A3981 Allegro Micro Systems, Inc., A3981 Datasheet - Page 30

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

A3981
The total power dissipation for each of the four decay modes,
P
tion, P
cycle. For slow decay the current will be rising for approximately
20% of the cycle and decaying for approximately 80%. For fast
decay the ratio will be approximately 50%. Note that these are
approximate figures and will vary slightly depending on the
motor characteristics and the use of synchronous rectification.
The following formulas may be used to estimate total power dis-
sipation:
• Synchronous slow decay mode
• Non-synchronous slow decay mode
D(TOT) XX
P
P
P
P
D(TOT)SS
D(TOT)SS
D(TOT)NS
D(TOT)NS
D
, and the drive current decay portion, P
, is the average power for the drive current ramp por-
= 0.2 × P
= 0.2 (I
= 0.2 × P
= 0.2 ( I
2
2
[R
[R
D
D
DS(on)H
+ 0.8 × P
DS(on)H
+ 0.8 × P
+ R
+ R
D(SS)
D(NS)
DS(on)L
DS(on)L
Figure 15. Allowable power dissipation, on typical PCBs
]) + 0.8 (I
]) + 0.8 (I
5
4
3
2
1
0
25
2
D(XX)
× R
2
R
(on 2-layer PCB)
× 2 × R
DS(on)L
Automotive, Programmable Stepper Driver
θJA
of the PWM
50
= 32 °C/W
DS(on)L
+ I × V
Ambient Temperature (°C)
75
F
)
)
• Synchronous fast decay mode
• Non-synchronous fast decay mode
An approximation of the total dissipation can be calculated by
summing the total power dissipated in both bridges and adding
the control circuit power due to V
The total power at the required ambient temperature can then be
compared to the allowable power dissipation shown in figure 15.
For critical applications, where the first order power estimate is
close to the allowable dissipation, the power calculation should
take several other parameters into account including: motor
parameters, dead time, and switching losses in the controller.
R
(on 4-layer PCB)
P
P
P
P
θJA
D(TOT)SF
D(TOT)SF
D(TOT)NF
D(TOT)NF
100
= 28 °C/W
= 0.5 × P
= I
= 0.5 × P
= 0.5( I
125
2
(R
DS(on)H
2
D
D
[R
+ 0.5 × P
+ 0.5 × P
150
DS(on)H
+ R
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
DS(on)L
+ R
D(SF)
D(NF)
BB
DS(on)L
)
× I
BB
] ) + 0.5( I × [V
and V
DD
× I
DD
FH
.
+ V
FL
] )
30