LP2980 datasheet(14/18 Pages) NSC

14 / 18 page

Typical Applications

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 IL values that may be safely used for

several ambient temperatures.

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

DS012078-51

FIGURE 5. 5V/400mA Regulator

DS012078-52

FIGURE 6. Safe Operation Boundary Curves for

Figure

DS012078-53

FIGURE 7. 5V to 3.3V @ 3A Converter

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