LT3010 LINER [Linear Technology], LT3010 Datasheet - Page 10

• Manufacturer Part Number: LT3010
• Description: 50mA, 3V to 80V Low Dropout Micropower Linear Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LT3010/LT3010-5
The LT3010 series regulators have internal thermal limit-
ing designed to protect the device during overload condi-
tions. For continuous normal conditions the maximum
junction temperature rating of 125 C must not be ex-
ceeded. It is important to give careful consideration to all
sources of thermal resistance from junction to ambient.
Additional heat sources mounted nearby must also be
considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
The following table lists thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with one ounce
copper.
Table 1. Measured Thermal Resistance
The thermal resistance junction-to-case (
at the exposed pad on the back of the die, is 16 C/W.
Continuous operation at large input/output voltage differ-
entials and maximum load current is not practical due to
thermal limitations. Transient operation at high input/
output differentials is possible. The approximate thermal
time constant for a 2500sq mm 3/32" FR-4 board with
10
2500 sq mm
1000 sq mm
225 sq mm
100 sq mm
TOPSIDE
COPPER AREA
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
BACKSIDE
U
BOARD AREA
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
U
W
(JUNCTION-TO-AMBIENT)
THERMAL RESISTANCE
40 C/W
45 C/W
50 C/W
62 C/W
JC
), measured
U
maximum topside and backside area for one ounce copper
is 3 seconds. This time constant will increase as more
thermal mass is added (i.e. vias, larger board, and other
components).
For an application with transient high power peaks, aver-
age power dissipation can be used for junction tempera-
ture calculations as long as the pulse period is significantly
less than the thermal time constant of the device and
board.
Calculating Junction Temperature
Example 1: Given an output voltage of 5V, an input voltage
range of 24V to 30V, an output current range of 0mA to
50mA, and a maximum ambient temperature of 50 C,
what will the maximum junction temperature be?
The power dissipated by the device will be equal to:
where:
So:
The thermal resistance will be in the range of 40 C/W to
62 C/W depending on the copper area. So the junction
temperature rise above ambient will be approximately
equal to:
I
I
V
I
OUT(MAX)
OUT(MAX)
GND
P = 50mA • (30V – 5V) + (1mA • 30V) = 1.28W
1.31W • 50 C/W = 65.5 C
IN(MAX)
at (I
= 30V
OUT
• (V
= 50mA
= 50mA, V
IN(MAX)
– V
IN
OUT
= 30V) = 1mA
) + (I
GND
• V
IN(MAX)
)
3010f