DCDC Converter Feedback Resistor Question - Page 1
Forum section: electronics / beginners

author: chancs
author_rank: Contributor
timestamp: December 25, 2021, 03:01:32 pm
content: I have used few buck converters before but this MP1477 from MPS suggests to put a resistor RT on the feedback pin which isn't part of the voltage divider form by R1 and R2. And they also suggest to use different RT value for different VOUT. So my questions are:1. What is this RT for, does it has anything to do with the IFB, the feedback pin current?2. Is the value for RT critical? If so, how to calculate it?

author: ledtester
author_rank: Super Contributor
timestamp: December 25, 2021, 03:39:18 pm
content: This datasheet shows an example of R3 set to 75K:https://www.monolithicpower.cn/cn/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP1477/document_id/3730I guess I would just extrapolate from the values given in "Table 1":

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 25, 2021, 04:20:20 pm
content: Shame, they don't show the internal configuration of that pin. Most likely it is an internal R+C feedback around the error amplifier, so that you can control compensation somewhat with the divider's Thevenin resistance.Which is all that really matters; once you know what values work well, you can increase the divider's resistance until the series resistor value is absorbed in their Thevenin equivalent, and save one part.The value depends on voltage, moreso because the L and C vary; the internal compensation is fixed, while the modulator gain varies (proportional to supply voltage), and the error amp compensation needs to include all three as factors. Basically, stick to recommended values, then if you like, evaluate the transient response (say 50 to 100% load step) and adjust L, C and RT for best results. Steps of say +50/-30% should be fine -- no need to be fancy, compensation is pretty forgiving once you're in the right area.Tim

author: chancs
author_rank: Contributor
timestamp: December 27, 2021, 01:15:25 pm
content: I still doesn't quite understand why RT is used for compensation. Isn't generally compensation is done with a feedforward capacitor parallels to R1 (top resistor). And according to datasheet, the feedback current (IFB) can be as large as 100nA, with RT = 75kOhm. It will introduce an offset voltage of 7.5mV, which is quite large in my option.

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2021, 04:13:04 pm
content: Quote from: T3sl4co1l on December 25, 2021, 04:20:20 pmThat is what I think is going on also.Rt could be replaced with a trimmer and adjusted while performing a transient response test to find out what it is doing and its optimum value.Quote from: chancs on December 27, 2021, 01:15:25 pmIt is done with either or both. 7.5 millivolts of error is likely insignificant compared to other error sources.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2021, 04:52:43 pm
content: Quote from: chancs on December 27, 2021, 01:15:25 pmDifferent kinds of compensation. When you have "type 2" (pole-zero) compensation, it's an R+C across the error amp (OUT to -IN, it's just a regular op-amp), with some series resistance to -IN, being that's the FB pin. (+IN is internally tied to the reference, or a divider of it.) This gives an integrator characteristic from low frequencies (like "type 1" i.e. no resistor, just C across the amp), but levels off at middle frequencies, leaving some loop gain and giving back some phase margin. (When this gain equals the modulator gain, or rather the inverse with respect to whatever total system gain you wanted, you get a sort of open-loop response where distortion may be higher but the overall gain can be flat from DC through this region.) The modulator's own cutoff then finishes the loop response (which may have multiple poles near cutoff, hence the need for extra phase margin -- a typical switching converter has at least the L (in current mode control), plus some effect due to the control method (peak current mode has a RHP zero), or the L and C together for voltage mode control, which is part of why VM is so terrible.The feedforward cap implements lead-lag compensation ("type 3"), where the feedback network itself puts a zero into the loop response, in addition to what's around the error amp. This is usually used in conjunction with a zero from the output filter cap ESR, to stabilize VM controls with a cutoff near the LC cutoff frequency (otherwise cutoff needs to be a modest fraction below, and transient response is even worse). It's not needed for current mode controls (where the error amp is mainly compensating for the modulator gain (as a transconductance stage: voltage input, current output) into the output filter cap), but can be used to further improve dynamics if desired. Or more generally, an R+C here, not just a pure C in parallel with the feedback resistor; it might be a good idea to put less switching ripple into the feedback node for example, or the feedback only needs to be peaked a modest amount (2x, 4x, etc.) rather than the maximum amount (1/(divider ratio)). These are all parameters to tweak based on the transient response and stability over all operating conditions, so, lots of load range and step testing.QuoteWhy? The rated spread is 789 to 821mV V_REF, a span of 32mV; and 100nA is the max, typ being 10 so the likely error is only 0.75mV due to that cause.Tim