For linear regulators like the 78xx series you have a minimum 2-volt drop to account for the B-E junctions of 2 transistors and the drop of a zener diode (emitter-follower series current pass element, error amplifier and voltage reference). So for a 5V regulator, you need to provide a minimum 7V input; for a 3.3V regulator you need a minimum of 5.3V, and so on.
There is a recent article in Circuit Cellar magazine, Issue 364, Nov. 2020: "Voltage Regulators: Beat The Heat", pg. 60 (From The Bench column by Jeff Bachiochi), that explains about this. On 11/15/20 8:07 PM, The MacDougals via TriEmbed wrote: > > Can’t find ams1117-3.3 TI’s L1117-3.3 is a direct replacement. > > > > The data sheet shows 15v (max) for Vin and nothing for min. What am I > missing? > > https://www.ti.com/lit/ds/symlink/lm1117.pdf > <https://www.ti.com/lit/ds/symlink/lm1117.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe&ts=1605410881526> > > > > ---> Paul > > > > > > *From:* Josh Wyatt <[email protected]> > *Sent:* Sunday, November 15, 2020 7:47 PM > > > > Why not use something like AMS1117-3.3? Minimum voltage on your 8 volt battery > is going to be something like 6VDC. > > > > -j > > > > On Sun, Nov 15, 2020 at 7:38 PM The MacDougals via TriEmbed > <[email protected] <mailto:[email protected]>> wrote: > > How do I find an LDO that operates on 8 – 8.8 v? > > I looked at a couple of datasheets and found linear regulators that take > 9v (max) input. > > I did not see a (min) spec for input v. > > > > How can I tell if it works at 8? > > > > For example: > > https://www.njr.com/electronic_device/PDF/NJM2888_E.pdf > > > > > > ---> Paul > > > > -- Scott G. Hall Raleigh, NC, USA [email protected] /Although kindness is rarely a job, no matter what you do it's always an option./
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