Datasheet

TPS2554
TPS2555
SLVSAM0 – JUNE 2011 www.ti.com
Power Dissipation and Junction Temperature
The low on resistance of the N-channel MOSFET allows small surface-mount packages to pass large currents. It
is good design practice to estimate power dissipation and junction temperature. The below analysis gives an
approximation for calculating junction temperature based on the power dissipation in the package. However, it is
important to note that thermal analysis is strongly dependent on additional system-level factors. Such factors
include air flow, board layout, copper thickness and surface area, and proximity to other devices dissipating
power. Good thermal-design practice must include all system-level factors in addition to individual component
analysis.
Begin by determining the R
DS(on)
of the MOSFET relative to the input voltage and operating temperature. As an
initial estimate, use the highest operating ambient temperature of interest and read R
DS(on)
from the typical
characteristics graph. Using this value, the power dissipation can be calculated by:
• P
D
= R
DS(on)
× I
OUT
2
Where:
• P
D
= Total power dissipation (W)
• R
DS(on)
= Power switch on-resistance (Ω)
• I
OUT
= Maximum current-limit threshold (A)
This step calculates the total power dissipation of the MOSFET.
Finally, calculate the junction temperature:
• T
J
= P
D
× θ
JA
+ T
A
Where:
• T
A
= Ambient temperature (°C)
• θ
JA
= Thermal resistance (°C/W)
• P
D
= Total power dissipation (W)
Compare the calculated junction temperature with the initial estimate. If they are not within a few degrees, repeat
the calculation using a "refined" R
DS(on)
based on the calculated MOSFET temperature from the previous
calculation as the new estimate. Two or three iterations are generally sufficient to achieve the desired result. The
final junction temperature is highly dependent on thermal resistance θ
JA
, and thermal resistance is highly
dependent on the individual package and board layout.
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