TOP232G can be used as Buck? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: December 27, 2021, 09:43:48 pm
content: Hi,Would you agree, the TOP232G can be used as a HV buck just like the LNK302?...for TOP232G, all that woudl be needed woudl be to feed a resistance from Vin to the M pin. ..Ans also, to use a bootstrap cap off the Vout liek in LNK302, and connect from this, an R/Zener, back to the control pin....which woudl also be fed by a 10u via a 10RCan you see any reason why this wouldn't work?BTW, has anybody noticed that the Power integrations forum is often messed up by hackers posts, which have somehow gotten through? Ive noticed this for many years now. Is someone trying to hack them up all the time?TOP232Ghttps://www.power.com/products/topswitch/topswitch-fx/top232gn?langcode=enLNK302https://www.power.com/sites/default/files/product-docs/lnk302_304-306.pdf

author: xavier60
author_rank: Super Contributor
timestamp: December 27, 2021, 11:11:47 pm
content: Like this but I think C2 should be shifted to where C3 is.https://restovarius.files.wordpress.com/2017/06/top224-buck1.png?w=676A resistor would need to be put between M and S and its value used to set the current limit to avoid possible inductor saturation.A control power supply circuit that I see with the LNK304 used in is actually Flyback, not Buck and produce a negative output rail.Edit: I need to rethink the arrangement on the C pin. Another 47uF and 10Ω resistor between C and S.

author: Faringdon
author_rank: Super Contributor
timestamp: December 27, 2021, 11:31:21 pm
content: Thanks, your link is great....i saw a schem like that....i think the caps are right as you show in the link ...the control pin needing some 10uF to 47uF.It makes you wonder why they brought about the LNK30X family?....because they only do what your link shows(i agree about the M pin resistor, and indeed, a resistor chain needed from vin to M too...so that it doesnt disable due to too low vin.)

author: xavier60
author_rank: Super Contributor
timestamp: December 28, 2021, 01:08:42 am
content: The C pin has the combined functions of the BP and FB pins on the LNK302 and draws a few mA.Im still not certain what is the best component arrangement is for the C pin for good stability.Some minimum load might be needed to prevent the output voltage climbing too high during startup.

author: Faringdon
author_rank: Super Contributor
timestamp: December 28, 2021, 10:11:33 am
content: QuoteThanks, i see your point, if the C pin cap is large, then whilst it charges up, the TOP232G is going to be on max duty cycle, and wont "know" that vout has risen until that big cap has charged....so this could result in vout overvoltage.......maybe this is why they brought out the LNK302 range.....since it does not need a large BP pin cap.

author: Faringdon
author_rank: Super Contributor
timestamp: December 29, 2021, 03:16:35 pm
content: Hi,What do you believe would be the output voltage of the attached TOP232G based Buck converter?Would you agree that due to the approximately 3mA of current that would be flowing into the C (Control) pin, then the Vout will be 5.8V + 9.1V + 3V = 17.9V?{ = [Control pin regulation voltage] + [3mA x 1k] + [9V1 zener] }Also, How do you think the TOP232G will manage with the fact that I(M) will drop below the shutdown threshold every time the internal power FET switches OFF?Also, since I(control) goes above 5.9mA during cycle skipping, do you agree that in such a time, vout will be 9.1V + 5.8V + 5.9V = 20.8VDoes this setup result in the Control pin capacitor constantly discharging below the Undervoltage threshold 4.8V, and thus making the TOP232G go through constant restarts? Does this affect the Vout ripple?

author: xavier60
author_rank: Super Contributor
timestamp: December 29, 2021, 10:58:27 pm
content: Quote from: Faringdon on December 29, 2021, 03:16:35 pmI does look like it should regulate about 18V. Having the series 1K resistor is unusual from the application examples I have seen.Stability is difficult to predict.R7 needs to be a low value, typically 5.8 0r 10 ohms to allow the control circuit to supply enough Gate current from the capacitor.A bypass capacitor could be placed across D7.

author: Faringdon
author_rank: Super Contributor
timestamp: December 30, 2021, 01:59:56 pm
content: Thanks Xavier60, you make excellent points.i agree that it is really odd that they have used the 1k....becuase it just introduces a potentially wide tolerance to the vout.Also, yes, the 210R ressitor is really weird esp due to gate drive situation.Also, Power integrations have the following app note for TOPswitch used as a Buck...so it makes you wonder why they brought out the LNK30X family.....(but maybe they have lower bias current...ahh yes, i think i just answered my own question)https://www.power.com/sites/default/files/forum/files/top%20switch%20Buckconverter.pdf

author: xavier60
author_rank: Super Contributor
timestamp: December 31, 2021, 08:35:19 am
content: One more possible complication. When used as a Buck regulator rather than Flyback, the IC needs to operate at a much lower duty cycle.An IC that has energy saving features might mistake this for low power demand and go into some green mode such as pulse skipping or burst mode.I stumbled across this,https://au.mouser.com/datasheet/2/328/linkswitch_tn2_family_datasheet-1951889.pdf