3952

FULL-BRIDGE

PWM MOTOR DRIVER

115 Northeast Cutoff, Box 15036

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

APPLICATION NOTES

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 as low as possible,

typically 90

°C to 125°C. The junction temperature can be

measured by attaching a thermocouple to the power tab/

batwing of the device and measuring the tab temperature,

T

using the formula

T

where V

determined from the electrical specification table for the

given level of I

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) con-

nected to the batwing terminals of the device.

The thermal performance in applications with high load

currents and/or high duty cycles can be improved by adding

external diodes in parallel with the internal diodes. In

internal PWM slow-decay applications, only the two top-side

(flyback) diodes need be added. For internal fast-decay

PWM, or external PHASE or ENABLE input PWM applica-

tions, all four external diodes should be added for maximum

junction temperature reduction.

imply that output short circuits are permitted. The hyster-

esis of the thermal shutdown circuit is approximately 15

°C.

If the internal current-control circuitry is not used; the

V

terminal should be connected to ground, and the RC

terminal should be left floating (no connection).

An internal under-voltage lockout circuit prevents

simultaneous conduction of the outputs when the device is

powered up or powered down.

Current Sensing

The actual peak load current (I

than the calculated value of I

OFF of the drivers. The amount of overshoot can be

approximated as

I

(V

L

LOAD

where V

back-EMF voltage of the load, R

resistance and inductance of the load respectively, and

t

characteristics table.

The reference terminal has an equivalent input resis-

tance of 50 k

Ω ±30%. This 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 IR drops, the current-sensing resistor should

have a separate return to the ground terminal of the

device. For low-value sense resistors, the IR drops in the

PCB 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

Larger values of R

but can result in excessive heating and power loss in the

sense resistor. The selected value of R

the SENSE terminal absolute maximum voltage rating to

be exceeded. The recommended value of R

range of

R