LT3014
APPLICATIO S I FOR ATIO
The LT3014 regulator has internal thermal limiting de-
signed to protect the device during overload conditions.
For continuous normal conditions the maximum junction
temperature rating of 125°C must not be exceeded. 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. SOT-23 Measured Thermal Resistance
COPPER AREA
TOPSIDE
2500 sq mm
1000 sq mm
225 sq mm
100 sq mm
50 sq mm
BACKSIDE
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
BOARD AREA
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
125°C/W
125°C/W
130°C/W
135°C/W
150°C/W
Table 2. DFN Measured Thermal Resistance
COPPER AREA
TOPSIDE
2500 sq mm
1000 sq mm
225 sq mm
100 sq mm
BACKSIDE
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
BOARD AREA
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
40°C/W
45°C/W
50°C/W
62°C/W
For the DFN package, the thermal resistance junction-to-
case (θ
JC
), measured at the exposed pad on the back of the
die, is 16°C/W.
U
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
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
20mA, 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:
I
OUT(MAX)
• (V
IN(MAX)
– V
OUT
) + (I
GND
• V
IN(MAX)
)
where:
I
OUT(MAX)
= 20mA
V
IN(MAX)
= 30V
I
GND
at (I
OUT
= 20mA, V
IN
= 30V) = 0.55mA
So:
P = 20mA • (30V – 5V) + (0.55mA • 30V) = 0.52W
The thermal resistance for the DFN package 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:
0.52W • 50°C/W = 26°C
3014fb
W
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