Datasheet

LM2931-N
SNOSBE5G –MARCH 2000–REVISED APRIL 2013 www.ti.com
APPLICATION HINTS
One of the distinguishing factors of the LM2931-N series regulators is the requirement of an output capacitor for
device stability. The value required varies greatly depending upon the application circuit and other factors. Thus
some comments on the characteristics of both capacitors and the regulator are in order.
High frequency characteristics of electrolytic capacitors depend greatly on the type and even the manufacturer.
As a result, a value of capacitance that works well with the LM2931-N for one brand or type may not necessary
be sufficient with an electrolytic of different origin. Sometimes actual bench testing, as described later, will be the
only means to determine the proper capacitor type and value. Experience has shown that, as a rule of thumb, the
more expensive and higher quality electrolytics generally allow a smaller value for regulator stability. As an
example, while a high-quality 100 μF aluminum electrolytic covers all general application circuits, similar stability
can be obtained with a tantalum electrolytic of only 47μF. This factor of two can generally be applied to any
special application circuit also.
Another critical characteristic of electrolytics is their performance over temperature. While the LM2931-N is
designed to operate to −40°C, the same is not always true with all electrolytics (hot is generally not a problem).
The electrolyte in many aluminum types will freeze around −30°C, reducing their effective value to zero. Since
the capacitance is needed for regulator stability, the natural result is oscillation (and lots of it) at the regulator
output. For all application circuits where cold operation is necessary, the output capacitor must be rated to
operate at the minimum temperature. By coincidence, worst-case stability for the LM2931-N also occurs at
minimum temperatures. As a result, in applications where the regulator junction temperature will never be less
than 25°C, the output capacitor can be reduced approximately by a factor of two over the value needed for the
entire temperature range. To continue our example with the tantalum electrolytic, a value of only 22μF would
probably thus suffice. For high-quality aluminum, 47μF would be adequate in such an application.
Another regulator characteristic that is noteworthy is that stability decreases with higher output currents. This
sensible fact has important connotations. In many applications, the LM2931-N is operated at only a few milliamps
of output current or less. In such a circuit, the output capacitor can be further reduced in value. As a rough
estimation, a circuit that is required to deliver a maximum of 10mA of output current from the regulator would
need an output capacitor of only half the value compared to the same regulator required to deliver the full output
current of 100mA. If the example of the tantalum capacitor in the circuit rated at 25°C junction temperature and
above were continued to include a maximum of 10mA of output current, then the 22μF output capacitor could be
reduced to only 10μF.
In the case of the LM2931CT adjustable regulator, the minimum value of output capacitance is a function of the
output voltage. As a general rule, the value decreases with higher output voltages, since internal loop gain is
reduced.
At this point, the procedure for bench testing the minimum value of an output capacitor in a special application
circuit should be clear. Since worst-case occurs at minimum operating temperatures and maximum operating
currents, the entire circuit, including the electrolytic, should be cooled to the minimum temperature. The input
voltage to the regulator should be maintained at 0.6V above the output to keep internal power dissipation and die
heating to a minimum. Worst-case occurs just after input power is applied and before the die has had a chance
to heat up. Once the minimum value of capacitance has been found for the brand and type of electrolytic in
question, the value should be doubled for actual use to account for production variations both in the capacitor
and the regulator. (All the values in this section and the remainder of the data sheet were determined in this
fashion.)
LM2931-N DSBGA Light Sensitivity
When the LM2931-N DSBGA package is exposed to bright sunlight, normal office fluorescent light, and other
LED's, it operates within the limits specified in the electrical characteristic table.
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