuous) . . . . . . . . . . . . . . 1.3 A*

The A3953S— is supplied in a choice of two power packages; a 16-pin

dual-in-line plastic package with copper heat-sink tabs, and a 16-lead plastic

SOIC with copper heat-sink tabs. For both package styles, the power tab is at

ground potential and needs no electrical isolation.

Logic Supply Voltage, V_CC. . . . . . . . . 7.0 V

Logic/Reference Input Voltage Range,

V_IN. . . . . . . . . . . -0.3 V to V_CC+ 0.3 V

Sense Voltage, V_SENSE

(V_CC= 5.0 V) . . . . . . . . . . . . . . . . 1.0 V

(V_CC= 3.3 V) . . . . . . . . . . . . . . . . 0.4 V

FEATURES

I

1.3 A Continuous Output Current

I Sleep (Low Current

Consumption) Mode

I Internal Transient-

Suppression Diodes

I Internal Thermal-

Shutdown Circuitry

I Crossover-Current

and UVLO Protection

Package Power Dissipation,

I 50 V Output Voltage Rating

P_D. . . . . . . . . . . . . . . . . . . . See Graph

I 3 V to 5.5 V Logic Supply Voltage

I Internal PWM Current Control

I Saturated Sink Drivers (Below 1 A)

I Fast and Slow Current-Decay Modes

I Automotive Capable

Operating Temperature Range,

T_A. . . . . . . . . . . . . . . . . -20˚C to +85˚C

Junction Temperature, T_J. . . . . . . +150˚C†

Storage Temperature Range,

T_S. . . . . . . . . . . . . . . . -55˚C to +150˚C

* Output current rating may be limited by duty

cycle, ambient temperature, and heat sinking.

Under any set of conditions, do not exceed the

specified current rating or a junction temperature

of 150˚C.

Always order by complete part number:

† Fault conditions that produce excessive junction

temperature will activate the device’s thermal

shutdown circuitry. These conditions can be

tolerated but should be avoided.

Part Number

Package

R_θJA

R_θJT

A3953SB

16-Pin DIP

43°C/W

67°C/W

6°C/W

A3953SLB

16-Lead SOIC

3953

FULL-BRIDGE

PWM MOTOR DRIVER

FUNCTIONAL BLOCK DIAGRAM

LOGIC

SUPPLY

6

V

CC

9

10

15

16

SLEEP &

STANDBY MODES

14

7

MODE

V

BB

PHASE

UVLO

& TSD

8

1

ENABLE

BRAKE

+

PWM LATCH

11

–

R

Q

SENSE

S

R

BLANKING

S

GROUND

V

CC

4

5

12

13

RC

3

+

–

2

GROUND

REF

V

TH

C

T

R

T

Dwg. FP-036-2A

TRUTH TABLE

BRAKE

ENABLE

PHASE

MODE

OUT_A

OUT_B

DESCRIPTION

H

L

H

L

X

H

L

H

L

Off

H

Off

L

Sleep Mode

Standby

H

L

Forward, Fast Current-Decay Mode

Forward, Slow Current-Decay Mode

Reverse, Fast Current-Decay Mode

Reverse, Slow Current-Decay Mode

Brake, Fast Current-Decay Mode

Brake, No Current Control

L

H

L

H

L

H

L

H

X

H

L

X = Irrelevant

115 Northeast Cutoff, Box 15036

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

2

3953

FULL-BRIDGE

PWM MOTOR DRIVER

5

4

3

R

= 6.0°C/W

θJT

SUFFIX 'B', R

= 43°C/W

θJA

2

1

0

SUFFIX 'LB', R

50

= 63°C/W

θJA

25

75

100

125

150

TEMPERATURE IN °C

Dwg. GP-049-2A

ELECTRICAL CHARACTERISTICS at T_J= 25˚C, V_BB= 5 V to 50 V, V_CC= 3.0 V to 5.5 V

(unless otherwise noted.)

Limits

Characteristic

Symbol

Test Conditions

Min.

Typ.

Max.

