LP2980-ADJ datasheet(8/9 Pages) NSC

8 / 9 page

Application Hints (Continued)

Off-brand capacitors should be avoided, as some poor qual-

ity Tantalums are seen with ESR’s > 10

Ω, and this usually

causes oscillation problems.

One caution about Tantalums if they are used on the input:

the ESR of a Tantalum is low enough that it can be destroyed

by surge current if powered up from a low impedance source

(like a battery) that has no limit on inrush current. In these

cases, use a ceramic input capacitor which does not have

this problem.

CERAMIC: Ceramics are generally larger and more costly

than Tantalums for a given amount of capacitance. Also, they

have a very low ESR which is quite stable with temperature.

Be warned that the ESR of a ceramic capacitor is typically

low enough to make an LDO oscillate: a 2.2 µF ceramic

demonstrated an ESR of about 15 m

Ω when tested. If used

as an output capacitor, this will cause instability (see ESR

Curves).

If a ceramic is used on the output of an LDO, a small resis-

tance (about 1

Ω) should be placed in series with the capaci-

tor. If it is used as an input capacitor, no resistor is needed as

there is no requirement for ESR on capacitors used on the

input.

EXTERNAL RESISTORS

The output voltage is set using two external resistors (see

Basic Application Circuit). It is recommended that the resis-

tor from the ADJ pin to ground be 51.1k.

The other resistor (R

ADJ) which connects between VOUT and

the ADJ pin is selected to set V

OUT as given by the formula:

OUT = 1.23 + 1.23 (RADJ/51.1k)

REVERSE CURRENT PATH

The power transistor used in the LP2980-ADJ has an inher-

ent diode connected between the input and output pin (see

below).

If the output is forced above the input by more than a V

BE,

this diode will become forward biased and current will flow

into the output pin and out the input pin. This current must be

limited to < 100 mA to prevent damage to the part.

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

is abruptly stepped down to a voltage which is a V

BE below

the output voltage. To prevent mis-operation, an external

Schottky diode (see below) must be used in applications

where the internal diode may be turned on.

Since the external Schottky diode turns on at a lower voltage

than the internal diode, the Schottky conducts all of the cur-

rent and prevents the internal diode from becoming forward

biased.

ON/OFF INPUT OPERATION

The LP2980-ADJ is shut off by driving the ON/OFF input low,

and turned on by pulling the ON/OFF input high. If this fea-

ture is not to be used, the ON/OFF input must be tied to V

to keep the regulator output on at all times (the ON/OFF in-

put 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 guar-

antee an ON or OFF state (see Electrical Characteristics).

It is also important that the turn-on (and turn-off) voltage sig-

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

greater than 40 mV/µs.

IMPORTANT: The shutdown function will not operate cor-

rectly if a slow-moving signal is used to drive the S/D input.

DS100001-24

DS100001-25

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