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Reverse current flow in linear regulator with PMOS pass element
Picture the circuit below. A dc supply of around 20 V is connected to both a load (with a wide input ranging power supply) and a linear regulator which is set to 13.8V and float charges a small SLA battery.
Clearly when the 20 V supply is available it powers the load and float-charges the battery. When that power supply is removed it's also clear that a diode across the regulator could ensure the load remains powered (rather than relying on the internal parasitic diode of the PMOS pass element). However it seems likely to me that some (many?) regulators would simply turn the internal PMOS hard on in this situation since the output is below the 13.8V regulated value. That would then allow current to flow in reverse through the pass element's very low on resistance and not experience the diode voltage drop. Perhaps a diode might be required to cover the transient situation before the battery voltage has fallen below 13.8V but once it has the regulator would conduct without significant voltage drop or power dissipation.
My question is if there are any regulators known to have this behaviour?
"... it seems likely to me that some (many?) regulators would simply turn the internal PMOS hard on in this situation since the output is below the regulated value..."
You are correct. This will situation continue until voltage at the input pin falls to either the 'UVLO threshold' or the ''Minimum Operating Voltage'. Below that point, the body diode of the pass element will come into play.
Some datasheets for PMOS LDOs describe this reversed Vin-Vout behavior, others ignore the behavior as they may assign this reverse voltage situation as outside the 'Guaranteed Operating Range'. For a descriptive example, see the 'REVERSE VOLTAGE' section in the LP38851 datasheet.
A similar situation can exist in Bipolar designs as well.
A series diode would be reccomended to prevent the battery from discharging through the regulator and R1.
Interesting! If I read you correctly, I would say that all pmos regulators operate the way you describe. There would need be a diode from OUT to IN, as you describe, with a lower Vfwd than the body diode of the internal FET (a schottky would do well) - because body diode conduction can cause unforeseen problems. This diode would turnon the LDO and allow battery current through the pass FET to power the load, as you also state.
I am sure that the internal current limit circuit would not work when there is reverse current through the FET. And I cannot predict what other aspects of the control circuit would respond piculiarly. At any rate, I doubt if any suppliers would guarantee operation when the lawyers get involved.
I think that even better than a diode, an "ideal diode" built around a MOSFET would avoid voltage drop and power waste on the series diode. The circuits for ideal oring diode control do exactly this.
Power waste will still exist, either across the regulator or divided between a diode and the regulator. If the voltage headroom exists, then the added cost/complexity needed for OR-ing diode should be weighed against the simplicty of a single diode.
You are right, I am sorry, in this case voltage has to be wasted anyway from 20 to 13V so there is no reason to use special oring diode.
It's one of those things that seems like it should work but is outside the specified operating conditions in the datasheet. I know that some regulators have additional circuitry to prevent reverse current flow which might interfere with this mode of operation but those that don't have any such protection don't really specify what happens when you try to do that.
With a schottky to initially turn on the LDO I would expect that the pass element would then conduct in the reverse direction (at which point the diode carries no current because the voltage drop across the P-FET would be lower than the schottky Vf). Unless I've missed something this allows a seamless transition from "battery charging, load running off external supply" to "load running off battery". It neatly removes the diode forward drop by using the LDO as an ideal diode.
It also requires that the LDOs circuitry continues to function as expected when run from a voltage significantly below the point where it drops out of regulation. An adjustable regulator presumably has to allow this but a fixed-voltage version might perhaps shutdown if the input voltage drops too far below the the point where it drops out.
The question becomes "what should I look for when trying to choose an LDO that would allow this?"
I would guess a (partial) list would include: Does not specify any reverse current protection or reverse leakage current, PMOS pass element, adjustable output
The way you describe your circuit is actually different than usual and so we are talking to some extent at cross purposes. By "reverse current " we are talking about only the current that goes through the internal body diode under the main pass fet from output to input. In your case, as I understand things, you bypass this body diode through the external schottky diode. Once you turn-on the part by way of the output through the schottky to VIN then you will not be now conducting what we define as "reverse current". The current will now flow "backwards", drain-source, through the FET - which is perfectly fine (from this narrow perspective). Since these LDOs are not at all characterized for this operation the only thing to do is to get hold of an Evaluation Module (EVM) and try it out.
Now, more to your point, the LDO will be "on" but not regulating - as you say. What I hope will happen is that the LDO will also see the battery voltage as slightly lower than the regulation point set by the feedback resistors (internal feedback or external), and then try to fully turn on the main pass fet in an effort increase output charge. But of course this will work to your advantage by allowing a low conduction path through the fet from battery to load.
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