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LP3982

SNVS185E – FEBRUARY 2002 – REVISED OCTOBER 2015

www.ti.com

(6)

Figure 15. Regulator Topology Simplified

Similarity in the output capabilities exists between op amps and linear regulators. Just as rail-to-rail output op

amps allow their output voltage to approach the supply voltage, low dropout regulators (LDOs) allow their output

voltage to operate close to the input voltage. Both achieve this by the configuration of their output transistors.

Standard operational amplifiers and regulator outputs are at the source (or emitter) of the output transistor. Rail-

to-rail op amp and LDO regulator outputs are at the drain (or collector) of the output transistor. This replaces the

There is a trade-off; the output impedance become significantly higher, thus providing a critically lower pole when

combined with the capacitive load. That is why rail-to-rail operational amplifiers are usually poor at driving

capacitive loads and a series output resistor recommended when doing so. LDOs require the same series

resistance except that the internal resistance of the output capacitor will usually suffice. Refer to the Output

Capacitance section for more information.

8.2.2.2 Output Capacitance

The LP3982 is specifically designed to employ ceramic output capacitors as low as 2.2

μF. Ceramic capacitors

below 10

μF offer significant cost and space savings, along with high frequency noise filtering. Higher values and

other types and of capacitor may be used, but their equivalent series resistance (ESR) must be maintained below

0.5

Ω.

Ceramic capacitor of the value required by the LP3982 are available in the following dielectric types: Z5U, Y5V,

X5R, and X7R. The Z5U and Y5V types exhibit a 50% or more drop in capacitance value as their temperature

increases from 25°C, an important consideration. The X5R generally maintain their capacitance value within

±20%. The X7R type are desirable for their tighter tolerance of 10% over temperature.

Ceramic capacitors pose a challenge because of their relatively low ESR. Like most other LDOs, the LP3982

relies on a zero in the frequency response to compensate against excessive phase shift in the feedback loop of

the regulator. If the phase shift reaches 360° (that is, becomes positive), the regulator oscillates. This

compensation usually resides in the zero generated by the combination of the output capacitor with its ESR. The

posed by low ESR capacitors is that the zero it generates can be too high in frequency for the pole it is intended

to compensate. The LP3982 overcomes this challenge by internally generating a strategically placed zero.

12

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