LP2980 Datasheet, PDF(13/18 Page) National Semiconductor (TI) – Micropower 50 mA Ultra Low-Dropout Regulator

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LP2980 Datasheet, PDF (13/18 Pages) National Semiconductor (TI) – Micropower 50 mA Ultra Low-Dropout Regulator

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In any application where the output voltage may be higher

than the input voltage, an external Schottky diode must be

connected from VIN to VOUT (cathode on VIN, anode on

VOUT. See Figure 5), to limit the reverse voltage across the

LP2980 to 0.3V (See Absolute Maximum Ratings).

1207850

FIGURE 3. 10 Î¼F ESR Range

REVERSE CURRENT PATH

The internal PNP power transistor used as the pass element

in the LP2980 has an inherent diode connected between the

regulator output and input. During normal operation (where

the input voltage is higher than the output) this diode is re-

verse biased (See Figure 4).

1207834

FIGURE 4. LP2980 Reverse Current Path

However, if the input voltage is more than a VBE below the

output voltage, this diode will turn ON and current will flow into

the regulator output. In such cases, a parasitic SCR can latch

which will allow a high current to flow into the VIN pin and out

the ground pin, which can damage the part.

The internal diode can also be turned on if the input voltage

is abruptly stepped down to a voltage which is a VBE below

the output voltage.

1207835

FIGURE 5. Adding External Schottky Diode Protection

ON/OFF INPUT OPERATION

The LP2980 is shut off by pulling the ON/OFF input low, and

turned on by driving the input high. If this feature is not to be

used, the ON/OFF input should be tied to VIN to keep the reg-

ulator on at all times (the ON/OFF input must not be left

floating).

To ensure proper operation, the signal source used to drive

the ON/OFF input must be able to swing above and below the

specified turn-on/turn-off voltage thresholds which guarantee

an ON or OFF state (see Electrical Characteristics).

The ON/OFF signal may come from either a totem-pole out-

put, or an open-collector output with pull-up resistor to the

LP2980 input voltage or another logic supply. The high-level

voltage may exceed the LP2980 input voltage, but must re-

main within the Absolute Maximum Ratings for the ON/OFF

pin.

It is also important that the turn-on/turn-off voltage signals

applied to the ON/OFF input have a slew rate which is greater

than 40 mV/Î¼s.

IMPORTANT: The regulator shutdown function will not oper-

ate correctly if a slow-moving signal is used to drive the ON/

OFF input.

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