Types and regulation schemes LDOs

Question

LDOs, or low-dropout linear regulators, come in various types, but it can be difficult to distinguish them, and to the novice engineer, this regulator is often made to seem deceptively simple.

Some LDOs didn't care about the ESR of the output capacitor (used with film capacitors). Later they were common with electrolytic output capacitors with some ESR and did not work well with ceramic capacitors (MLCC). And now it seems to be common for them to be perfectly stable with MLCCs but have a maximum ESR requirement of a few Ohms.

My understanding, and correct me if I'm wrong, is that this difference must be due to differences in their control functions, that instability is caused by insufficient phase margin, and that this instability should be countered by introducing one or more zeros to the control function (adding ESR to Cout and/or feed-forward capacitance across R1 in the control loop).

But I'm missing the bigger picture as to what the differences are with regards to their build and regulation schemes, how to spot the different types, other than look for the ESR requirements, and what other factors, related to build and regulation scheme, might be prudent to consider when choosing an LDO.

Can someone provide me with a good overview?

Answer 1

It can be hard to identify each type, but I will try to provide a guide.

Many newer devices have specifically addressed the loop stability problem with different manufacturers taking different approaches:

Analog devices has their range of AnyCap devices, and you can read about how they achieve operation insensitive to output ESR at this page.

Linear technology has a new range of regulators (typical part linked) where a current source is used for the internal reference, which permits the loop gain of the device to be constant regardless of \$Z{in}\$, and \$V_{in}\$ to \$V_{out}\$ ratio making loop stabilisation much more straightforward; in fact LT claim this part requires no output capacitance, although the dropout voltage is a little high compared to some competing parts.

Generally, most parts that have a lineage of more than 10 years age will exhibit loop stability issues without the proper amount of output ESR (the venerable LM1117 for instance is a relatively low dropout part with similar operation to the even more venerable LM117 but can be troublesome in this regard).

The lineage is important; many older devices were very simple to use and then the drop-out became an issue so manufacturers made parts that operated in a virtually identical manner but with lower drop-out (the operation is not identical without proper ESR, though but at the time of part release many capacitors had the right amount of ESR).

A quick glance at the datasheet original issue date is usually a good first step; if it predates 2006, be suspicious. That said, the easiest way is to simply search for "ESR" in the datasheet which may (or may not) uncover a stability requirement on ESR.

Some regulators require an ESR with an upper bound but no low bound.

It can be difficult to navigate this sea of parts, and experience is a key element; that said, LDO manufacturers that have devices without ESR requirements advertise it far and wide, so looking for parts that are insensitive to output ESR can be as simple as a quick search with your favourite search engine.

Answer 2

It sounds like you've got a solid understanding of control loop theory, and this is certainly something that needs to be addressed in the design. In my opinion, however, with and LDO regulator this is secondary concern. With over 20 years experience as a professional EE, here's my process for choosing an LDO for an application...

1. Find a device that provides the required load voltage and current with the given input voltage. With an LDO, the load current the device can supply should be de-rated by at least 20%.

2. Perform a thorough thermal analysis, where the LDO is operating at maximum load current at the maximum input voltage at the maximum ambient temperature. Consideration needs to be given to the type of heat-sink being used and the sum of the thermal impedances to ensure that the junction temperature of the device never exceeds the max allowed. If a device cannot be found that will operate comfortably under these conditions, it's time to consider a switching supply.

3. Now, follow the datasheet regarding the input and output capacitor requirements. If the capacitor manufacturer doesn't provide information regarding ESR, use an LCR meter to measure the ESR of the capacitors.