lt3008ets8-trpbf Linear Technology Corporation, lt3008ets8-trpbf Datasheet - Page 11

• Manufacturer Part Number: lt3008ets8-trpbf
• Description: 3?a Iq, 20ma, 45v Low Dropout Linear Regulato
• Manufacturer: Linear Technology Corporation

Table 2: Measured Thermal Resistance for DC Package
Table 3: Measured Thermal Resistance for TSOT-23 Package
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.
*Device is mounted on the topside.
*Device is mounted on the topside.
TOPSIDE*
TOPSIDE*
2500mm
1000mm
2500mm
1000mm
225mm
100mm
225mm
100mm
50mm
50mm
COPPER AREA
COPPER AREA
2
2
2
2
2
2
2
2
2
2
BACKSIDE
BACKSIDE
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2
2
2
2
2
2
2
2
2
2
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
BOARD
BOARD
AREA
AREA
2
2
2
2
2
2
2
2
2
2
(JUNCTION-TO-AMBIENT)
(JUNCTION-TO-AMBIENT)
THERMAL RESISTANCE
THERMAL RESISTANCE
65°C/W
70°C/W
75°C/W
80°C/W
85°C/W
65°C/W
67°C/W
70°C/W
75°C/W
85°C/W
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:
where,
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:
The maximum junction temperature equals the maximum
junction temperature rise above ambient plus the maximum
ambient temperature or:
I
I
V
I
0.1898W(75°C/W) = 14.2°C
T
OUT(MAX)
OUT(MAX)
GND
J(MAX)
IN(MAX)
at (I
= 85°C + 14.2°C = 99.2°C
= 12.6V
OUT
(V
= 20mA
IN(MAX)
= 20mA, V
– V
OUT
IN
= 12.6V) = 0.3mA
) + I
GND
(V
IN(MAX)
LT3008
)
11
3008f