LT1965
APPLICATIONS INFORMATION
The following tables list thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 1/16" FR-4 board with one ounce
copper.
Calculating Junction Temperature
Example:Givenanoutputvoltageof2.5V, aninputvoltage
range of 3.3V 5%, an output current range of 0mA to
500mA and a maximum ambient temperature of 85°C,
what will the maximum junction temperature be?
Table 1. Measured Thermal Resistance for DFN Package
Copper Area
Topside* Backside
Thermal Resistance
The power dissipated by the device equals:
Board Area (Junction-to-Ambient)
2
2
2
2
2
2
2
2500mm
1000mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
60°C/W
62°C/W
65°C/W
68°C/W
70°C/W
I
• (V
– V ) + I
• V
OUT(MAX)
IN(MAX)
OUT
GND IN(MAX)
2
2
2
2
2
2
where:
225mm
100mm
2
I
= 500mA
= 3.465V
OUT(MAX)
2
50mm
V
*Device is mounted on topside
IN(MAX)
I
at (I = 500mA, V = 3.465V) = 8.2mA
OUT IN
GND
Table 2. Measured Thermal Resistance for MSOP Package
Copper Area
Topside* Backside
Thermal Resistance
So,
Board Area (Junction-to-Ambient)
P = 500mA(3.465V – 2.5V) + 8.2mA(3.465V) = 0.511W
2
2
2
2
2
2
2
2500mm
1000mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
55°C/W
57°C/W
60°C/W
65°C/W
68°C/W
2
2
2
2
2
Using a DFN package, the thermal resistance will be in
the range of 60°C/W to 70°C/W depending on the cop-
per area. So the junction temperature rise above ambient
approximately equals:
2
225mm
100mm
2
2
50mm
*Device is mounted on topside
0.511W • 65°C/W = 33.22°C
Table 3. Measured Thermal Resistance for DD-PAK Package
The maximum junction temperature equals the maximum
ambienttemperatureplusthemaximumjunctiontempera-
ture rise above ambient or:
Copper Area
Topside* Backside
Thermal Resistance
Board Area (Junction-to-Ambient)
2
2
2
2
2
2500mm
1000mm
125mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
25°C/W
30°C/W
35°C/W
2
2
2
T
= 85°C + 33.22°C = 118.22°C
JMAX
2
*Device is mounted on topside
Measured Thermal Resistance for TO-220 Package
Thermal Resistance (Junction-to-Case) = 3°C/W
1965f
11
Linear [ Linear ]
