Datasheet
LT1965 Series
15
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applications inForMation
The following tables list thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on a 2-layer 1/16" FR-4 board with
one ounce copper.
Table 1. Measured Thermal Resistance for DFN Package
Copper Area Thermal Resistance
Topside* Backside Board Area (Junction-to-Ambient)
2500mm
2
2500mm
2
2500mm
2
60°C/W
1000mm
2
2500mm
2
2500mm
2
62°C/W
225mm
2
2500mm
2
2500mm
2
65°C/W
100mm
2
2500mm
2
2500mm
2
68°C/W
50mm
2
2500mm
2
2500mm
2
70°C/W
*Device is mounted on topside
Table 2. Measured Thermal Resistance for MSOP Package
Copper Area Thermal Resistance
Topside* Backside Board Area (Junction-to-Ambient)
2500mm
2
2500mm
2
2500mm
2
55°C/W
1000mm
2
2500mm
2
2500mm
2
57°C/W
225mm
2
2500mm
2
2500mm
2
60°C/W
100mm
2
2500mm
2
2500mm
2
65°C/W
50mm
2
2500mm
2
2500mm
2
68°C/W
*Device is mounted on topside
Table 3. Measured Thermal Resistance for DD-Pak Package
Copper Area Thermal Resistance
Topside* Backside Board Area (Junction-to-Ambient)
2500mm
2
2500mm
2
2500mm
2
25°C/W
1000mm
2
2500mm
2
2500mm
2
30°C/W
125mm
2
2500mm
2
2500mm
2
35°C/W
*Device is mounted on topside
Measured Thermal Resistance for TO-220 Package
Thermal Resistance (Junction-to-Case) = 3°C/W
Calculating Junction Temperature
Example: Given an output voltage of 2.5V, an input volt
-
age 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?
The power dissipated by the device equals:
I
OUT(MAX)
• (V
IN(MAX)
– V
OUT
) + I
GND
• V
IN(MAX)
where:
I
OUT(MAX)
= 500mA
V
IN(MAX)
= 3.465V
I
GND
at (I
OUT
= 500mA, V
IN
= 3.465V) = 8.2mA
So,
P = 500mA(3.465V – 2.5V) + 8.2mA(3.465V) = 0.511W
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:
0.511W • 65°C/W = 33.22°C
The maximum junction temperature equals the maximum
ambient temperature plus the maximum junction tempera
-
ture rise above ambient or:
T
JMAX
= 85°C + 33.22°C = 118.22°C
Protection Features
The LT
1965 regulators incorporate several protection
features that makes them ideal for use in battery-powered
circuits. In addition to the normal protection features
associated with monolithic regulators, such as current
limiting and thermal limiting, the devices also protect
against reverse-input voltages, reverse-output voltages
and reverse output-to-input voltages.
Current-limit protection and thermal overload protection
protect the device against current overload conditions
at its output. For normal operation, do not exceed the
maximum rated junction temperature of 125°C (LT1965E,
LT1965I) or 150°C (LT1965H).