LP2975 Datasheet, PDF (16/37 Pages) National Semiconductor (TI) – MOSFET LDO Driver/Controller

LP2975
SNVS006F – SEPTEMBER 1997 – REVISED APRIL 2013
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DESIGN EXAMPLE: A design is to be done with V_IN = 5V and V_OUT = 3.3V with a maximum load current of 300
mA. Based on these conditions, power dissipation in the FET during normal operation would be:
P_D = (V_IN − V_OUT) × I_LOAD
Solving, we find that P_D = 0.51W. Assuming that the maximum allowable value of T_J is 150°C and the maximum
T_A is 70°C, the value of θ_J-A is found to be 157°C/W.
However, if this design must survive a continuous short on the output, the power dissipated in the FET is higher:
P_D(SC) = V_IN × I_SC = 5 × 0.33 = 1.65W
(This assumes the current sense resistor is selected for an I_SC value that is 10% higher than the required 0.3A).
The value of θ_J-A required to survive continuous short circuit is calculated to be 49°C/W.
Having solved for the value(s) of θ_J-A, a FET can be selected. It should be noted that a FET must be used with a
θ_J-A value less than or equal to the calculated value.
HIGH POWER (≥2W) APPLICATIONS: As power dissipation increases above 2W, a FET in a larger package
must be used to obtain lower values of θ_J-A. The same formulae derived in the previous section are used to
calculate P_D and θ_J-A.
Having found θ_J-A, it becomes necessary to calculate the value of θ_S-A (the heatsink-to-ambient thermal
resistance) so that a heatsink can be selected:
θ_S-A = θ_J-A − (θ_J-C + θ_C-S)
where
• θ_J-C is the junction-to-case thermal resistance. This parameter is the measure of thermal resistance between
the semiconductor die inside the FET and the surface of the case of the FET where it mounts to the heatsink
(the value of θ_J-C can be found on the data sheet for the FET). A typical FET in a TO-220 package will have a
θ_J-C value of approximately 2–4°C/W, while a device in a TO-3 package will be about 0.5–2°C/W.
• θ_C-S is the case-to-heatsink thermal resistance, which measures how much thermal resistance exists between
the surface of the FET and the heatsink. θ_C-S is dependent on the package type and mounting method. A TO-
220 package with mica insulator and thermal grease secured to a heatsink will have a θ_C-S value in the range
of 1–1.5°C/W. A TO-3 package mounted in the same manner will have a θ_C-S value of 0.3–0.5°C/W. The best
source of information for this is heatsink catalogs (Wakefield, AAVID, Thermalloy) since they also sell
mounting hardware.
• θ_S-A is the heatsink-to-ambient thermal resistance, which defines how well a heatsink transfers heat into the
air. Once this is determined, a heatsink must be selected which has a value which is less than or equal to the
computed value.
The value of θ_S-A is usually listed in the manufacturer's data sheet for a heatsink, but the information is
sometimes given in a graph of temperature rise vs. dissipated power.
DESIGN EXAMPLE: A design is to be done which takes 3.3V in and provides 2.5V out at a load current of 7A.
The power dissipation will be calculated for both normal operation and short circuit conditions.
For normal operation:
P_D = (V_IN − V_OUT) × I_LOAD = 5.6W
If the output is shorted to ground:
P_D(SC) = V_IN × I_SC = 3.3 × 7.7 = 25.4W
(Assuming that a sense resistor is selected to set the value of I_SC 10% above the nominal 7A).
θ_J-A will be calculated assuming a maximum T_A of 70°C and a maximum T_J of 150°C:
θ_J-A = (T_J − T_A)/P_D(MAX)
For normal operation:
θ_J-A = (150 − 70) / 5.6 = 14.3°C/W
For designs which must operate with the output shorted to ground:
θ_J-A = (150 − 70) / 25.4 = 3.2°C/W
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