Units

Power Outputs

Load Supply Voltage Range

Output Leakage Current

V_BB

I_CEX

Operating, I_OUT= 1.3 A, L = 3 mH

V_OUT= V_BB

V_CC

—

<1.0

<-1.0

33

50

V

µA

mA

V

_OUT= 0 V

I_SENSE- I_OUT1, I_OUT= 850 mA,

_SENSE= 0 V, V_CC= 5 V

—

-50

38

Sense Current Offset

I_SO

22

V

Output Saturation Voltage

BRAKE = H

V_CE(SAT)

V_SENSE= 0.4 V, V_CC= 3.0 V:

Source Driver, I_OUT= -0.85 A

Source Driver, I_OUT= -1.3 A

Sink Driver, I_OUT= 0.85 A

Sink Driver, I_OUT= 1.3 A

V_SENSE= 0.4 V, V_CC= 3.0 V:

Sink Driver, I_OUT= 0.85 A

Sink Driver, I_OUT= 1.3 A

I_F= 0.85 A

—

1.0

1.7

0.4

1.1

1.9

0.5

1.3

V

(Forward/Reverse Mode)

Output Saturation Voltage

BRAKE = L

V_CE(SAT)

—

1.0

1.3

1.2

1.4

1.2

1.5

1.4

1.6

V

(Brake Mode)

Clamp Diode Forward Voltage

(Sink or Source)

V_F

I_F= 1.3 A

Continued next page…

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3

3953

FULL-BRIDGE

PWM MOTOR DRIVER

ELECTRICAL CHARACTERISTICS at T_J= 25˚C, V_BB= 5 V to 50 V, V_CC= 3.0 V to 5.5 V

(unless otherwise noted.)

Limits

Characteristic

AC Timing

Symbol

Test Conditions

Min.

Typ.

Max.

Units

PWM RC Fixed Off-time

PWM Turn-Off Time

t_{OFF RC}

C_T= 680 pF, R_T= 30 kΩ, V_CC= 3.3 V

18.3

20.4

1.0

22.5

1.5

µs

t_PWM(OFF)

Comparator Trip to Source Off,

I_OUT= 25 mA

—

Comparator Trip to Source Off,

I_OUT= 1.3 A

—

1.8

2.6

µs

PWM Turn-On Time

t_PWM(ON)

I_RCCharge On to Source On,

—

0.4

0.7

µs

I

_OUT= 25 mA

I_RCCharge On to Source On,

I_OUT= 1.3 A

0.55

0.85

PWM Minimum On Time

Propagation Delay Times

t_ON(min)

V_CC= 3.3 V, R_T≥ 12 kΩ, C_T= 680 pF

V_CC= 5.0 V, R_T≥ 12 kΩ, C_T= 470 pF

I_OUT= 1.3 A, 50% to 90%:

ENABLE On to Source On

ENABLE Off to Source Off

ENABLE On to Sink On

0.8

1.4

1.6

1.9

2.0

µs

t_pd

—

0.3

70

1.0

0.8

2.4

0.8

2.0

1.7

1.5

—

3.0

—

µs

kHz

ENABLE Off to Sink Off (MODE = L)

PHASE Change to Sink On

PHASE Change to Sink Off

PHASE Change to Source On

PHASE Change to Source Off

1 kΩ Load to 25 V, V_BB= 50 V

I_OUT= 1.3 A

Crossover Dead Time

t_CODT

Maximum PWM Frequency

f_PWM(max)

Continued next page…

115 Northeast Cutoff, Box 15036

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

4

3953

FULL-BRIDGE

PWM MOTOR DRIVER

ELECTRICAL CHARACTERISTICS at T_J= 25˚C, V_BB= 5 V to 50 V, V_CC= 3.0 V to 5.5 V

(unless otherwise noted. )

Limits

Characteristic

Symbol

Test Conditions

Min.

Typ.

Max.

Units

Control Circuitry

Thermal Shutdown Temp.

