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LP3982

SNVS185D – FEBRUARY 2002 – REVISED APRIL 2013

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Figure 17. 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 op amps 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 threshold (or

off, of course. The output impedance become significantly higher, thus providing a critically lower pole when

combined with the capacitive load. That's why rail-to-rail op amps are usually poor at driving capacitive loads and

recommend a series output resistor 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.

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) should 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 regulator's

feedback loop. If the phase shift reaches 360° (i.e.; becomes positive), the regulator will oscillate. This

compensation usually resides in the zero generated by the combination of the output capacitor with its equivalent

series resistance (ESR). The zero is intended to cancel the effects of the pole generated by the load capacitance

regulator. The challenge posed by low ESR capacitors is that the zero it generates can be too high in frequency

for the pole that it's intended to compensate. The LP3982 overcomes this challenge by internally generating a

strategically placed zero.

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