LP2950-N, LP2951-N

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SNVS764N – JANUARY 2000 – REVISED MAY 2013

APPLICATION HINTS

Output Capacitor Requirements

A 1.0

μF (or greater) capacitor is required between the output and ground for stability at output voltages of 5V or

higher. At lower output voltages, more capacitance is required (2.2

μF or more is recommended for 3.0V and

3.3V versions). Without this capacitor the part will oscillate. Most types of tantalum or aluminum electrolytic work

fine here; even film types work but are not recommended for reasons of cost. Many aluminum electrolytics have

electrolytes that freeze at about

−30°C, so solid tantalums are recommended for operation below −25°C. The

important parameters of the capacitor are an ESR of about 5

Ω or less and a resonant frequency above 500 kHz.

The value of this capacitor may be increased without limit.

Figure 44. Output Capacitor ESR Range

The reason for the lower ESR limit is that the loop compensation of the feedback loop relies on the capacitance

value and the ESR value of the output capacitor to provide the zero that gives added phase lead (See

Figure 44).

(1)

estimated:

(2)

(3)

For ceramic capacitors, the low ESR produces a zero at a frequency that is too high to be useful, so meaningful

phase lead does not occur. A ceramic output capacitor can be used if a series resistance is added

(recommended value of resistance about 0.1

Ω to 2Ω) to simulate the needed ESR. Only X5R, X7R, or better,

At lower values of output current, less output capacitance is required for stability. The capacitor can be reduced

to 0.33

μF for currents below 10 mA or 0.1 μF for currents below 1 mA. Using the adjustable versions at voltages

below 5V runs the error amplifier at lower gains so that more output capacitance is needed. For the worst-case

situation of a 100 mA load at 1.23V output (Output shorted to Feedback) a 3.3

μF (or greater) capacitor should

be used.

Unlike many other regulators, the LP2950-N will remain stable and in regulation with no load in addition to the

internal voltage divider. This is especially important in CMOS RAM keep-alive applications. When setting the

output voltage of the LP2951-N versions with external resistors, a minimum load of 1

μA is recommended.

Applications having conditions that may drive the LP2950-N/51 into nonlinear operation require special

consideration. Nonlinear operation will occur when the output voltage is held low enough to force the output

stage into output current limiting while trying to pull the output voltage up to the regulated value. The internal loop

response time will control how long it takes for the device to regain linear operation when the output has returned

to the normal operating range. There are three significant nonlinear conditions that need to be considered, all can

force the output stage into output current limiting mode, all can cause the output voltage to over-shoot with low

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