T_J

—

2.5

0.12

—

3.0

2.0

—

0

165

8.0

2.75

0.17

42

—

°C

V

Thermal Shutdown Hysteresis

UVLO Enable Threshold

UVLO Hysteresis

∆T_J

3.0

0.25

50

V

Logic Supply Current

I_CC(ON)

I_CC(OFF)

I_CC(Brake)

I_CC(Sleep)

I_BB(ON)

I_BB(OFF)

I_BB(Brake)

I_BB(Sleep)

V_CC

V_ENABLE= 0.8 V, V_BRAKE= 2.0 V

V_ENABLE= 2.0 V, V_MODE= 0.8 V

V_BRAKE= 0.8 V

mA

µA

mA

µA

V

12

15

42

50

V_ENABLE= V_MODE= V_BRAKE= 2.0 V

V_ENABLE= 0.8 V

500

2.5

1.0

5.0

—

800

4.0

50

Motor Supply Current

(No Load)

V_ENABLE= 2.0 V, V_MODE= 0.8 V

V_BRAKE= 0.8 V

50

V_ENABLE= V_MODE= 2.0 V

Operating

50

Logic Supply Voltage Range

Logic Input Voltage

5.5

—

V_IN(1)

V

V_IN(0)

—

0.8

20

V

Logic Input Current

I_IN(1)

V_IN= 2.0 V

<1.0

<-2.0

—

µA

V

I_IN(0)

V_IN= 0.8 V

-200

0.4

1.0

5.0

V_SENSEVoltage Range

V_SENSE(3.3)

V_SENSE(5.0)

I_REF

V_CC= 3.0 V to 3.6 V

V_CC= 4.5 V to 5.5 V

V_REF= 0 V to 1 V

V_REF= 0 V

0

—

V

Reference Input Current

—

µA

mV

Comparator Input Offset Volt.

V_IO

2.0

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5

3953

FULL-BRIDGE

PWM MOTOR DRIVER

FUNCTIONAL DESCRIPTION

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

tor (C_T) to determine the time period (t_OFF= R_Tx C_T)

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).

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 (I_TRIP) is set

by the selection of an external current sensing resistor

(R_S) and reference input voltage (V_REF). 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-

Figure 2

Fast and Slow Current-Decay Waveforms

ENABLE

mated by:

V_REF

R_S

I_TRIP

≈

– I_SO

MODE

where I_SOis the offset due to base drive current.

I

TRIP

RC

In forward or reverse mode the current-control cir-

cuitry limits the load current as follows: when the load

current reaches I_TRIP, 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.

LOAD

CURRENT

RC

Dwg. WP-015-1

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.

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

I_TRIPlevel 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.

Figure 1 — Load-Current Paths

V

BB

DRIVE CURRENT

RECIRCULATION (SLOW-DECAY MODE)

RECIRCULATION (FAST-DECAY MODE)

As in the case of forward/reverse operation, the

drivers are enabled after a time given by t_OFF= R_Tx C_T

(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 I_TRIP. If so,

the PWM cycle will repeat, limiting the peak load current

to the desired value.

R

S

Dwg. EP-006-13A

115 Northeast Cutoff, Box 15036

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

6

3953

FULL-BRIDGE

PWM MOTOR DRIVER

During braking, when the MODE input is high, the

peak current limit can be approximated by:

RC Blanking. In addition to determining the fixed off-time

of the PWM control circuit, the C_Tcomponent sets the

comparator 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.

V_REF

I_{TRIP BRAKE MH}

≈

R_S

CAUTION: Because the kinetic energy stored in the motor

and load inertia is being converted into current, which

charges the V_BBsupply bulk capacitance (power supply

output and decoupling capacitance), care must be taken

to ensure the capacitance is sufficient to absorb the

energy without exceeding the voltage rating of any

devices connected to the motor supply.

During internal PWM operation, at the end of the t_OFF

time, the comparator’s output is blanked and C_Tbegins to

be charged from approximately 0.22V_CCby an internal

current source of approximately 1 mA. The comparator

output remains blanked until the voltage on C_Treaches

Brake Operation - MODE Input Low. During braking,

with the MODE input low, the internal current-control

circuitry is disabled. Therefore, care should be taken to

ensure that the motor’s current does not exceed the

ratings of the device. The braking current can be mea-

sured 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:

approximately 0.60V_CC

.

When a transition of the PHASE input occurs, C_Tis

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_Tis charged by an internal

current source of approximately 1 mA. The comparator

output remains blanked until the voltage on C_Treaches

V_BEMF– 1V

I_{PEAK BRAKE ML}

≈

R_LOAD

approximately 0.60V_CC

.

