MIC5239_07 Datasheet, PDF (11/14 Pages) Micrel Semiconductor – Low Quiescent Current 500mA μCap LDO Regulator

Micrel
∆T = T_J(max) – T_A(max)
T_J(max) = 125°C
T_A(max) = maximum ambient operating
temperature
For example, the maximum ambient temperature is
50°C, the ∆T is determined as follows:
∆T = 125°C – 50°C
∆T = 75°C
Using Figure 6, the minimum amount of required copper
can be determined based on the required power
dissipation. Power dissipation in a linear regulator is
calculated as follows:
P_D = (V_IN – V_OUT) I_OUT + V_IN × I_GND
If we use a 3V output device and a 28V input at
moderate output current of 25mA, then our power
dissipation is as follows:
P_D = (28V – 3V) × 25mA + 28V 250µA
P_D = 625mW + 7mW
P_D = 632mW
From Figure 6, the minimum amount of copper required
to operate this application at a ∆T of 75°C is 110mm².
Quick Method
Determine the power dissipation requirements for the
design along with the maximum ambient temperature at
which the device will be operated. Refer to Figure 7,
which shows safe operating curves for three different
ambient temperatures: 25°C, 50°C and 85°C. From
these curves, the minimum amount of copper can be
determined by knowing the maximum power dissipation
required. If the maximum ambient temperature is 50°C
and the power dissipation is as above, 639mW, the
curve in Figure 7 shows that the required area of copper
is 110mm².
The θ_JA of this package is ideally 80°C/W, but it will vary
depending upon the availability of copper ground plane
to which it is attached.
Figure 7. Copper Area vs. Power-MSOP
Power Dissipation (T_A)
MIC5239
Figure 8. Copper Area vs. Power-SOIC
Power Dissipation (∆T_JA)
Figure 9. Copper Area vs. Power-SOIC
Power Dissipation (T_A)
The same method of determining the heatsink area used
for the power MSOP-8 can be applied directly to the
power SOIC-8. The same two curves showing power
dissipation versus copper area are reproduced for the
power SOIC-8 and they can be applied identically.
Power SOIC-8 Thermal Characteristics
The power SOIC-8 package follows the same idea as
the power MSOP-8 package, using four ground leads
with the die attach paddle to create a single-piece
electrical and thermal conductor, reducing thermal
resistance and increasing power dissipation capability.
Quick Method
Determine the power dissipation requirements for the
design along with the maximum ambient temperature at
which the device will be operated. Refer to Figure 9,
which shows safe operating curves for three different
ambient temperatures, 25°C, 50°C, and 85°C. From
these curves, the minimum amount of copper can be
determined by knowing the maximum power dissipation
required. If the maximum ambient temperature is 50°C,
and the power dissipation is 632mW, the curve in Figure
9 shows that the required area of copper is less than
100mm², when using the power SOIC-8.
December 2007
11
M9999-121007