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Diode OR on FB pin of switching controller to add constant current regulation

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What are the pitfalls of using a diode OR to combine output voltage and output current into the FB pin of a switching controller?
Obviously there is the diode v-drop to consider and that will vary a little with current, though that should be small.
And there's some reverse leakage current between the diodes so one will offset the other a little.

The current signal would be from a high side current sense chip with analog output and voltage from simple voltage divider.

I'm just wondering if this is valid approach to add basic current regulation. (Doesn't need to be super accurate. +/- 5% variation on voltage and current regulation is fine)

I'm guessing the correct way to do it is with two opamps as ideal diodes?

One thought: be careful about introducing gain into the FB loop.  I know you're measuring current and turning it into a voltage (so 'gain' is a bit ephemeral) but you still might end up with something that oscillates.  Many switching controller example circuits already put things like resistors in series with the FB pin to (I presume) try and avoid this problem at higher freqs.

(Mind you, my bench power supply oscillates when it hits current limit!  So maybe it's a standard feature :D)

The big one is probably the lag introduced to the control loop by the current sense IC.  A lot of those have fairly poor bandwidth, combined with a relatively high frequency switcher that could eat too much phase margin and cause loop stability problems in current regulation mode.  Since the current loop will be slower than the voltage loop (because the current loop is just the voltage loop but with more stages in the feedback path) you'll need to figure out a way to compensate the former without (ideally) slowing down the latter too much. 

If you're thinking of using a voltage divider from the output of the current sense amp and then a diode from the output of that divider to the FB pin then of course the impedance of the current feedback divider needs to be substantially lower than the impedance of the voltage feedback divider, otherwise the current feedback can't sufficiently override the voltage feedback.  You could add an additional op amp as an error amp with one input from the current sense amplifier, the other input from a voltage representing the current setpoint, and the output driving the FB pin via a diode to solve the impedance problem, substantially eliminate the diode Vf from the equation, and maybe make it a bit easier to set the current limit.  This would incur additional phase lag in the current regulation loop, but probably less than the current sense amp does.  Since the op amp will be saturated when the current is less than the setpoint you'll need to select an op amp that can come out of saturation and back into linear operation quickly if a fast response to overcurrent is important. 

This will effectively be a battery charger, so the output ripple is not all that critical, neither is fast response to current regulation.

Yeah, the chips i'm looking at have resistors/cap on COMP pin for FB loop compensation. so may need some tweaking

You can't just directly place a diode in there because the diode voltage drop is unstable with temperature and current. But diodes are indeed a valid way of "summing together" two error amplifiers, commonly used in discrete bench PSU designs.

In the case of a switcher IC you might directly connect the voltage feedback divider to FB then just add in the current override signal trough a diode and generated by a separate opamp acting as a error amplifier.

As for stability, yes you want to have enough gain for good regulation but not too much. What helps the most having a switcher chip that has an external COMP pin. This lets you slow down its feedback loop in case it gets too nervous.

Stable current regulation can be tough however. Hard shorts or large reactive components can send things into oscillation. So make sure to test it with a wide range of loads.


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