Datasheet
P
D
+
ǒ
V
IN
* V
OUT
Ǔ
I
OUT
160
140
120
100
80
60
40
20
0
q
JA
( C/W)
°
0 1 2 3
4 5
6
7
8 9 10
Board Copper Area ( )in
2
DCQ
DRB
KTT
R
qJA
+
(
)125
O
C * T
A
)
P
D
TPS786xx
SLVS389L –SEPTEMBER 2002–REVISED OCTOBER 2010
www.ti.com
THERMAL INFORMATION
Knowing the maximum R
qJA
, the minimum amount of
POWER DISSIPATION
PCB copper area needed for appropriate heatsinking
can be estimated using Figure 26.
Knowing the device power dissipation and proper
sizing of the thermal plane that is connected to the
tab or pad is critical to avoiding thermal shutdown
and ensuring reliable operation.
Power dissipation of the device depends on input
voltage and load conditions and can be calculated
using Equation 4:
(4)
Power dissipation can be minimized and greater
efficiency can be achieved by using the lowest
possible input voltage necessary to achieve the
required output voltage regulation.
On the SON (DRB) package, the primary conduction
path for heat is through the exposed pad to the
printed circuit board (PCB). The pad can be
Note: q
JA
value at board size of 9in
2
(that is, 3in ×
connected to ground or be left floating; however, it
3in) is a JEDEC standard.
should be attached to an appropriate amount of
copper PCB area to ensure the device does not
Figure 26. q
JA
vs Board Size
overheat. On both SOT-223 (DCQ) and DDPAK
(KTT) packages, the primary conduction path for heat
Figure 26 shows the variation of q
JA
as a function of
is through the tab to the PCB. That tab should be
ground plane copper area in the board. It is intended
connected to ground. The maximum
only as a guideline to demonstrate the effects of heat
junction-to-ambient thermal resistance depends on
spreading in the ground plane and should not be
the maximum ambient temperature, maximum device
used to estimate actual thermal performance in real
junction temperature, and power dissipation of the
application environments.
device and can be calculated using Equation 5:
NOTE: When the device is mounted on an
application PCB, it is strongly recommended to use
(5)
Ψ
JT
and Ψ
JB
, as explained in the Estimating Junction
Temperature section.
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