A3980 datasheet(14/18 Pages) ALLEGRO

14 / 18 page

Automotive DMOS Microstepping Driver

with Translator

A3980

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

Application Information

The A3980 is a power circuit, therefore careful consider-

ation must be given to power dissipation and the effects

of high currents on interconnect and supply wiring.

Power Dissipation. A ﬁrst order approximation of

the power dissipation in the A3980 can be determined

by examining the power dissipation in each of the

two full-bridges during each of the operation modes.

When synchronous rectiﬁcation is used, current ﬂow

most of the time through the DMOS FETs that are

switched on. When synchronous rectiﬁcation is not

used, the current ﬂows through the body diode of

the DMOS FETs during the decay phase. The use of

fast or slow decay also affects the dissipation. All the

above combinations can be calculated from ﬁve basic

DMOS output states, shown in the following illustrations.

Drive Current Ramp

Diagonally opposite DMOS

output transistors are on. Cur-

rent ﬂows from positive supply

through load to ground. Used in

all combinations.

Dissipation is I2R losses in the

DMOS transistors:

PD = I2(RDSONH+RDSONL)

Synchronous Slow Decay

Both low-side DMOS output

transistors are on. Current circu-

lates through both transistors and

the load.

Dissipation is I2R losses in the

DMOS transistors:

PSS = I2(2

× RDSONL)

Synchronous Fast Decay

Diagonally opposite DMOS output

transistors are on. Current ﬂows

from ground through load to posi-

tive supply.

Dissipation is I2R losses in the

DMOS transistors:

PSF = I2(RDSONH + RDSONL)

Non-Synchronous Slow Decay

One low-side DMOS output tran-

sistor and one body diode conduct-

ing. Current circulates through the

diode, the transistor, and the load.

Dissipation is I2R losses in the

DMOS transistors plus IV loss in

the diode:

PNS = (I2

× RDSONL)+ (I × VF)

Non-Synchronous Fast Decay

Diagonally opposite body diodes

conducting. Current ﬂows from

ground through load to positive

supply.

Dissipation is IV losses in the

diodes:

PNF = I(VFH + VFl)

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