LM2940 low dropout voltage regulator specs?

I was looking at the LM2940T-5.0 low dropout 5v voltage regulator spec sheet and i'm puzzled! The dropout voltage is listed at 0.5v for a 1A load, at 0.15v for a 200ma load, and at 0.1v for a 100mA load. Yet the minimum input voltage is listed as 6.25V for a 5V output!

Why wouldn't the minimum input voltage be 5.5v for a 1A load or 5.15v for a 200mA load?

The reason i'm asking is that i want to use a 6V input to get a 5V output for a load of about 100 or 200mA. 6V is of course < 6.25v, but it is >> 5.15v. Will this work? What am i not understanding here?

Also (minor), is there any difference between "dropout voltage" and "input-output differential"? The reason i'm asking is that there are side-by-side plots on page 8 of the TI spec sheet that use these two different terms, but the values in the plots seem 100% compatible!

I will assume you are referring to this datasheet.

Under Section 6.5 Electrical Characteristics, the minimum input voltage is specified for 5mA <= Iout <= 1A over the recommended operating temperature range. You must give it 6.25V minimum for it to be able to provide that range of current over the temperature range.

The comparable number for 100mA is 5.35V (200mV max dropout + 5V + 150mV) over the temperature range . It would be more for 200mA. You don't really want to go right down to the minimum dropout, which is why it's specified with a bit more than the absolute minimum input voltage.

This is generally true of all linear ICs- if you run it right up against the maximum limits (for example, a rail-to-rail amplifier right near the supply rails) the performance will deteriorate noticeably.

• Thanks forth great answer! So are you saying that a 6v input would almost certainly be ok? I understand the gist of your answer but i don't see why you added BOTH 200mV and 150mV to 5V for a 100mA current load?? Commented Sep 3, 2020 at 15:21
• Yes i was indeed referring to the datasheet you linked! About the 5.35v = 0.2 + 5 + 0.15 caluation you made: When i look at the datasheet i see a dropout voltage of 0,1v @ currentt=100mA, so i'm not sure what the 200mV and 150mV numbers correspond to (i'd assume one is an approximation of the dropout voltage, but what is the other - just curious?) Commented Sep 3, 2020 at 15:39
• Maximum dropout voltage at 100mA over temperature is 200mV (second line from the bottom). 150mV is the margin they added between the 1A number (1.0V, 4th line from the bottom) and the input voltage to meet all specifications (6.15V). Commented Sep 3, 2020 at 15:48
• I see. That makes sense. Thank you Commented Sep 3, 2020 at 19:28

The minimum input voltage of 6.25 volts is required to ensure that all the other specifications listed in the data sheet are met such as

• output level accuracy,
• line regulation,
• output impedance,
• quiescent current,
• output noise voltage and
• ripple rejection.

If you want to forgo some or all of these niceties then sure, you can run the input down to a minimum of 1 volt difference (0.5 volts typically) but be prepared to be dissapointed.

• Thanks for the great answer! But if what you're saying is true, what's the point of having a regulator that is spec'd at dropout voltages of 0.1v to 0.5v for currents of 0.1A to 1A, if this regulator requires an input-output differential of 1.25v to be "fully functional"? The main feature, after all, of a low drop-out voltage regulator, is its low drop-out voltage, but the input-output differential of the minimum operating voltage spec is 12x greater than the dropout voltage for a 0.1A current load (and 2.5x greater for a 1A load). Am i missing something here? Commented Sep 3, 2020 at 15:34
• @Maurice just think about the accuracy of the device's output voltage. It is rated to produce between 4.75 volts and 5.25 volts with an input as low as 6.25 volts. That's a headroom of only 1 volt guaranteed if the device's output voltage happens to be a little high at 5.25 volts. Then consider the worst case dropout voltage of 1 volt. The numbers add up perfectly. If you picked a regulator that happens to produce 4.75 volts on it's output, I would expect the input voltage to be able to get down to 5.75 volts (1 amp load). Commented Sep 3, 2020 at 15:44
• ok that makes sense now. Thanks Andy Commented Sep 3, 2020 at 19:29

Why wouldn't the minimum input voltage be 5.5v for a 1A load or 5.15v for a 200mA load?

The minimum supply voltage (6.5V) probably refers to the minimum voltage required for the internal logic of the IC to be fully functional. The dropout voltage is essentially how much voltage drop the PNP (depicted in the following image) is going to have for a given load.

• Thanks for the response! But that seems at odds with the fact that minimum supply voltage is 6.25v for the 5v version of LM2940 and 9.4v for the 8v version. I would think that the internal logic would be the same for those two versions. Am i wrong about that? Commented Sep 3, 2020 at 15:18

I bought one and measured it. :) Stable 5 V output at 100 mA requires a 5.4 V input at 27 degrees Celsius.

• Wow. Awesome. No i know for sure, i guess (+/- individual device variation)!! Commented Sep 3, 2020 at 19:32
• I thought about it, I checked three more and the result was + 0.35 ... + 0.4V@100mA load. Commented Sep 3, 2020 at 22:28