Datasheet
TPS2556
TPS2557
www.ti.com
SLVS931A – NOVEMBER 2009– REVISED FEBRUARY 2012
Table 1. Common R
ILIM
Resistor Selections (continued)
1750 63.1 63.4 62.8 64.0 1489 1742 1965
2000 55.2 54.9 54.4 55.4 1737 2012 2252
2250 49.1 48.7 48.2 49.2 1975 2269 2523
2500 44.2 44.2 43.8 44.6 2191 2501 2765
2750 40.2 40.2 39.8 40.6 2425 2750 3025
3000 36.9 36.5 36.1 36.9 2689 3030 3315
3250 34.0 34.0 33.7 34.3 2901 3253 3545
3500 31.6 31.6 31.3 31.9 3138 3501 3800
3750 29.5 29.4 29.1 29.7 3390 3764 4068
4000 27.7 27.4 27.1 27.7 3656 4039 4349
4250 26.0 26.1 25.8 26.4 3851 4241 4554
4500 24.6 24.9 24.7 25.1 4050 4446 4761
4750 23.3 23.2 23.0 23.4 4369 4773 5091
5000 22.1 22.1 21.9 22.3 4602 5011 5331
5250 21.1 21.0 20.8 21.2 4861 5274 5595
5500 20.1 20.0 19.8 20.2 5121 5539 5859
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 N-channel 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 N-channel 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 the "refined" r
DS(on)
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. The Dissipating Rating Table provides examples of thermal resistance for specific packages and board
layouts.
Copyright © 2009–2012, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): TPS2556 TPS2557