LP2980 Datasheet, PDF (14/18 Pages) National Semiconductor (TI) – Micropower 50 mA Ultra Low-Dropout Regulator

Typical Applications
FIGURE 5. 5V/400mA Regulator
DS012078-51
The LP2980 can be used to control higher-current regula-
tors, by adding an external PNP pass device. With the PNP
transistors shown in , the output current can be as high as
400 mA, as long as the input voltage is held within the Safe
Operation Boundary Curves shown below in.
To ensure regulation, the minimum input voltage of this regu-
lator is 6V. This “headroom” is the sum of the V_BE of the ex-
ternal transistor and the dropout voltage of the LP2980.
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. Recommended devices (other PNP transistors can be
used if the current gain and voltage ratings are similar).
C. Capacitor is required for regulator stability. Minimum size
is shown, and may be increased without limit.
D. Increasing the output capacitance improves transient re-
sponse and increases phase margin.
E. Maximum safe input voltage and load current are limited
by power dissipation in the PNP pass transistor and the
maximum ambient temperature for the specific application. If
a TO-92 transistor such as the MPS2907A is used, the ther-
mal resistance from junction-to-ambient is 180˚C/W in still
air.
Assuming a maximum allowable junction temperature of
150˚C for the MPS2907A device, the following curves show
the maximum V_IN and I_L values that may be safely used for
several ambient temperatures.
DS012078-52
FIGURE 6. Safe Operation Boundary Curves for Figure
5
FIGURE 7. 5V to 3.3V @ 3A Converter
DS012078-53
With limited input voltage range, the LP2980 can control a
3.3V, 3A regulator with the use of a high current-gain exter-
nal PNP pass transistorias shown in . 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 Requirements,
below. For best load regulation at the high load current, the
LP2980 output voltage connection should be made as close
to the load as possible.
Although this regulator can handle a much higher load cur-
rent than can the LP2980 alone, it can be shut down in the
same manner as the LP2980. When the ON/OFF control is
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