LP3981

SNVS159H – OCTOBER 2001 – REVISED JULY 2015

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9.2.2.5 No-Load Stability

The LP3981 remains stable and in regulation with no external load. This is specially important in CMOS RAM

keep-alive applications.

9.2.2.6 Noise Bypass Capacitor

Connecting a 0.033-µF capacitor between the BYPASS pin and ground significantly reduces noise on the

regulator output. This capacitor is connected directly to a high impedance node in the bad gap reference circuit.

Any significant loading on this node causes a change on the regulated output voltage. For this reason, DC

leakage current through this pin must 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. Hight-quality ceramic

capacitors with either NPO or COG dielectric typically have very low leakage. Polypropolene and polycarbonate

film capacitors are available in small surface-mount packages and typically have extremely low leakage current.

Unlike many other LDOs, addition of a noise reduction capacitor does not effect the transient response of the

device.

9.2.2.7 Capacitor Characteristics

The LP3981 is designed to work with ceramic capacitors on the output to take advantage of the benefits they

offer: for capacitance values in the range of 1 µF to 4.7 µF, ceramic capacitors are the smallest, least expensive

and have the lowest ESR values (which makes them best for eliminating high frequency noise). The ESR of a

typical 1µF ceramic capacitor is in the range of 20 m

Ω to 40 mΩ, which easily meets the ESR requirement for

stability by the LP3981.

Capacitance of a ceramic capacitor can vary with temperature. Most large value ceramic capacitors (

≊ 2.2 µF)

are manufactured with Z5U or Y5V temperature characteristics, which results in the capacitance dropping by

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

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

±15%.

Tantalum capacitors are less desirable than ceramic 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 equivalently sized

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 must also be noted that the ESR of a typical tantalum increases about 2:1

as the temperature goes from 25°C down to

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

9.2.3 Application Curves

VIN = VOUT + 1 V to VOUT + 1.6 V

VIN = VOUT + 1V to VOUT + 1.6 V

Figure 18. Line Transient Response

Figure 19. Line Transient Response

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