LT3008 データシートの表示（PDF） - Linear Technology
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LT3008 Datasheet PDF : 16 Pages
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LT3008
APPLICATIONS INFORMATION
The following tables list thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 two-layer boards with
one ounce copper.
PCB layers, copper weight, board layout and thermal vias
affect the resultant thermal resistance. Although Tables
2 and 3 provide thermal resistance numbers for 2-layer
boards with 1 ounce copper, modern multi-layer PCBs
provide better performance than found in these tables.
For example, a 4-layer, 1 ounce copper PCB board with
3 thermal vias from the DFN exposed backside or the
3 fused TSOT-23 GND pins to inner layer GND planes
achieves 45°C/W thermal resistance. Demo circuit DC
1388A’s board layout achieves this 45°C/W performance.
This is approximately a 30% improvement over the lowest
numbers shown in Tables 2 and 3.
Table 2: Measured Thermal Resistance for DC Package
COPPER AREA
TOPSIDE*
2500mm2
BACKSIDE
2500mm2
BOARD
AREA
2500mm2
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
65°C/W
1000mm2 2500mm2 2500mm2
70°C/W
225mm2
100mm2
2500mm2
2500mm2
2500mm2
2500mm2
75°C/W
80°C/W
50mm2
2500mm2 2500mm2
85°C/W
*Device is mounted on the topside.
Table 3: Measured Thermal Resistance for TSOT-23 Package
COPPER AREA
TOPSIDE*
2500mm2
BACKSIDE
2500mm2
BOARD
AREA
2500mm2
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
65°C/W
1000mm2 2500mm2 2500mm2
67°C/W
225mm2 2500mm2 2500mm2
70°C/W
100mm2 2500mm2 2500mm2
75°C/W
50mm2
2500mm2 2500mm2
85°C/W
*Device is mounted on the topside.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input volt-
age range of 12V ±5%, an output current range of 0mA
to 20mA and a maximum ambient temperature of 85°C,
what will the maximum junction temperature be for an
application using the DC package?
The power dissipated by the device is equal to:
IOUT(MAX) (VIN(MAX) – VOUT) + IGND (VIN(MAX))
where,
IOUT(MAX) = 20mA
VIN(MAX) = 12.6V
IGND at (IOUT = 20mA, VIN = 12.6V) = 0.3mA
So,
P = 20mA(12.6V – 3.3V) + 0.3mA(12.6V) = 189.8mW
The thermal resistance ranges from 65°C/W to 85°C/W
depending on the copper area. So the junction temperature
rise above ambient approximately equals:
0.1898W(75°C/W) = 14.2°C
The maximum junction temperature equals the maximum
junction temperature rise above ambient plus the maximum
ambient temperature or:
TJ(MAX) = 85°C + 14.2°C = 99.2°C
3008f
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