LT3013
15
3013fe
APPLICATIONS INFORMATION
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 generated
by power devices.
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 board with one ounce
copper.
Table 1. TSSOP Measured Thermal Resistance
COPPER AREA
TOPSIDE BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
2500 sq mm 2500 sq mm 40°C/W
1000 sq mm 2500 sq mm 45°C/W
225 sq mm 2500 sq mm 50°C/W
100 sq mm 2500 sq mm 62°C/W
Table 2. DFN Measured Thermal Resistance
COPPER AREA
TOPSIDE BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
2500 sq mm 2500 sq mm 40°C/W
1000 sq mm 2500 sq mm 45°C/W
225 sq mm 2500 sq mm 50°C/W
100 sq mm 2500 sq mm 62°C/W
The thermal resistance junction-to-case (θ
JC
), measured
at the exposed pad on the back of the die, is 16°C/W.
Continuous operation at large input/output voltage dif-
ferentials 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 three 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, average
power dissipation can be used for junction temperature
calculations if the pulse period is signifi cantly 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 8V to 12V, an output current range of 0mA to
250mA, and a maximum ambient temperature of 30°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)
= 250mA
V
IN(MAX)
= 12V
I
GND
at (I
OUT
= 250mA, V
IN
= 12V) = 8mA
So:
P = 250mA • (12V – 5V) + (8mA • 12V) = 1.85W
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:
1.85W • 50°C/W = 92.3°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
T
JMAX
= 30°C + 92.3°C = 122.3°C
Example 2: Given an output voltage of 5V, an input voltage
of 48V that rises to 72V for 5ms(max) out of every 100ms,
and a 5mA load that steps to 200mA for 50ms out of
every 250ms, what is the junction temperature rise above
ambient? Using a 500ms period (well under the time
constant of the board), power dissipation is as follows:
P1(48V in, 5mA load) = 5mA • (48V – 5V)
+ (200µA • 48V) = 0.23W
P2(48V in, 50mA load) = 200mA • (48V – 5V)
+ (8mA • 48V) = 8.98W
P3(72V in, 5mA load) = 5mA • (72V – 5V)
+ (200µA • 72V) = 0.35W
P4(72V in, 50mA load) = 200mA • (72V – 5V)
+ (8mA • 72V) = 13.98W
