A3950SEUTR-T Allegro, A3950SEUTR-T Datasheet - Page 9

• Manufacturer Part Number: A3950SEUTR-T
• Manufacturer: Allegro

A3950
Power Dissipation. First order approximation of power
dissipation in the A3950 can be calculated by first examining
the power dissipation in the full-bridge during each of the
operation modes. The A3950 features synchronous rectifica-
tion, a feature that effectively shorts out the body diode by
turning on the low R
cycle. This significantly reduces power dissipation in the
full-bridge. In order to prevent shoot-through, where both
Figure 1. Current Decay Patterns
1
2
3
Drive current
Fast decay with synchronous rectification (reverse)
Slow decay with synchronous rectification (brake)
DS(on)
DMOS driver during the decay
VBB
1
3
2
Applications Information
DMOS Full-Bridge Motor Driver
source and sink driver are on at the same time, the A3950
implements a 500 ns typical crossover delay time. For this
period, the body diode in the decay current path conducts
the current until the DMOS driver turns on. This does affect
power dissipation and should be considered in high current,
high ambient temperature applications. In addition, motor
parameters and switching losses can add power dissipation
that could affect critical applications.
Drive Current. This current path is through source DMOS
driver, motor winding, and sink DMOS driver. Power dissi-
pation is I
as shown in the following equation:
Fast Decay with Synchronous Rectification. This
decay mode is equivalent to a phase change where the oppo-
site drivers are switched on. When in fast decay, the motor
current is not allowed to go negative (direction change).
Instead, as the current approaches zero, the drivers turn off.
The power calculation is the same as the drive current calcu-
lation, equation 1:
Slow Decay SR (Brake Mode). In this decay mode, both
sink drivers turn on, allowing the current to circulate through
the sink drivers and the load. Power dissipation is I
in the two sink DMOS drivers:
2
P
R loses in one source and one sink DMOS driver,
D
=
P =
I
D
2
(
R
I
DS(on)Source
2
(
2 ×
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
R
DS(on)Sink
+
R
DS(on)Sink
)
)
2
R loses
(1)
(2)
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