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 (C_T) connected in parallel from the RC

timing terminal to ground. The fixed off-time, over a range

of values of C_T= 470 pF to 1500 pF and R_T= 12 kΩ to

100 kΩ, is approximated by:

When the device is disabled, via the ENABLE input,

C_Tis discharged to near ground. When the device is re-

enabled, C_Tis charged by an internal current source of

approximately 1 mA. The comparator output remains

blanked until the voltage on C_Treaches approximately

0.60V_CC

.

For 3.3 V operation, the minimum recommended

value for C_Tis 680 pF 5 %. For 5.0 V operation, the

minimum recommended value for C_Tis 470 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_Tare recommended and the value of

R_Tcan be sized to determine t_OFF. For more information

regarding load current regulation, see below.

t_OFF≈ R_Tx C_T

The operation of the circuit is as follows: when the

PWM latch is reset by the current comparator, the voltage

on the RC terminal will begin to decay from approximately

0.60V_CC. When the voltage on the RC terminal reaches

approximately 0.22V_CC, the PWM latch is set, thereby

enabling the driver(s).

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7

3953

FULL-BRIDGE

PWM MOTOR DRIVER

LOAD CURRENT REGULATION

WITH INTERNAL PWM

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).

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

the user-selected value of t_OFFand the minimum on-time

pulse t_ON(min)max 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, a brush dc motor when

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

regulation can be approximated by:

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

[(V_BB– V_{SAT(source+sink)}) x t_ON(min)max] – (1.05(V_SAT(sink)+ V_F) x t_OFF

)

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

I_AVE

≈

1.05 x (t_ON(min)max + t_OFF) x R_LOAD

where t_OFF= R_Tx C_T, R_LOADis the series resistance of the

load, V_BBis 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

section on stepper motors below.

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.

The following procedure can be used to evaluate the

worst-case slow current-decay internal PWM load current

regulation in the system:

Set V_REFto 0 volts. With the load connected and the

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

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

C_Tthen should be increased until the measured value of

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

characteristics table. When the new value of C_Thas been

set, the value of R_Tshould be decreased so the value for

t_OFF= R_Tx C_T(with the artificially increased value of C_T) is

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

current regulation then can be measured in the system

under operating conditions.

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 t₁should be a

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

R_Tand C_Tcomponents 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.

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

8

3953

FULL-BRIDGE

PWM MOTOR DRIVER

Figure 3

To minimize current-sensing inaccuracies caused by

ground trace I x R drops, the current-sensing resistor

should have a separate return to the ground terminal of

the device. For low-value sense resistors, the I x R drops

in the printed wiring board can be significant and should

be taken into account. The use of sockets should be

avoided as their contact resistance can cause variations in

the effective value of R_S.

Synchronous Fixed-Frequency Control Circuit

V

CC

t

2

RC

1

Generally, larger values of R_Sreduce the aforemen-

tioned effects but can result in excessive heating and

power loss in the sense resistor. The selected value of R_S

should not cause the absolute maximum voltage rating of

1.0 V (0.4 V for V_CC= 3.3 V operation), for the SENSE

terminal, to be exceeded.

1N4001

2N2222

N

t

1

Dwg. EP-060

Miscellaneous Information. A logic high applied to both

the ENABLE and MODE terminals puts the device into a

sleep mode to minimize current consumption when not in

use.

The current-sensing comparator functions down to

ground allowing the device to be used in microstepping,

sinusoidal, and other varying current-profile applications.

An internally generated dead time prevents crossover

currents that can occur when switching phase or braking.

Thermal Considerations. For reliable operation it is

recommended that the maximum junction temperature be

kept below 110°C to 125°C. The junction temperature can

be measured best by attaching a thermocouple to the

power tab/batwing of the device and measuring the tab

temperature, T_TAB. The junction temperature can then be

approximated by using the formula:

Thermal protection circuitry turns off all drivers should

the junction temperature reach 165°C (typical). This is

intended only to protect the device from failures due to

excessive junction temperatures and should not imply that

output short circuits are permitted. The hysteresis of the

thermal shutdown circuit is approximately 15°C.

T_J≈ T_TAB+ (I_LOADx 2 x V_Fx R_θJT

)

where V_Fmay be chosen from the electrical specification

table for the given level of I_LOAD. The value for R_θJTis

given in the package thermal resistance table for the

appropriate package.

