LP2986

www.ti.com

SNVS137H – MARCH 1999 – REVISED APRIL 2013

Be warned that the ESR of a ceramic capacitor can be low enough to cause instability: a 2.2 µF ceramic was

measured and found to have an ESR of about 15 m

Ω.

If a ceramic capacitor is to be used on the LP2986 output, a 1

Ω resistor should be placed in series with the

capacitor to provide a minimum ESR for the regulator.

Another disadvantage of ceramic capacitors is that their capacitance varies a lot with temperature:

Large ceramic capacitors are typically manufactured with the Z5U temperature characteristic, which results in the

capacitance dropping by a 50% as the temperature goes from 25°C to 80°C.

ALUMINUM: The large physical size of aluminum electrolytics makes them unattractive for use with the LP2986.

Their ESR characteristics are also not well suited to the requirements of LDO regulators.

The ESR of an aluminum electrolytic is higher than a tantalum, and it also varies greatly with temperature.

A typical aluminum electrolytic can exhibit an ESR increase of 50X when going from 20°C to

−40°C. Also, some

aluminum electrolytics can not be used below

−25°C because the electrolyte will freeze.

USING AN EXTERNAL RESISTIVE DIVIDER

The LP2986 output voltage can be programmed using an external resistive divider (see Basic Application

Circuits).

The resistor connected between the Feedback pin and ground should be 51.1k. The value for the other resistor

(R1) connected between the Feedback pin and the regulated output is found using the formula:

(1)

It should be noted that the 25 µA of current flowing through the external divider is approximately equal to the

current saved by not connecting the internal divider, which means the quiescent current is not increased by using

external resistors.

(2)

significantly over temperature. Mica or ceramic capacitors can be used, assuming a tolerance of ±20% or better

is selected.

If a ceramic is used, select one with a temperature coefficient 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

To assure proper operation, the signal source used to drive the Shutdown input must be able to swing above and

Characteristics).

Since the Shutdown input comparator does not have hysteresis, It is also important that the turn-on (and turn-off)

voltage signals applied to the Shutdown input have a slew rate which is not less than 40 mV/µs when moving

CAUTION: The regulator output state (either On or Off) can not be specified if a slow-moving AC (or DC) signal

Copyright © 1999–2013, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LP2986