Why would I need a low-dropout regulator?
To answer this you start by considering "Why would I need a voltage regulator?. You need a voltage regulator to provide a constant voltage to a circuit. More often than not, a constant voltage supply improves circuit performance and allows you to use components that cannot work on lower or higher voltages. Not all circuits require voltage regulation however.
A voltage regulator isn't perfect - if you put 5.1 volts at the input, most will not produce a reliable 5 volts out under all (or any) load conditions so, manufacturers specify the "drop-out" voltage such as: -
With a 1 V (minimum) drop-out voltage the device can regulate the output from loads requiring 10 mA up to 1 amp.
This is just a made up example. Low drop out regulators are generally classified as working with an input-output voltage lower than 1 volt. Having said that, some manufacturers will call their devices "LDO" if the input-output voltage has to be 2 volts.
Notice also that in the above made-up specification I implied there was a minimum load of 10 mA - watch out for this as it can bite you i.e. you buy an LDO regulator and hook it up but instead of producing 5 volts, it's producing 5.75 volts - the smallprint usually informs that the minimum load is x mA and with no load connected, the output doesn't regulate very well.
why does this particular application (a blood pressure monitor)
Some designs do, some designs don't. Without going into great detail, as I said above, a voltage regulator can improve circuit performance and this can mean: -
- Better stability
- Lower noise
- More predictable signals i.e. more accuracy
- Less drift over time
Using an LDO regulator offers exactly the same but allows the output voltage to remain regulated when the input voltage is quite close to the output voltage value.