Type 3 feedback loop compensator...pole and zero positions - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 05, 2020, 06:22:38 pm
content: HiThe following are application notes by Renesas and ti.com. They each give the expressions for poles and zeros of a type 3 feedback loop compensator.However, they are offering different pole and zero frequencies to each other. That is, they have the poles and zero’s swapped. For example, the ti.com app note has the high frequency pole as an expression involving the opamp’s feedback components…whereas the intersil app note has the high frequency pole expression using the input components.Also, the zero frequencies are similarly not correspondant between Renesas and ti.comWho is right?Type 3 compensator according to ti.comPage 8 and 9https://www.ti.com/lit/an/slva662/slva662.pdf?ts=1607167216358&ref_url=https%253A%252F%252Fwww.google.de%252FType 3 compensator according to RenesasPage 6https://www.renesas.com/eu/en/document/tb417-designing-stable-compensation-networks-single-phase-voltage-mode-buck-regulatorsQuote ReplyReport Edit

author: Andy Watson
author_rank: Super Contributor
timestamp: December 05, 2020, 07:17:33 pm
content: Noting that Renesas and TI have used different labels for their capacitors I think that the other main difference is that Renesas are giving an expression for the complete transfer function of the system whereas TI are giving the response of the compensation network - \$H(s)\$. If we can assume that the rest of the system (or op-amp) is perfect then the transfer fuctnion will be \$\frac{1}{H(s)}\$.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 06, 2020, 01:25:09 am
content: Also, concerning page 6 of the Above (and below) Renesas app note, why does the phase plot start atminus90 degrees?........it should be plus 90 degrees.Renesas app note:https://www.renesas.com/eu/en/document/tb417-designing-stable-compensation-networks-single-phase-voltage-mode-buck-regulatorsThe transfer function of that opamp feedback circuit gives a phase shift of plus 90 degrees near the zero Hertz axis...............__________............................____________________________-------Also, regarding equation 38, page 9 of the ti.com application note (shown again below), this equation is nonsense as it has "fp0" in the denominator....and having zero in the denominator is nonsense....and gives a value of infinite for the capacitor...would you agree? [fp0 = zero Hertz, or vanishingly small Hertz]....In fact, "fp0" appears throughout equations 36 to 39 of page 9, and makes nonsense of each equation, would you agree?ti.com app note.https://www.ti.com/lit/an/slva662/slva662.pdf?ts=1607167216358&ref_url=https%253A%252F%252Fwww.google.de%252F

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 06, 2020, 04:13:28 pm
content: ...Actually, would you agree with the following........the Transfer function (of the type 3 opamp compensator) near the f=0 axis boils down to a simple feedback cap and an input resistor.The transfer function of this is...Vout/Vin = -j/wCR.and Atan ((-1/wCR)/ ZERO) = Atan (-infinite)..and this equals to [plus or minus] n.pi/2 where n = 1,3,5,7...So it looks like the phase can start off at -90, or +90, or -270, or +270....etc etc etc.................