Thought I gave a reasonable background, limiting error amp outputs for instance. The error amp has high DC gain. It is desirable not to require additional amps, and to prevent any amp from saturating (to prevent excessive slewing and the attendant recovery time penalty).
In general, the error amp will have +in = reference, -in = feedback. An inverting amp is sometimes possible, when the reference is available inverted or when the process* is inverting.
*Process, "plant", the thing under control.
If you'd like something even more concrete, suppose you have this:
- 0-5V triangle wave, 100kHz
- PWM comparator, gate drive, switcher -- buck converter let's say, and current sense. Let's say the current sense uses a Kelvin connected shunt and a differential sense amp.
- Inner error amp: output to PWM comp; -in to ISENSE, +in to IREF
- Outer error amp: output to IREF (sets inner loop variable), -in to VOUT, +in to VREF (maybe we've got a potentiometer hanging off a TL431 so it's an adjustable regulated DC output)
And the amps are running from +/-15V supplies.
There are two things wrong with this:
1. The inner error amp can slew outside the range of the triangle wave. So for -15 to 0V, it's saturated, 0% PWM. Only the 0-5V range is linear. Then 5-15V it does 100%, saturated again. The inner current loop needs to be as fast as possible, in order to allow the outer voltage loop to be as fast as possible. Letting it span excess range wastes time slewing.
2. The outer error amp has wasted range -15 to 0V, since the output current can never be negative (assuming transistor + diode type buck converter; a synchronous buck could actually go bidirectional, as long as the current sense amp supports it). And it will saturate at +15V, whereas it would be desirable (or required even) to have an adjustable limit. That limit must be accurate if a repeatable current limit is required (say for a lab supply with square SOA -- tight voltage and current regulation).
Leaving the outer voltage amp alone is especially awful, as going from -15V to 0V will take an extremely long time (tens of miliseconds?). And don't say it won't happen, because it will, each and every time the load current drops suddenly. And then the load starts back up and the voltage drops out (potentially followed by extreme overshoot as it attempts to recover). So some sort of limiting is required there, and the output of that limiter has to be able to drive the compensation network (let's say it's 10kohms + 0.01uF).
By the way, this is a known and familiar failing of many power supplies, and a primary reason they are often rated for regulation, and transients if at all, at two relatively linear points, like 10 and 100%, or even 50 and 100% load. Well what happens at 0%-100%, or even 0%-10%? You probably don't want to know.
Both loops illustrate the problem, and each suggests different options.
For the inner loop, a fixed limit is fine. The simplest is a voltage divider (to +V and -V or GND, as needed), so that the guaranteed output range (V_OH to V_OL, at the same or higher load resistance) gets reduced to the desired range. A couple oddball resistor values are sometimes required, and you'll still end up with dead zones (because the real amp will saturate better than the guaranteed limits), but it's a far sight better than nothing.
You could also use an R2R amp on a restricted supply, but then you need another chip, and you need (in this case) a 5V power rail, and the inputs are no longer +/-15V compatible. (If the entire circuit operated on +5/0V rails, this satisfies the fixed limits for both amps, at the expense of some 10s of mV saturation -- if accuracy down to low levels is required, rails just beyond +5/0V are required. +/-15V would be a far sight from there, but illustrates the need better.)
Certainly, neither is acceptable for the voltage amp, if an adjustable current limit is required.
...Is "dur, use a diode" really the extent of original thought here? Come on guys, let's see some brain storming!
Tim