LT1963A Series
applicaTions inForMaTion
The following tables list thermal resistance for several The power dissipated by the device will be equal to:
different board sizes and copper areas. All measurements
I
(V
– V ) + I (V
)
OUT(MAX) IN(MAX)
OUT
GND IN(MAX)
were taken in still air on 1/16" FR-4 board with one ounce
copper.
where,
I
= 500mA
= 6V
OUT
OUT(MAX)
Table 3. Q Package, 5-Lead DD
V
IN(MAX)
COPPER AREA
THERMAL RESISTANCE
I
at (I
= 500mA, V = 6V) = 10mA
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
GND
IN
2
2
2
2
2
2
2
2
So,
2500mm
1000mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
23°C/W
25°C/W
33°C/W
P = 500mA(6V – 3.3V) + 10mA(6V) = 1.41W
2
125mm
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 4. S0-8 Package, 8-Lead SO
COPPER AREA
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
THERMAL RESISTANCE
1.41W(28°C/W) = 39.5°C
2
2
2
2
2
2
2
2
2
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
55°C/W
55°C/W
63°C/W
69°C/W
2
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
1000mm
2
225mm
125mm
2
*Device is mounted on topside
T
JMAX
= 50°C + 39.5°C = 89.5°C
Table 5. SOT-223 Package, 3-Lead SOT-223
Protection Features
COPPER AREA
THERMAL RESISTANCE
The LT1963A regulators incorporate several protection
featureswhichmakethemidealforuseinbattery-powered
circuits. In addition to the normal protection features
associated with monolithic regulators, such as current
limiting and thermal limiting, the devices are protected
against reverse input voltages, reverse output voltages
and reverse voltages from output to input.
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
2
2
2
2500mm
1000mm
2500mm
2500mm
2500mm
2500mm
42°C/W
42°C/W
2
2
2
2
2
2
2
2
2
2
2
225mm
100mm
2500mm
2500mm
1000mm
2500mm
2500mm
1000mm
1000mm
50°C/W
56°C/W
49°C/W
52°C/W
2
2
2
1000mm
1000mm
2
0mm
Current limit protection and thermal overload protection
areintendedtoprotectthedeviceagainstcurrentoverload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125°C.
*Device is mounted on topside
T Package, 5-Lead TO-220
Thermal Resistance (Junction-to-Case) = 4°C/W
The input of the device will withstand reverse voltages of
20V. Current flow into the device will be limited to less
than 1mA (typically less than 100µA) and no negative
voltage will appear at the output. The device will protect
both itself and the load. This provides protection against
batteries that can be plugged in backward.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input volt-
age 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?
1963aff
19
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