LT1963 Series
APPLICATIO S I FOR ATIO
Thermal Considerations
The power handling capability of the device is limited by the
maximum rated junction temperature (125°C). The power
dissipated by the device is made up of two components:
1. Output current multiplied by the input/output voltage
differential: (I
OUT
)(V
IN
– V
OUT
), and
2. GND pin current multiplied by the input voltage:
(I
GND
)(V
IN
).
The GND pin current can be found using the GND Pin
Current curves in the Typical Performance Characteris-
tics. Power dissipation will be equal to the sum of the two
components listed above.
The LT1963 series regulators have internal thermal lim-
iting designed to protect the device during overload
conditions. For continuous normal conditions, the maxi-
mum junction temperature rating of 125°C must not be
exceeded. It is important to give careful consideration to
all sources of thermal resistance from junction to ambi-
ent. Additional heat sources mounted nearby must also
be considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
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.
Table 1. Q Package, 5-Lead DD
COPPER AREA
TOPSIDE*
BACKSIDE
2500mm
2
1000mm
2
125mm
2
2500mm
2
2500mm
2
2500mm
2
BOARD AREA
2500mm
2
2500mm
2
2500mm
2
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
23°C/W
25°C/W
33°C/W
*
Device is mounted on topside
U
Table 2. SO-8 Package, 8-Lead SO
COPPER AREA
TOPSIDE*
BACKSIDE
2500mm
2
1000mm
225mm
100mm
2
2
2
W
U U
BOARD AREA
2500mm
2
2500mm
2500mm
2500mm
2
2
2
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
55°C/W
55°C/W
63°C/W
69°C/W
2500mm
2
2500mm
2500mm
2500mm
2
2
2
*
Device is mounted on topside
Table 3. SOT-223 Package, 3-Lead SOT-223
COPPER AREA
TOPSIDE*
BACKSIDE
2500mm
2
BOARD AREA
2500mm
2
2500mm
2
2500mm
2
2500mm
2
1000mm
2
1000mm
2
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
42°C/W
42°C/W
50°C/W
56°C/W
49°C/W
52°C/W
2500mm
2
1000mm
2
225mm
2
100mm
2
1000mm
2
1000mm
2
2500mm
2
2500mm
2
2500mm
2
1000mm
2
0mm
2
*
Device is mounted on topside
Table 4. FE Package, 16-Lead TSSOP
COPPER AREA
TOPSIDE*
BACKSIDE
2500mm
2
1000mm
2
225mm
2
100mm
2
2500mm
2
2500mm
2
2500mm
2
2500mm
2
BOARD AREA
2500mm
2
2500mm
2
2500mm
2
2500mm
2
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
38°C/W
43°C/W
48°C/W
60°C/W
*
Device is mounted on topside
T Package, 5-Lead TO-220
Thermal Resistance (Junction-to-Case) = 4°C/W
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
range of 4V to 6V, an output current range of 0mA to
500mA and a maximum ambient temperature of 50°C,
what will the maximum junction temperature be?
The power dissipated by the device will be equal to:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
where,
I
OUT(MAX)
= 500mA
V
IN(MAX)
= 6V
I
GND
at (I
OUT
= 500mA, V
IN
= 6V) = 10mA
So,
P = 500mA(6V – 3.3V) + 10mA(6V) = 1.41W
1963fc
13
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