LP2985-N

SNVS018V – MARCH 2000 – REVISED JUNE 2013

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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.

CAPACITOR CHARACTERISTICS

The LP2985-N 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 4.7 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 2.2

µF ceramic capacitor is in the range of 10 m

Ω to 20 mΩ, which easily meets the ESR limits required for stability

by the LP2985-N.

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 2.2 µF capacitor were used on the output since it will drop down to approximately

1 µF at high ambient temperatures (which could cause the LM2985 to oscillate). If Z5U or Y5V capacitors are

used on the output, a minimum capacitance value of 4.7 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.

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.

On/Off Input Operation

The LP2985-N is shut off by driving the ON/OFF 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 ON/OFF input must be able to swing above and

below the specified turn-on/turn-off voltage thresholds listed in the ELECTRICAL CHARACTERISTICS section

must have a slew rate which is

≥ 40 mV/µs.

CAUTION

The regulator output voltage cannot be ensured if a slow-moving AC (or DC) signal is

applied that is in the range between the specified turn-on and turn-off voltages listed

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