LP2951JAN Datasheet, PDF (13/24 Pages) Texas Instruments – Adjustable Micropower Voltage Regulators

Thermal Response
Output Capacitor ESR Range
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Application Hints
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 alu-
minum 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 curve Output Capacitor ESR Range).
The reason for the lower ESR limit is that the loop compen-
sation 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 (recom-
mended value of resistance 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 capaci-
tance 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 us-
ing high value external resistors to set the output voltage.
Adding a 100pF capacitor between Output and Feedback and
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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 approxi-
mately 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 in the front of the datasheet.) 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 out-
put. The out of regulation condition may be due either to low
input voltage, current limiting, or thermal limiting.
Figure 1 below gives a timing diagram depicting the ER-
ROR 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).
Since the LP2951's dropout voltage is load-dependent (see
curve in typical performance characteristics), the input volt-
age 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 re-
quires an external pullup resistor. This resistor may be re-
turned to the output or some other supply voltage depending
on system requirements. In determining a value for this re-
sistor, 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 100k to 1 MΩ. The resistor is
not required if this output is unused.
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