[TriEmbed] Powering ESP32 from an 8v golf cart battery

[John Wettroth]

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:triembed-bounces at triembed.org] On Behalf Of Pete
Soper via TriEmbed
Sent: Monday, November 16, 2020 10:46 AM
To: triembed at triembed.org
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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