DMOS Microstepping Driver with Translator

A3979

10

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

cycle is triggered by an internal fixed–off-time cycle, load

current recirculates according to the decay mode selected

by the control logic. The A3979 synchronous rectification

feature turns on the appropriate MOSFETs during the decay

of the current, and effectively shorts out the body diodes

significantly and eliminates the need for external Schottky

diodes for most applications. The synchronous rectification

can be set to either Active mode or Disabled mode:

• Active Mode. When the SR input is logic low, Active

mode is enabled and synchronous rectification can occur.

This mode prevents reversal of the load current by turning

off synchronous rectification when a zero current level is

detected. This prevents the motor winding from conduct-

ing in the reverse direction.

• Disabled Mode. When the SR input is logic high, syn-

chronous rectification is disabled. This mode is typi-

cally used when external diodes are required to transfer

power dissipation from the A3979 package to the external

diodes.

mounted should have a heavy ground plane. For optimum

electrical and thermal performance, the A3979 should be

soldered directly onto the board.

The load supply terminals, VBBx, should be decoupled with

an electrolytic capacitor (>47 μF is recommended), placed

as close to the device as possible.

To avoid problems due to capacitive coupling of the high

dv / dt switching transients, route the bridge-output traces

away from the sensitive logic-input traces.

Always drive the logic inputs with a low source impedance

to increase noise immunity.

PGND (power ground) terminal must be connected together

externally.

All ground lines should be connected together and be as

short as possible. A star ground system, centered under the

device, is an optimum design.

The copper ground plane located under the exposed thermal

pad is typically used as the star ground.

ground-trace IR drops in sensing the output current level,

ground return to the star ground of the device. This path

should be as short as possible.

For low-value sense resistors, the IR drops in the printed cir-

cuit board sense resistor traces can be significant and should

be taken into account.

The use of sockets should be avoided as they can introduce

all drivers when the junction temperature reaches 165°C,

typical. It is intended only to protect the device from failures

due to excessive junction temperatures and should not imply

that output short circuits are permitted. Thermal shutdown

has a hysteresis of approximately 15°C.

Applications Information