Capacitor Characteristics

Ceramics

Tantalum

Aluminum

ON/OFF Operation

LP2981

Due to their very low ESR values, ceramic capacitors are not suitable for use as the output capacitor. For

instance, a typical 2.2-

µF ceramic capacitor has an ESR in the range of 10 mΩ to 20 mΩ and, thus, easily can

fall out of minimum ESR requirements under certain operating conditions.

If a ceramic capacitor is used at the output, a 1-

Ω resistor should be placed in series with the capacitor to raise

the ESR seen by the regulator.

Solid tantalum capacitors are optimal choices for the LP2981, but they still must meet the minimum ESR

requirement. Note that the ESR of a tantalum capacitor increases as temperature drops, as much as doubling

from 25°C to –40°C. Thus, ESR margins must be maintained over the temperature range to prevent regulator

instability. For operation at very low temperatures, paralleling a tantalum capacitor with a ceramic one keeps the

combined ESR from increasing near the upper limit of the ESR curve.

Aluminum capacitors can be used, but use with the LP2981 is impractical due to their large physical dimensions.

They also must meet the ESR requirements over the full temperature range. In this regard, aluminium capacitors

are at a big disadvantage due to their sharp ESR increase as temperature drops. For example, over a

temperature drop from 20°C to –40°C, the ESR of an aluminum electrolytic capacitor can increase by a factor of

50. In addition, some of the electrolytes used in these capacitors can freeze at –25°C, making the capacitor

nonoperational.

The LP2981 allows for a shutdown mode via the ON/OFF pin. If the shutdown feature is not used, ON/OFF

should be connected to the input to ensure that the regulator is on at all times. To drive ON/OFF:

• A LOW (≤0.3 V) turns the regulator OFF; a HIGH (≥1.6 V) turns it ON.

• Use either a totem-pole output or an open-collector output with a pullup resistor tied to V

ON/OFF pin.

• Apply a signal with a slew rate of ≥40 mV/µs. A slow slew rate can cause the shutdown function to operate

incorrectly.

Copyright © 2004–2008, Texas Instruments Incorporated

Submit Documentation Feedback

7