Electronic – LM2940 low dropout voltage regulator specs

Yes, the dropout voltage is the headroom the regulator needs to work with. This is the minimum amount the input voltage must be above the output voltage for the regulator to work properly. So as you say, a 3.3 V regulator with 1.1 V dropout requires at least 4.4 V input.

Stuff happens, so it's usually a good idea to give the regulator a little more room. However, going too far will be less efficient, so there is a tradeoff. The range you quote of 4.75 to 5.25 volts could well be a reasonable tradeoff. Basically, that's a "5 V" supply.

The output current of a linear regulator is its input current (minus the small current on the ground pin, but that is small enough to ignore for the point I'm trying to make). Since the input voltage is higher than the output but the currents are the same, the output has less power. The difference in input power and output power is wasted in the regulator as heat. Another way to look at this is that the regulator will dissipate the difference between the input and output voltages times the current as heat. For example, consider a case where the input is 5.0 V, the output 3.3 V, and the current 500 mA. The voltage accross the regulator is 5.0V - 3.3V = 1.7V. That times 500 mA is 850 mW, which will go into heating the regulator. That would be fine for something like a TO-220 package, but a SOT-23, for example, would burn up at 850 mW.

I think you may have already checked for most of the topics in the list bellow. However, I have tried to make an exhaustive list that will surely help others arriving this question.

The usual suspects when dealing with issues with the 78xx standard linear regulators:

1. The output may be oscillating, which may be difficult to verify if you only have a digital multimeter at hand. Check, if possible, with an oscilloscope. Possible culprits:
   • Follow the manufacturer's recommendations for input and output capacitors. Not all capacitor technologies (Aluminum, Tantalum, ceramic) are allowed. Not all values are allowed. Check also the range of input/output capacitance for stable operation. There is usually at least one section in the datasheet devoted to stability.
   • Check that you are respecting the minimum/maximum input voltage for the regulator. Minimum input voltage could be as high as Vout + 2.5V, depending on the device. Maximum input voltage varies hugely among different devices. Again, check the datasheet.
   • Check that a minimum load current is required from the regulator. Some devices are happy with very low load current (<1mA), however, many 78xx devices will require a minimum load current for stable operation.
2. If the output voltage is higher that it should be:
   • Check that the ground is not being "pulled up". Most 78xx devices can effectively work with a "virtual ground", that is, if you insert a resistance from the common return to ground, the reference voltage and the output voltage will both rise by V=R*I, being R the common path resistance to ground and I the quiescent current plus load current.
   • The device may be broken, faulty or a counterfeit. Simply check with another sample, even best if you can try a sample from another batch.
3. If the output voltage is lower that it should be:
   • Check that you are respecting the minimum voltage drop required by the regulator. Check the datasheet. Some non LDO devices may require the input voltage to be up to several volts higher than the specified output voltage.
4. Check your measurement devices. If possible, try more than one DMM. Check your batteries. Even better, check the signals with an oscilloscope.
5. Check your power supply: real output voltage, noise, current capabilities...
6. Check your connections. Twice. Lots of the electronic issues during the prototyping phase can be traced down to bad connections or mafunctioning breadboards.
7. Check the pinout of your device. Make sure you are wiring correctly the ground, reference and output connections.
8. Beware of shorting the input of your voltage regulator. Many of the 78xx devices cannot withstand a "short" in the input capacitor while the output capacitor is still charged. This will usually damage permanently your device, totally or partially (even worse), which could explaing the strange behaviour of the device from than point on.