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A3953

FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems 

3953

FULL-BRIDGE

PWM MOTOR DRIVER

Figure 3

Synchronous Fixed-Frequency Control Circuit

V CC

t2

RC1

1N4001

2N2222

t

1

RC N

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.

An internally generated dead time prevents crossover

currents that can occur when switching phase or braking.

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.

APPLICATION NOTES

Current Sensing. The actual peak load current (IPEAK)

will be above the calculated value of ITRIP due to delays in

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

be approximated by:

(VBB – [(ITRIP x RLOAD) + VBEMF]) x tPWM(OFF)

IOS ≈

LLOAD

where VBB is the motor supply voltage, VBEMF is the back-

EMF voltage of the load, RLOAD and LLOAD are the resis-

tance and inductance of the load respectively, and

tPWM(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.

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 RS.

Generally, larger values of RS reduce the aforemen-

tioned effects but can result in excessive heating and

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

should not cause the absolute maximum voltage rating of

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

terminal, to be exceeded.

The current-sensing comparator functions down to

ground allowing the device to be used in microstepping,

sinusoidal, and other varying current-profile applications.

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, TTAB. The junction temperature can then be

approximated by using the formula:

TJ ≈ TTAB + (ILOAD x 2 x VF x RθJT)

where VF may be chosen from the electrical specification

table for the given level of ILOAD. The value for RθJT is

given in the package thermal resistance table for the

appropriate package.

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.

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.

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

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

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