1. System requirements:
VOUT = 5.0V; VIN (MAX) = 9.0V; VIN (MIN) = 5.6V; IOUT = 700mA; Operating cycle = 100%; TA = 50 °C According to the above system requirements select 750mA MIC2937A5.0BU regulator, Its parameters are:
VOUT = 5V ± 2% (worst case when overheated) TJ MAX = 125°C. Using TO263 package, θJC=3°C/W; θCS≈0°C/W (directly soldered on circuit board).
2. Preliminary calculation:
VOUT(MIN)=5V5×2%=4.9VPD=(VIN(MAX)VOUT(MIN))+IOUT+(VIN(MAX)×I)=[9V4.9V]×700mA+(9V×15mA) = 3W maximum value of temperature rise, ΔT = TJ(MAX)  TA = 125°C  50°C = 75°C; thermal resistance θJA (worst case): ΔT/PD = 75°C/3.0W = 25°C/W.
Thermal resistance of the heat sink, θSA=θJA(θJC+θCS); θSA=25(3+0)=22°C/W (maximum).
3. Determine the physical dimensions of the radiator:
A square, singlesided, horizontal solder mask with a copper foil heatsink and a heatdissipating copper foil covered with a black oil paint, with a 1.3 m/sec air cooling solution, is better than the latter.
With the solidline approach, a conservative design requires 5,000mm2 of heatdissipating copper foil, a 71mm x 71mm (2.8 inches on each side) square.
4. Thermal requirements for SO8 and SOT223 packages:
Calculate the thermal dissipation area under the following conditions: VOUT=5.0V; VIN(MAX)=14V; VIN(MIN)=5.6V; IOUT=150mA; Duty Cycle=100%; TA=50°C. Circuit board production equipment is more likely to handle dualinline SO8 packaged devices under permissible conditions. Can SO8 meet this requirement? With the MIC295103BM (SO8 package), the following parameters can be obtained:
TJ MAX=125°C; θJC≈100°C/W.
5. Calculate the parameters using the SO8 package:
PD = [14V5V] x 150mA + (14V x 8mA) = 1.46W; elevated temperature = 125°C  50°C = 75°C; thermal resistance θJA (worst case):
ΔT/PD = 75°C/1.46W = 51.3°C/W; θSA = 51100 = 49°C/W (maximum).
Obviously, SO8 can not meet the design requirements without refrigeration. Consider the MIC52015.0BS regulator in the SOT223 package. The package is smaller than the SO8, but its three pins have good heat dissipation. Select MIC52013.3BS, its related parameters are as follows:
TJ MAX=125°C The thermal resistance of OT223 is θJC=15°C/WθCS=0°C/W (directly soldered on the board).
6. Calculate the result of using SOT223 package:
PD = [14V4.9V] × 150mA + (14V × 1.5mA) = 1.4W rising temperature = 125°C  50°C = 75°C; thermal resistance θJA (worst case):
ΔT/PD=75°C/1.4W=54°C/W; θSA=5415=39°C/W (maximum). According to the above data, referring to Figure 1, the use of 1,400 mm2 heatdissipating copper foil (a square with a side length of 1.5 inches) can meet the design requirements.
The above design results can be used as a rough reference. The actual design needs to understand the thermal characteristics of the circuit board and obtain more accurate results that meet the actual design.
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