Datasheet

Y Y
JT J T JT D
:T =T + P·
Y Y
JB J B JB D
:T =T + P·
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
q
JA
(KTW)
q
JA
(RGW)
TPS749xx
www.ti.com SBVS082G –JUNE 2007–REVISED NOVEMBER 2010
NOTE: When the device is mounted on an
application PCB, it is strongly recommended to use
Ψ
JT
and Ψ
JB
, as explained in the Estimating Junction
Temperature section.
ESTIMATING JUNCTION TEMPERATURE
Using the thermal metrics Ψ
JT
and Ψ
JB
, shown in the
Thermal Information table, the junction temperature
can be estimated with corresponding formulas (given
in Equation 6). For backwards compatibility, an older
q
JC
,Top parameter is listed as well.
(6)
Where P
D
is the power dissipation shown by
Note: q
JA
value at board size of 9in
2
(that is, 3in ×
Equation 4, T
T
is the temperature at the center-top of
3in) is a JEDEC standard.
the IC package, and T
B
is the PCB temperature
Figure 29. q
JA
vs Board Size
measured 1mm away from the IC package on the
PCB surface (refer to Figure 30).
Figure 29 shows the variation of q
JA
as a function of
NOTE: Both T
T
and T
B
can be measured on actual
ground plane copper area in the board. It is intended
application boards using a thermo-gun (an infrared
only as a guideline to demonstrate the effects of heat
thermometer).
spreading in the ground plane and should not be
For more information about measuring T
T
and T
B
, see
used to estimate actual thermal performance in real
the application note Using New Thermal Metrics
application environments.
(SBVA025), available for download at www.ti.com.
Copyright © 2007–2010, Texas Instruments Incorporated Submit Documentation Feedback 15