LP2992

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SNVS171H – NOVEMBER 2001 – REVISED JANUARY 2015

The output capacitor must be located not more than 1 cm from the output pin and returned to a clean analog

ground.

8.2.2.1.3

Noise Bypass Capacitor

Connecting a 10-nF capacitor to the BYPASS pin significantly reduces noise on the regulator output. It should be

noted that the capacitor is connected directly to a high-impedance circuit in the bandgap reference.

Because this circuit has only a few microamperes flowing in it, any significant loading on this node will cause a

change in the regulated output voltage. For this reason, dc leakage current through the noise bypass capacitor

must never exceed 100 nA, and should be kept as low as possible for best output voltage accuracy.

The types of capacitors best suited for the noise bypass capacitor are ceramic and film. High-quality ceramic

capacitors with either NPO or COG dielectric typically have very low leakage. 10-nF polypropolene and

polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low

leakage current.

8.2.2.2 Capacitor Characteristics

The LP2992 was designed to work with ceramic capacitors on the output to take advantage of the benefits they

offer. For capacitance values in the 2.2-µF to 10-µF range, ceramics are the least expensive and also have the

lowest ESR values (which makes them best for eliminating high-frequency noise). The ESR of a typical 4.7-µF

ceramic capacitor is in the range of 5 m

Ω to 10 mΩ, which easily meets the ESR limits required for stability by

the LP2992.

One disadvantage of ceramic capacitors is that their capacitance can vary with temperature. Most large value

ceramic capacitors (

≥ 2.2 µF) are manufactured with the Z5U or Y5V temperature characteristic, which results in

the capacitance dropping by more than 50% as the temperature goes from 25°C to 85°C.

This could cause problems if a 4.7-µF capacitor were used on the output because it will drop down to

approximately 2.3 µF at high ambient temperatures (which could cause the LP2992 to oscillate). If Z5U or Y5V

capacitors are used on the output, a minimum capacitance value of 10 µF must be observed.

A better choice for temperature coefficient in ceramic capacitors is X7R, which holds the capacitance within

±15%. Unfortunately, the larger values of capacitance are not offered by all manufacturers in the X7R dielectric.

8.2.2.2.1

Tantalum

Tantalum capacitors are less desirable than ceramics for use as output capacitors because they are more

expensive when comparing equivalent capacitance and voltage ratings in the 1-µF to 4.7-µF range.

Another important consideration is that Tantalum capacitors have higher ESR values than equivalent size

ceramics. This means that while it may be possible to find a Tantalum capacitor with an ESR value within the

stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic

capacitor with the same ESR value.

It should also be noted that the ESR of a typical Tantalum will increase about 2:1 as the temperature goes from

25°C down to

−40°C, so some guard band must be allowed.

8.2.2.3 Reverse Input-Output Voltage

The PNP power transistor used as the pass element in the LP2992 has an inherent diode connected between

the regulator 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

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

0.3 V (see Absolute Maximum Ratings).

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