LT1963A Series
W U U
APPLICATIO S I FOR ATIO
2. GND pin current multiplied by the input voltage:
(IGND)(VIN).
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Table 3. SOT-223 Package, 3-Lead SOT-223
COPPER AREA
TOPSIDE*
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
BACKSIDE
2500mm2
2500mm2
2500mm2
2500mm2
1000mm2
0mm2
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.
2500mm2
1000mm2
225mm2
100mm2
1000mm2
1000mm2
2500mm2
2500mm2
2500mm2
2500mm2
1000mm2
1000mm2
42∞C/W
42∞C/W
50∞C/W
56∞C/W
49∞C/W
52∞C/W
The LT1963A series regulators have internal thermal
limiting 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.
*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?
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 power dissipated by the device will be equal to:
IOUT(MAX)(VIN(MAX) – VOUT) + IGND(VIN(MAX)
)
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.
where,
IOUT(MAX) = 500mA
VIN(MAX) = 6V
IGND at (IOUT = 500mA, VIN = 6V) = 10mA
Table 1. Q Package, 5-Lead DD
COPPER AREA
THERMAL RESISTANCE
So,
BOARD AREA (JUNCTION-TO-AMBIENT)
TOPSIDE*
BACKSIDE
2500mm2
2500mm2
2500mm2
P = 500mA(6V – 3.3V) + 10mA(6V) = 1.41W
2500mm2
1000mm2
125mm2
2500mm2
2500mm2
2500mm2
23∞C/W
25∞C/W
33∞C/W
Using a DD package, the thermal resistance will be in the
range of 23∞C/W to 33∞C/W depending on the copper
area. So the junction temperature rise above ambient will
be approximately equal to:
*Device is mounted on topside
Table 2. SO-8 Package, 8-Lead SO
COPPER AREA
1.41W(28∞C/W) = 39.5∞C
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
TOPSIDE*
2500mm2
1000mm2
225mm2
BACKSIDE
2500mm2
2500mm2
2500mm2
2500mm2
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
2500mm2
2500mm2
2500mm2
2500mm2
55∞C/W
55∞C/W
63∞C/W
69∞C/W
100mm2
TJMAX = 50∞C + 39.5∞C = 89.5∞C
*Device is mounted on topside.
1963af
13
Linear [ Linear ]
