A3981 Allegro Micro Systems, Inc., A3981 Datasheet - Page 29

• Manufacturer Part Number: A3981
• Manufacturer: Allegro Micro Systems, Inc.

A3981
Power Dissipation
The A3981 is a power circuit, therefore careful consideration
must be given to power dissipation and the effects of high cur-
rents on interconnect and supply wiring.
A first order approximation of the power dissipation in the A3981
can be determined by examining the power dissipation in each of
the two bridges during each of the operation modes. When syn-
Synchronous Slow Decay
Synchronous Fast Decay
• Diagonally opposite DMOS output
• Current flows from ground through
• Both low-side DMOS output
• Current circulates through both
Dissipation is I
P
Dissipation is I
P
transistors are on
load to positive supply
DMOS transistors:
transistors are on
transistors and the load
transistors:
D(SF)
D(SS)
= I
= I
2
2
Figure 14. Basic output states
× (R
× (2 × R
2
2
DS(on)H
R losses in the
R losses in the DMOS
DS(on)L
+R
Automotive, Programmable Stepper Driver
Drive Current Ramp-up
DS(on)L
)
)
• Diagonally opposite DMOS output
• Current flows from positive supply
(Used in all combinations)
Dissipation is I
P
transistors are on
through load to ground
DMOS transistors:
D
= I
chronous rectification is used current will flow most of the time
through the DMOS transistors that are switched on. When syn-
chronous rectification is not used the current will flow through
the body diode of the DMOS transistors during the decay phase.
The use of fast or slow decay will also affect the dissipation. All
the above combinations can be calculated from five basic DMOS
output states as shown in figure 14.
2
× (R
DS(on)H
2
R losses in the
+ R
Non-Synchronous Slow Decay
Non-Synchronous Fast Decay
DS(on)L
)
• Diagonally opposite body diodes
• Current flows from ground through
• One low-side DMOS output transistor
• Current circulates through the diode,
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
conducting
Dissipation is IV losses in the diodes:
P
and one body diode conducting
Dissipation is I
P
load to positive supply
the transistor and the load
transistors plus IV loss in the diode:
D(NF)
D(NS)
= I × ( V
= (I
2
× R
FH
2
DS(on)L
R losses in the DMOS
+ V
FL
) ⁄ ( I × V
)
F
)
29