Datasheet

LM1117-N, LM1117I
SNOS412M –FEBRUARY 2000–REVISED MARCH 2013 www.ti.com
P
D
= (V
IN
-V
OUT
)I
L
+ V
IN
I
G
(3)
Figure 24 shows the voltages and currents which are present in the circuit.
Figure 24. Power Dissipation Diagram
The next parameter which must be calculated is the maximum allowable temperature rise, T
R
(max):
T
R
(max) = T
J
(max)-T
A
(max)
where
• T
J
(max) is the maximum allowable junction temperature (125°C) which will be encountered in the application
• T
A
(max) is the maximum ambient temperature which will be encountered in the application (4)
Using the calculated values for T
R
(max) and P
D
, the maximum allowable value for the junction-to-ambient
thermal resistance (θ
JA
) can be calculated:
θ
JA
= T
R
(max)/P
D
(5)
If the maximum allowable value for θ
JA
is found to be ≥136°C/W for PFM package or ≥79°C/W for TO-220
package or ≥92°C/W for TO-263 package, no heatsink is needed since the package alone will dissipate enough
heat to satisfy these requirements. If the calculated value for θ
JA
falls below these limits, a heatsink is required.
As a design aid, Table 1 shows the value of the θ
JA
of SOT-223 and PFM for different heatsink area. The copper
patterns that we used to measure these θ
JA
s are shown at the end of APPLICATION INFORMATION. Figure 25
and Figure 26 reflects the same test results as what are in the Table 1
Figure 27 and Figure 28 shows the maximum allowable power dissipation vs. ambient temperature for the SOT-
223 and PFM device. Figure 29 and Figure 30 shows the maximum allowable power dissipation vs. copper area
(in
2
) for the SOT-223 and PFM devices. Please see AN1028 for power enhancement techniques to be used with
SOT-223 and PFM packages.
Application Note AN-1187 (SNOA401) discusses improved thermal performance and power dissipation for the
WSON.
Table 1. θ
JA
Different Heatsink Area
Layout Copper Area Thermal Resistance
Top Side (in
2
)
(1)
Bottom Side (in
2
) (θ
JA
,°C/W) SOT-223 (θ
JA
,°C/W) PFM
1 0.0123 0 136 103
2 0.066 0 123 87
3 0.3 0 84 60
4 0.53 0 75 54
5 0.76 0 69 52
6 1 0 66 47
7 0 0.2 115 84
8 0 0.4 98 70
9 0 0.6 89 63
10 0 0.8 82 57
11 0 1 79 57
12 0.066 0.066 125 89
13 0.175 0.175 93 72
(1) Tab of device attached to topside copper
12 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated
Product Folder Links: LM1117-N LM1117I