Datasheet

LT3009 Series
15
3009fd
The following tables list thermal resistance for several dif-
ferent board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with one ounce
copper.
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:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
where,
I
OUT(MAX)
= 20mA
V
IN(MAX)
= 12.6V
I
GND
at (I
OUT
= 20mA, V
IN
= 12.6V) = 0.45mA
So,
P = 20mA(12.6V – 3.3V) + 0.45mA(12.6V) = 191.7mW
The thermal resistance will be in the range of 65°C/W to
85°C/W depending on the copper area. So the junction
temperature rise above ambient will be approximately
equal to:
0.1917W(75°C/W) = 14.4°C
The maximum junction temperature equals the maximum
junction temperature rise above ambient plus the maximum
ambient temperature or:
T
J(MAX)
= 85°C + 14.4°C = 99.4°C
Table 2: Measured Thermal Resistance for DC Package
COPPER AREA
BOARD
AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)TOPSIDE* BACKSIDE
2500mm
2
2500mm
2
2500mm
2
65°C/W
1000mm
2
2500mm
2
2500mm
2
70°C/W
225mm
2
2500mm
2
2500mm
2
75°C/W
100mm
2
2500mm
2
2500mm
2
80°C/W
50mm
2
2500mm
2
2500mm
2
85°C/W
*Device is mounted on the topside.
Table 3: Measured Thermal Resistance for SC70 Package
COPPER AREA
BOARD
AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)TOPSIDE* BACKSIDE
2500mm
2
2500mm
2
2500mm
2
75°C/W
1000mm
2
2500mm
2
2500mm
2
80°C/W
225mm
2
2500mm
2
2500mm
2
85°C/W
100mm
2
2500mm
2
2500mm
2
90°C/W
50mm
2
2500mm
2
2500mm
2
95°C/W
*Device is mounted on the topside.
APPLICATIONS INFORMATION