LP2950-N, LP2951-N
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SNVS764Q –JANUARY 2000–REVISED DECEMBER 2017
Figure 37. 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 37).
fZ = (1 / (2 × π × COUT × ESR))
(1)
Using the 2.2 µF value from the Output Capacitor ESR Range curve (Figure 37), a useful range for fZ can be
estimated:
fZ(MIN)= (1 / (2 x π × 2.2 µF x 5 Ω)) = 14.5 kHz
fZ(MAX)= (1 / (2 x π × 2.2 µF x 0.05 Ω)) = 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 0.1 Ω to 2 Ω) 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 VOUT(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 5 V 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 1.23 V output (output shorted to Feedback) a 3.3-µF (or greater) capacitor should
be used.
Unlike many other regulators, the LP2950-N remains 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 occurs 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 controls 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
value output capacitors when the condition is removed, and the recommended generic solution is to set the
output capacitor to a value not less than 10 µF. Although the 10 µF value for COUT may not eliminate the output
voltage over-shoot in all cases, it should lower it to acceptable levels (< 10% of VOUT(NOM)) in the majority of
cases. In all three of these conditions, applications with lighter load currents are more susceptible to output
voltage over-shoot than applications with higher load currents.
1. At power-up, with the input voltage rising faster than output stage can charge the output capacitor.
VIN tRISE(MIN) > ((COUT / 100 mA) × ΔVIN)
where
•
ΔVIN = VOUT(NOM) + 1 V
(4)
(5)
2. Recovery from an output short circuit to ground condition.
C
OUT(MIN) ≈ (160 mA – ILOAD(NOM))/((VOUT(NOM)/10)/25 µs))
3. Toggling the LP2951-N SHUTDOWN pin from high (OFF) to low (ON).
C
OUT(MIN) ≈ (160 mA – ILOAD(NOM))/((VOUT(NOM)/10)/25 µs))
(6)
21
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