LP2986 datasheet(12/14 Pages) NSC

12 / 14 page

Application Hints (Continued)

A lead compensation capacitor (C

F) must also be used to

place a zero in the loop response at about 50 kHz. The value

for C

F can be found using:

F = 1/(2π x R1 x 50k)

A good quality capacitor must be used for C

F to ensure that

the value is accurate and does not change significantly over

temperature. Mica or ceramic capacitors can be used, as-

suming a tolerance of ±20% or better is selected.

If a ceramic is used, select one with a temperature coeffi-

cient of NPO, COG, Y5P, or X7R. Capacitor types Z5U, Y5V,

and Z4V can not be used because their value varies more

that 50% over the −25˚C to +85˚C temperature range.

SHUTDOWN INPUT OPERATION

The LP2986 is shut off by driving the Shutdown input low,

and turned on by pulling it high. If this feature is not to be

used, the Shutdown input should be tied to V

IN to keep the

regulator output on at all times.

To assure proper operation, the signal source used to drive

the Shutdown input must be able to swing above and below

the specified turn-on/turn-off voltage thresholds listed as V

and V

L, respectively (see Electrical Characteristics).

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

nals applied to the Shutdown input have a slew rate which is

not less than 40 mV/µs.

CAUTION: the regulator output state can not be guaranteed

if a slow-moving AC (or DC) signal is applied that is in the

range between V

H and VL.

REVERSE INPUT-OUTPUT VOLTAGE

The PNP power transistor used as the pass element in the

LP2986 has an inherent diode connected between the regu-

lator output and input.

During normal operation (where the input voltage is higher

than the output) this diode is reverse-biased.

However, if the output is pulled above the input, 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 V

IN (and out the ground pin), which

can damage the part.

In any application where the output may be pulled above the

input, an external Schottky diode must be connected from

IN to VOUT (cathode on VIN, anode on VOUT), to limit the re-

verse voltage across the LP2986 to 0.3V (see Absolute

Maximum Ratings).

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