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I am creating a device utilizing multiple components, all of which have an acceptable operaating voltage of about 5 V. One component has an operating voltage of about 6 V.

I am looking at relevant schematics online, and noticed that this device utilizes a component* I didn't think I needed (TPS763XX, a low-dropout linear regulator).

This is the first time I've ever heard of this component; I've done brief research on it and don't understand it much. Is it used to regulate input voltages for certain components (i.e. were my device to have a 6 V supply, this particular component would create an output of 5 V to be the voltage supply for those devices requiring it?)?

What is this component used for in general, and why does this particular application (a blood pressure monitor) require it?

*Reference component U1 in Table 2 at the top of p.17.

EDIT: fixed link

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  • \$\begingroup\$ low dropout, as opposed to 'high dropout', which term is not used, but all regulators made in the 1970s needed 3v minimum between input and output. An LDO needs much less, often 100mV or so. \$\endgroup\$ – Neil_UK Feb 23 '17 at 14:26
  • \$\begingroup\$ @howland12 link [1] is broken (should be .pdf not .puff) \$\endgroup\$ – jfowkes Feb 23 '17 at 14:37
  • \$\begingroup\$ Low Dropout will give you better efficiency if you can tweak the power supply output. But you gave almost details about your design. Like current required for each voltage, battery powered? \$\endgroup\$ – Misunderstood Feb 23 '17 at 14:50
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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) require it?

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.

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"Low dropout" means you just need less headroom above your regulated voltage than a "normal" regulator. For example, the minimum input voltage for a 7805 to produce 5V is about 7V -- or 2V of dropout. Let's say that a "conventional" 3.3V regulator also needs 2V. Well, if you have a 5V supply, and need 3.3V for some of your circuit, you're out of luck -- unless you can find an LDO regulator with a dropout of less than 1.7V (which is no problem, these days)

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Well, a voltage regulator has an important characteristic called dropout, which is the minimum voltage between input and output for proper regulation.

LM317 drops 1.5 to 2V between 20mA and 1A for example.

"LDO" or "low drop out" regulators are designed for much lower dropout voltage, like 0.2V or even a couple tens of mV.

You don't need a LDO to make 3V3 from a 6V battery. A LM317 would work just fine.

However, if you are looking for a voltage regulator with...

  • Low standby current
  • Features (like an enable pin)
  • Low noise and such
  • Stable with small output capacitors like ceramic
  • Cheap

Then, it'll probably also be a LDO, because pretty much all chip makers only make LDOs these days.

And... in your case, if you want 5V from a 6V supply, you will need a LDO too.

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I get the sense you are asking more about what a voltage regulator is as opposed to specifically a low-dropout regulator. These are used very commonly in electronic circuits to maintain a fixed voltage, usually stepping down from some higher voltage source. So in your case if you have a device that can't operate above 5V and you have a 6V power source, you would use a regulator to step the voltage down from 6V to 5V.

Regulators come in various fixed voltages, and some can provide adjustable voltage.

As others have noted, a low-dropout regulator is just one where the minimum input to output voltage difference is low. In the 6V to 5V case, many standard regulators would not be able to do this because the dropout voltage is more than 1V. A low dropout regulator would probably be needed.

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A linear voltage regulator, when properly used, will provide a regulated (within limits) voltage from a varying input voltage (within limits).

They are used because:

  • ICs/microcontrollers etc. tend to have quite narrow acceptable voltage ranges, and power supplies can have quite a wide possible output range, it is often necessary to use a regulator to ensure that the supply to those components is within limits.

  • A typical design may have components that require different voltages. (5V, 3V3, 3V etc.). Different regulators can supply these voltage from a single input supply.

The "low dropout" part simply means that these parts will work down to a "low" difference between input and output voltages.

In the case of the heartbeat monitor you have linked to, they have just used 4xAA batteries to power it (roughly 6V), so the regulator was necessary to provide the correct voltage for the rest of the system. If this were an actual low power product, the power supply would probably be very different.

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