LP2951JAN_15 Datasheet, PDF (13/25 Pages) Texas Instruments – Series of Adjustable Micropower Voltage Regulators

LP2951JAN
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APPLICATION HINTS
SNVS408A – NOVEMBER 2010 – REVISED APRIL 2013
External Capacitors
A 1.0μF (or greater) capacitor is required between the output and ground for stability at output voltages of 5V or
more. At lower output voltages, more capacitance is required (2.2μF or more is recommended for 3V and 3.3V
versions). Without this capacitor the part will oscillate. Most types of tantalum or aluminum electrolytics 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 500kHz.
The value of this capacitor may be increased without limit.
Ceramic capacitors whose value is greater than 1000pF should not be connected directly from the LP2951
output to ground. Ceramic capacitors typically have ESR values in the range of 5 to 10mΩ, a value below the
lower limit for stable operation (see Figure 33).
The reason for the lower ESR limit is that the loop compensation of the part relies on the ESR of the output
capacitor to provide the zero that gives added phase lead. The ESR of ceramic capacitors is so low that this
phase lead does not occur, significantly reducing phase margin. A ceramic output capacitor can be used if a
series resistance is added (recommended value of resistance is about 0.1Ω to 2Ω).
At lower values of output current, less output capacitance is required for stability. The capacitor can be reduced
to 0.33μF for currents below 10mA or 0.1μF for currents below 1mA. 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 100mA load at 1.23V output (Output shorted to Feedback) a 3.3μF (or greater) capacitor should be
used.
When setting the output voltage of the LP2951 with external resistors, a minimum load of 1μA is recommended.
A 1μF tantalum, ceramic or aluminum electrolytic capacitor should be placed from the LP2951 input to ground if
there is more than 10 inches of wire between the input and the AC filter capacitor or if a battery is used as the
input.
Stray capacitance to the LP2951 Feedback terminal can cause instability. This may especially be a problem
when using high value external resistors to set the output voltage. Adding a 100pF capacitor between Output and
Feedback and increasing the output capacitor to at least 3.3μF will fix this problem.
Error Detection Comparator Output
The comparator produces a logic low output whenever the LP2951 output falls out of regulation by more than
approximately 5%. This figure is the comparator's built-in offset of about 60mV divided by the 1.235 reference
voltage. (Refer to the Block Diagram) This trip level remains 5% below normal regardless of the programmed
output voltage of the 2951. For example, the error flag trip level is typically 4.75V for a 5V output or 11.4V for a
12V output. The out of regulation condition may be due either to low input voltage, current limiting, or thermal
limiting.
Figure 34 below gives a timing diagram depicting the ERROR signal and the regulated output voltage as the
LP2951 input is ramped up and down. For 5V versions, the ERROR signal becomes valid (low) at about 1.3V
input. It goes high at about 5V input (the input voltage at which V_OUT = 4.75V). Because the LP2951's dropout
voltage is load-dependent (see Figure 12), the input voltage trip point (about 5V) will vary with the load current.
The output voltage trip point (approx. 4.75V) does not vary with load.
The error comparator has an open-collector output which requires an external pullup resistor. This resistor may
be returned to the output or some other supply voltage depending on system requirements. In determining a
value for this resistor, note that while the output is rated to sink 400μA, this sink current adds to battery drain in a
low battery condition. Suggested values range from 100kΩ to 1 MΩ. The resistor is not required if this output is
unused.
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