I'll put my 2 cents in here for a few subtleties. I defined probably 1000 different linear and switching regulators at Maxim in my 25 years. Vout plus dropout is pretty good overall. But don't forget that dropout is defined where the output voltage drops 100 mV. Its coming out of regulation and all the goodness that linears give you stops happening. You also want to do this at max load, max output tolerance and worst temp. The drop out for bipolars decreases for higher temps which helps but at very cold temps, it can grow- a lot a very cold. This is not allways well specified. There are several classes of dropout that are driven by the design of the ouput stage Vdropout 2v standard bipolar linears like the 7805 use an NPN darlington output- nice low impedance and easy to use. 1v LM1117 type bipolar linears use a Sziklai modified darlington with an NPN follower driven by a PNP- pretty good comprimise .5v LM2940 PNP pass element parts have low dropout but some squirelly stability issues at times and can have high quiescent at dropout .1v PMOS or charge pumped NPN pass element types that looks like a small resistance in dropout. Quiescent can be very low. . Depending on the type of regulator, there are subtlties that happen around dropout. Old bipolar regulators like the 7805 have a drop out of about 2v conservatively. The output is an emitter follower darlington stage which is 2 vbe's (.7v each) and 2 Vce sat (about .2v each). This is about 1.8v. Since the output is a follower, it has a gain of 1 and are generally very well behaved with very little thought given to bypassing and stability. High frequency rejection is poor and accuracy is somewhat poor. First generation bipolar LDO's like the LM2940 etc, used a PNP output stage with a grounded NPN pulling its base down. These parts have two Vce sats in the dropout path (about .5v). These transistors have gain on top of the error amp gain and get unstable without following the the bypassing instructions closely. The output cap becomes the dominant pole and the ESR of the output cap has to in a specific range- neither two small or two large. The other annoying feature of this class is as you approach dropout- the beta provided by the PNP pass element goes south and they can draw lots of current at or near dropout trying to keep the PNP in saturation. In low power circuits, this can cause a sort of latching action and flatten a battery in no time. The LM1117 type second gen bipolar LDO's have a NPN follower ouput. These have the benefit of a follower but only moderate dropout performance. They were basically invented to make 3.3v from 5v which a 7805 couldn't do. Good for point of load but kind of mediocre otherwise. Modern BiCMOS LDO's generally have a PMOS pass element and some MOS and Bipolar circuits.. These can have very low quiescent, very low noise and the lowest dropout possible. They are also pretty stable with most loads but take a signicant cap on the output usually. Something like a 10 uF ceramic. Microchip make some good low cost parts in this class. Probably my favorites. There are all CMOS LDO's that share most of features of the last category but don't get the low noise and high accurancy generally. One other issue is your 8v battery. The charging voltage on a Lead Acid could be over 10V which is a common abs max for many linears. My 2 cents.
Regards, John M. Wettroth (984) 329-5420 (home) (919) 349-9875 (cell) _____ From: TriEmbed [mailto:[email protected]] On Behalf Of Pete Soper via TriEmbed Sent: Monday, November 16, 2020 10:46 AM To: [email protected] Subject: Re: [TriEmbed] Powering ESP32 from an 8v golf cart battery On 11/15/20 10:34 PM, The MacDougals via TriEmbed wrote:
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