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 5Ω 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
f
Z
= (1 / (2 x
π
x C
OUT
x ESR) )
(1)
Using the 2.2 µF value from the
Output Capacitor ESR Range
curve (Figure
a useful range for f
Z
can be
estimated:
f
Z(MIN)
= (1 / (2 x
π
x 2.2 µF x 5Ω) ) = 14.5 kHz
f
Z(MAX)
= (1 / (2 x
π
x 2.2 µF x 0.05Ω) ) = 318 kHz
(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 0.1Ω to 2Ω) to simulate the needed ESR. Only X5R, X7R, or better,
MLCC types should be used, and should have a DC voltage rating at least twice the V
OUT(NOM)
value.
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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