lt3009 Linear Technology Corporation, lt3009 Datasheet - Page 12

• Manufacturer Part Number: lt3009
• Description: 3?a Iq, 20ma Low Dropout Linear Regulators
• Manufacturer: Linear Technology Corporation

LT3009 Series
APPLICATIONS INFORMATION
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.
Table 2: Measured Thermal Resistance for DC Package
Table 3: Measured Thermal Resistance for SC70 Package
12
*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
75°C/W
80°C/W
85°C/W
90°C/W
95°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) + 0.45mA(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:
The maximum junction temperature equals the maximum
junction temperature rise above ambient plus the maximum
ambient temperature or:
I
I
V
I
0.1917W(75°C/W) = 14.4°C
T
OUT(MAX)
OUT(MAX)
GND
J(MAX)
IN(MAX)
at (I
= 85°C + 14.4°C = 99.4°C
= 12.6V
OUT
(V
= 20mA
IN(MAX)
= 20mA, V
– V
OUT
IN
= 12.6V) = 0.45mA
) + I
GND
(V
IN(MAX)
)
3009fa