Will LM317 circuit maintain steady output voltage in face of a change in input

[MrAl]

Neat discussion about the exact meaning of "dropout voltage"

But regardless of what the datasheet claim (or even what you measure) you probably should not operate your linear regulators so close to the dropout limit. Dropout voltage are usually only measured in one dimension, mostly input voltage where output start to change xx%. Other specs however start to degrade long before dropout voltage, sometimes to the point of uselessness. PSRR for example are getting worse the closer you get to the dropout voltage limit.

Hi,

Yes there is a lot more to be considered, but when we talk about a particular aspect of a circuit we usually concentrate on that first so we can isolate the effects of that mode of operation.  We can always add other aspects later, but we would also consider them separately.
This is the way a lot of analysis is done, where we try to concentrate the analysis on one dimension of the problem alone, then we can always sum the individual analysis results to figure out the entire set of problems.

When we consider the drop out voltage alone, we get a good understanding of how that works, period.  When we add in the other issues, we may see that the drop out voltage increase probably falls under the noise floor.  I can only say 'probably' here though because all that would have to be analyzed as well.

The main point here is that when we talk about drop out voltage, we talk about drop out voltage.  That's because we want to know the effects of that aspect of the design alone.  This allows us to assess the workings of that part of it all by itself.  By doing this we can determine that the above statement about the drop out voltage (probably falls below the noise floor) is true, but if we didn't do that, we would not know that yet.  It could have turned out that the drop out voltage was the main problem and everything else was insignificant.  After the analysis we find out one way or the other.

[MrAl]

The drop-out voltage is the difference between the input and output voltage, at which the regulator stops working properly. In the case of the LM317, it'll still output a voltage in the drop-out region, but it will no longer be regulated and changes in the supply voltage and load current will affect the output voltage.

There's a lot of talk about 2V or 3V, but in reality, it depends on the load current, temperature and the LM317 itself. Note when temperature is mentioned, it applies to the LM317 chip itself, which means the effects of self-heating need to be taken into account, at higher currents.

Here's a graph from a data sheet (ST I think) showing the drop-out voltage of the LM317 at different currents and temperatures. If it's always going to be operated in a high temperature environment and the output current is under 20mA, then you'll get away with 1.5V. but if it's operated at the full 1.5A and over the entire temperature range, then it's 2.5V.

To answer the original question: it depends. At normal room temperature, say around 20°C, the drop-out voltage will be around 1.6V at 20mA and 2.3V at 1.5A, so when set to 12V, it'll stop regulating when the input voltage falls below 13.6V when the load current is 20mA and 14.3V, when it's 1.5A.

Hi,

Well what has been brought to light here is that "not working properly" is not an accurate way to interpret a low dropout voltage specification.  There's no way around this because the same part may not work in one application, but may work just fine in another, and that's given the exact conditions of input and output, voltage and current.  We also want to know the significance of the spec.

What we were seeking is an accurate way to understand this phenomenon so we can properly figure out what it is all about, even if nothing else.  What seems to have come of it is that the change in the meaning was not that significant.  However, we would have not been able to know that for sure until after the analysis.

The charts you had shown may help with this little investigation also.

There is another aspect of this too unfortunately. That is that different manufacturers specify the drop out voltage under different test conditions.  One may use a voltage drop of 100mv, while others may use a drop down to 95 percent of nominal, just for two examples.

[MrAl]

Note that the criterion for regulator drop-out in the data sheet graph above is a change in output voltage of 100 mV to indicate regulation failure.

Hello,

See, we are trying to avoid vague statements like that, even though they are interesting too.
"Regulation failure" is application dependent.
For example, if the nominal was 5.0000 volts and the voltage dropped to 4.9000 volts, it would be a disaster for a voltage reference application, but a TTL application would be very happy with that 4.9000 volts.

So it is not really a failure, it's just a spec like any other spec that lets us know what we are dealing with during the design process.

[MrAl]

100mV error is always a problem because it means that the control loop is far from its normal operating point, loop gain is down the drain and the pass device is turned hard on so there is zero ripple rejection. Basically the regulator is a constant voltage between the input and the output.

Frankly, specifying dropout voltage under such conditions is plain cheating.

Hi,

Yes that's a good point.  If we have a resistor between the input and output (which is the first approximation) then small differences in the input are felt immediately at the output.

What is coming to light in this thread now is that the drop out voltage just has to be understood a little better.

[Gyro]

If the OP is really worried about dropout voltage, he would probably be better going for an LM2940-12 (12V version). This has a guaranteed <200mV dropout at 100mA. Less components, hassle, and better performance. Available in the same packages as the LM317.