LP2975 Datasheet, PDF (17/37 Pages) National Semiconductor (TI) – MOSFET LDO Driver/Controller

LP2975
www.ti.com
SNVS006F – SEPTEMBER 1997 – REVISED APRIL 2013
The value of 14.3°C/W can be easily met using a TO-220 device. Calculating the value of θ_S-A required
(assuming a value of θ_J-C = 3°C/W and θ_C-S = 1°C/W):
θ_S-A = θ_J-A − (θ_J-C + θ_C-S)
θ_S-A = 14.3 − (3 + 1) = 10.3°C/W
Any heatsink may be used with a thermal resistance ≤ 10.3°C/W @ 5.6W power dissipation (refer to
manufacturer's data sheet curves). Examples of suitable heatsinks are Thermalloy #6100B and IERC
#LATO127B5CB.
However, if the design must survive a sustained short on the output, the calculated θ_J-A value of 3.2°C/W
eliminates the possibility of using a TO-220 package device.
Assuming a TO-3 device is selected with a θ_J-C value of 1.5°C/W and θ_C-S = 0.4°C/W, we can calculate the
required value of θ_S-A:
θ_S-A = θ_J-A − (θ_J-C + θ_C-S)
θ_S-A = 3.2 − (1.5 + 0.4) = 1.3°C/W
A θ_S-A value ≤1.3°C/W would require a relatively large heatsink, or possibly some kind of forced airflow for
cooling.
SHORT-CIRCUIT CURRENT LIMITING
Short-circuit current limiting is easily implemented using a single external resistor (R_SC). The value of R_SC can be
calculated from:
R_SC = V_CL / I_SC
where
• I_SC is the desired short circuit current.
• V_CL is the current limit sense voltage.
The value of V_CL is 57 mV (typical), with specified limits listed in the ELECTRICAL CHARACTERISTICS section.
When doing a worst-case calculation for power dissipation in the FET, it is important to consider both the
tolerance of V_CL and the tolerance (and temperature drift) of R_SC.
For maximum accuracy, the INPUT and CURRENT LIMIT pins must be Kelvin connected to R_SC, to avoid errors
caused by voltage drops along the traces carrying the current from the input supply to the Source pin of the FET.
EXTERNAL CAPACITORS
The best capacitors for use in a specific design will depend on voltage and load current (examples of tested
circuits for several different output voltages and currents are provided in a previous section.)
Information in the next sections is provided to aid the designer in the selection of the external capacitors.
Input Capacitor
Although not always required, an input capacitor is recommended. Good bypassing on the input assures that the
regulator is working from a source with a low impedance, which improves stability. A good input capacitor can
also improve transient response by providing a reservoir of stored energy that the regulator can utilize in cases
where the load current demand suddenly increases. The value used for C_IN may be increased without limit. Refer
to the REFERENCE DESIGNS section for examples of input capacitors.
Output Capacitor
The output capacitor is required for loop stability (compensation) as well as transient response. During sudden
changes in load current demand, the output capacitor must source or sink current during the time it takes the
control loop of the LP2975 to adjust the gate drive to the pass FET. As a general rule, a larger output capacitor
will improve both transient response and phase margin (stability). The value of C_OUT may be increased without
limit.
OUTPUT CAPACITOR AND COMPENSATION: Loop compensation for the LP2975 is derived from C_OUT and, in
some cases, the feed-forward capacitor C_F (see next section).
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