+125 C = +85 C + P (65 C/W)
D
°
°
°
(2)
P = (V
V
)
I
= (
-
OUT
D
IN
OUT
V
3.3 V)
0.08 A = 0.615 W
IN
-
´
´
(3)
5
Board Layout, Schematic, and Parts List
5.1
PCB Layouts
Board Layout, Schematic, and Parts List
www.ti.com
Example 1. Maximum Power Dissipation Calculation
What is the maximum input voltage that can be applied to a TPS71401 with the output voltage
configured to 3.3 V if the ambient temperature is +85
°
C and the full 80 mA of load current is required?
Given:
T
J
= +125
°
C, T
A
= +85
°
C,
θ
JA
= 65
°
C/W
Using
, we substitute in the given values above and find that the maximum power dissipation
for the part is P
D
= 0.615 W.
This result means that the total power dissipation of the TPS71401 must be less than 0.615 W. Now the
input voltage can be calculated:
Therefore, the maximum input voltage should be 10.98 V or less in order to maintain a safe junction
temperature. However, the recommended maximum input voltage is 10.0 V, so the device can be used
over the entire recommended input voltage range.
Similar analyses can be performed to determine the maximum ambient temperature allowed and still
provide the full output current while maintaining the junction temperature at or below +125
°
C. The
maximum ambient temperature depends on the output voltage.
lists the maximum ambient
temperature allowed for the pre-programmed output voltages.
Table 3. Output Voltage and Maximum Ambient
Temperature
Maximum Ambient
Output Voltage (V)
Temperature (
°
C)
5.0
99
3.3
90
2.7
87
2.5
86
This section provides the TPS71401DRVEVM-426 board layout and illustrations. It also includes the
schematic and bill of materials for the EVM.
through
show the layout for the TPS71401DRVEVM-426 PCB.
Note:
Board layouts are not to scale. These figures are intended to show how the board is laid out;
they are not intended to be used for manufacturing TPS71401DRVEVM-426 PCBs.
4
TPS71401DRVEVM-426
SBVU013 – January 2009
