LP2980 Datasheet, PDF (15/18 Pages) National Semiconductor (TI) – Micropower 50 mA Ultra Low-Dropout Regulator

FIGURE 8. 5V to 3.3V @ 3A Converter
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With limited input voltage range, the LP2980 can control a
3.3V, 3A regulator with the use of a high current-gain external
PNP pass transistor as shown in Figure 8. If the regulator is
to be loaded with the full 3A, heat sinking will be required on
the pass transistor to keep it within its rated temperature
range. Refer to the Heatsink Thermal Resistance Require-
ments, below. For best load regulation at the high load cur-
rent, the LP2980 output voltage connection should be made
as close to the load as possible.
Although this regulator can handle a much higher load current
than can the LP2980 alone, it can be shut down in the same
manner as the LP2980. When the ON/OFF control is brought
low, the converter will be in shutdown, and will draw less than
1 μA from the source.
Notes:
A. Drive this input with a logic signal (see Application Hints).
If the shutdown function is not to be used, tie the ON/OFF pin
directly to the V_IN pin.
B. Capacitor is required for regulator stability. Minimum size
is shown, and may be increased without limit.
C. Increasing the output capacitance improves transient re-
sponse and increases phase margin.
D. A heatsink may be required for this transistor. The maxi-
mum allowable value for thermal resistance of the heatsink is
dependent on ambient temperature and load current (see
curves in Figure 9). Once the value is obtained from the graph,
a heatsink must be selected which has a thermal resistance
equal to or lower than this value. If the value is above 60°C/
W, no heatsink is required (the TO-220 package alone will
safely dissipate this).
For these curves, a maximum junction temperature of 150°C
is assumed for the pass transistor. The case-to-heatsink at-
tachment thermal resistance is assumed to be 1.5°C/W. All
calculations are for 5.5V input voltage (which is worst-case
for power dissipation).
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FIGURE 9. Heatsink Thermal Resistance Requirements for
Figure 8
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