APPLICATION NOTES

Current Sensing. The actual peak load current (I_PEAK

)

will be above the calculated value of I_TRIPdue to delays in

the turn off of the drivers. The amount of overshoot can

be approximated by:

The power dissipation of the batwing packages can be

improved by 20% to 30% by adding a section of printed

circuit board copper (typically 6 to 18 square centimeters)

connected to the batwing terminals of the device.

(V_BB– [(I_TRIPx R_LOAD) + V_BEMF]) x t_PWM(OFF)

I_OS

≈

L_LOAD

The thermal performance in applications that run at

high load currents and/or high duty cycles can be im-

proved by adding external diodes in parallel with the

internal diodes. In internal PWM slow-decay applications,

only the two ground clamp diodes need be added. For

internal fast-decay PWM, or external PHASE or ENABLE

input PWM applications, all four external diodes should be

added for maximum junction temperature reduction.

where V_BBis the motor supply voltage, V_BEMFis the back-

EMF voltage of the load, R_LOADand L_LOADare the resis-

tance and inductance of the load respectively, and

t_PWM(OFF)is specified in the electrical characteristics table.

The reference terminal has a maximum input bias

current of 5 µA. This current should be taken into

account when determining the impedance of the external

circuit that sets the reference voltage value.

PCB Layout. The load supply terminal, V_BB, should be

decoupled with an electrolytic capacitor (>47 µF is recom-

www.allegromicro.com

9

3953

FULL-BRIDGE

PWM MOTOR DRIVER

the motor is more linear than in the case of ENABLE

PWM control (which produces a discontinuous current at

low current levels).

mended) placed as close to the device as is physically

practical. To minimize the effect of system ground I x R

drops on the logic and reference input signals, the system

ground should have a low-resistance return to the motor

supply voltage.

With bidirectional dc servo motors, the PHASE

terminal can be used for mechanical direction control.

Similar to when braking the motor dynamically, abrupt

changes in the direction of a rotating motor produces a

current generated by the back-EMF. The current gener-

ated will depend on the mode of operation. If the internal

current control circuitry is not being used, then the maxi-

mum load current generated can be approximated by

I_LOAD= (V_BEMF+ V_BB)/R_LOADwhere V_BEMFis proportional to

the motor’s speed. If the internal slow current-decay

control circuitry is used, then the maximum load current

See also “Current Sensing” and “Thermal Consider-

ations” above.

Fixed Off-Time Selection. With increasing values of t_OFF,

switching losses will decrease, low-level load-current

regulation will improve, EMI will be reduced, the PWM

frequency will decrease, and ripple current will increase.

The value of t_OFFcan be chosen for optimization of these

parameters. For applications where audible noise is a

concern, typical values of t_OFFare chosen to be in the

range of 15 ms to 35 ms.

generated can be approximated by I_LOAD= V_BEMF/R_LOAD

For both cases care must be taken to ensure that the

.

maximum ratings of the device are not exceeded. If the

internal fast current-decay control circuitry is used, then

the load current will regulate to a value given by:

Stepper Motor Applications. The MODE terminal can

be used to optimize the performance of the device in

microstepping/sinusoidal stepper-motor drive applications.

When the load current is increasing, slow decay mode is

used to limit the switching losses in the device and iron

losses in the motor. This also improves the maximum rate

at which the load current can increase (as compared to

I_LOAD= V_REF/R_S.

CAUTION: In fast current-decay mode, when the direction

of the motor is changed abruptly, the kinetic energy stored

in the motor and load inertia will be converted into current

that charges the V_BBsupply bulk capacitance (power

supply output and decoupling capacitance). Care must be

taken to ensure that the capacitance is sufficient to absorb

the energy without exceeding the voltage rating of any

devices connected to the motor supply.

fast decay) due to the slow rate of decay during t_OFF

.

When the load current is decreasing, fast-decay mode is

used to regulate the load current to the desired level. This

prevents tailing of the current profile caused by the back-

EMF voltage of the stepper motor.

In stepper-motor applications applying a constant

current to the load, slow-decay mode PWM is typically

used to limit the switching losses in the device and iron

losses in the motor.


型号：	A3953
厂家：	ALLEGRO MICROSYSTEMS
描述：	FULL-BRIDGE PWM MOTOR DRIVER 全桥PWM电机驱动器驱动器电机
文件：	总14页 (文件大小：229K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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