Application Hints

One of the distinguishing factors of the LM2931 series regu-

lators 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 com-

ments on the characteristics of both capacitors and the

regulator are in order.

High frequency characteristics of electrolytic capacitors de-

pend greatly on the type and even the manufacturer. As a

result, a value of capacitance that works well with the

LM2931 for one brand or type may not necessary be suffi-

cient 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 cir-

cuits, similar stability can be obtained with a tantalum elec-

trolytic of only 47µF. This factor of two can generally be

applied to any special application circuit also.

Another critical characteristic of electrolytics is their perfor-

mance over temperature. While the LM2931 is designed to

operate to −40˚C, the same is not always true with all elec-

trolytics (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 coinci-

dence, worst-case stability for the LM2931 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 tempera-

ture range. To continue our example with the tantalum elec-

trolytic, 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 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 tem-

perature 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 mini-

mum 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 oper-

ating 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 deter-

mined in this fashion.)

LM2931 micro SMD Light Sensitivity

When the LM2931 micro SMD package is exposed to bright

sunlight, normal office fluorescent light, and other LED’s, it

operates within the guaranteed limits specified in the electri-

cal characteristic table.

Definition of Terms

Dropout Voltage: The input-output voltage differential at

which the circuit ceases to regulate against further reduction

in input voltage. Measured when the output voltage has

dropped 100 mV from the nominal value obtained at 14V

input, dropout voltage is dependent upon load current and

junction temperature.

Input Voltage: The DC voltage applied to the input terminals

with respect to ground.

Input-Output Differential: The voltage difference between

the unregulated input voltage and the regulated output volt-

age for which the regulator will operate.

Line Regulation: The change in output voltage for a change

in the input voltage. The measurement is made under con-

ditions of low dissipation or by using pulse techniques such

that the average chip temperature is not significantly af-

fected.

Load Regulation: The change in output voltage for a

change in load current at constant chip temperature.

Long Term Stability: Output voltage stability under accel-

erated life-test conditions after 1000 hours with maximum

rated voltage and junction temperature.

Output Noise Voltage: The rms AC voltage at the output,

with constant load and no input ripple, measured over a

specified frequency range.

Quiescent Current: That part of the positive input current

that does not contribute to the positive load current. The

regulator ground lead current.

Ripple Rejection: The ratio of the peak-to-peak input ripple

voltage to the peak-to-peak output ripple voltage at a speci-

fied frequency.

Temperature Stability of V

output voltage for a thermal variation from room temperature

to either temperature extreme.

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