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LP2981-33 Datasheet(PDF) 7 Page - Texas Instruments

Part No. LP2981-33
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com

LP2981-33 Datasheet(HTML) 7 Page - Texas Instruments

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Capacitor Characteristics
ON/OFF Operation
www.ti.com ......................................................................................................................................................... SLVS521F – JULY 2004 – REVISED AUGUST 2008
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
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
IN (or another logic
supply). The HIGH signal can exceed VIN, but must not exceed the absolute maximum ratings of 20 V for the
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
Copyright © 2004–2008, Texas Instruments Incorporated
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