Position of filter capacitor not right at signal pin of dcdc module...why? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: March 22, 2020, 01:26:13 pm
content: Hi,We are using the DCM3623T50M53C2T00 as a DCDC module. This module has some signal pins. Its usual to provide noise-proofing for signal pins with a capacitor connected right at the pin.However, page 39 and 40 of the “DCM design guide” show the “TR” and “EN” pins being filtered with a resistor at the pin…..the resistor then leading into a capacitor…why is the capacitor not at the pin itself?…the “DCM design guide” says that the resistor is there to damp oscillations between the lead inductance and the external filter capacitor…but all ICs have lead inductance…it doesn’t mean you put a resistor in there…the filter cap is always connected right at the pin…surely?Interestingly, on page 39, the filter capacitor at the “TR” pin is recommended to be 100nF. However, on page 77 of the “DCM design guide”, this same filter capacitor is recommended to be 1nF. Do you know which is the best value?I hope it isn’t 100nF….because we are connecting our external feedback loop to the TR pin and 100nF would slow things down too much.“DCM design guide"http://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdfDCM3623T50M53C2T00 datasheethttp://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdf

author: dmills
author_rank: Super Contributor
timestamp: March 22, 2020, 11:41:36 pm
content: What does the Vicor FAE have to say about all this?You are trying to do something far enough 'off label' that I would absolutely be talking to Vicor about this, I don't think the datasheets will really tell you what you need, you need the FAE.My reading is that the 330ohm resistors are there to give an internal over voltage clamp something to work against, basically protection from the inductive kick that can happen if hot plugging a module with reasonably long power and sense wiring. These are the sort of detail that make Vicors stuff as good as it is.As to cap value? Who knows (Vicor does), FAE again.

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2020, 07:15:58 pm
content: Quote from: dmills on March 22, 2020, 11:41:36 pmThe documents show that TRIM is referenced and bypassed to Vin- and has an internal 10 kilohm pull-up to 3.3 volts so there is nowhere for an inductive kick to get into it.Vicor says the filter is to limit noise which makes sense since TRIM is referenced to Vin- but resonance with lead inductance is rarely a problem with high impedance inputs. For instance high impedance inputs on oscilloscopes generally have like a 50 ohm resistor to quash resonance but they operate to 100s of MHz. An exception is when driving the base/gate of an emitter/source follower which has a low AC load impedance but if that was the case, Vicor should have added the resistance internally in series with the base/gate.Maybe the resistance is needed to prevent the filter capacitor from discharging into the TRIM pin and damaging something if Vin+ is shorted to Vin-, perhaps by a crowbar circuit. This is a common problem with bypassing the adjust pin of integrated regulators and why a protection diodes are suggested if their input can be shorted.

author: ocset
author_rank: Super Contributor
timestamp: March 24, 2020, 06:04:28 am
content: QuoteThanks, i think, in line with what you say, we will add a capacitor at the pin, and a diode to mitigate the circumstance you kindly speak of

author: Damianos
author_rank: Frequent Contributor
timestamp: March 25, 2020, 12:07:40 am
content: Instead of guessing, try to read the text just above the figures that show these resistors.In both cases they explain that these resistors are used to damp the peak at resonant frequency, of the capacitor and parasitic inductance.

author: ocset
author_rank: Super Contributor
timestamp: March 25, 2020, 09:11:25 am
content: QuoteThanks, but this is not normal to have those kind of resonances on a signal pin...the parasitic inductance is tiny. In any electronics signals, the filter capacitor is right on the pin of the IC...otherwise it is far less effective.....think when we you filter a signal into a microcontroller...you put the filter cap right on the pin of the micro.

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2020, 04:25:05 am
content: Quote from: Damianos on March 25, 2020, 12:07:40 amYes, that is what Vicor says but it conflicts with the circuit details they give; they do not show the input circuit structures.

author: Damianos
author_rank: Frequent Contributor
timestamp: March 28, 2020, 05:03:29 pm
content: Quote from: treez on March 25, 2020, 09:11:25 amSorry for the looong delay to respond!As it is more like a module and not a monolithic IC, there maybe exist a little more inductance than what we expect. Also the internal Vcc is from a switching circuit, that has some remaining ripple that has its consequences.Quote from: David Hess on March 26, 2020, 04:25:05 amI don`t see a conflict, even if they don`t give the details of the circuit.Probably the auxiliary and the main regulator are operating on the same frequency. This can produce a slow variation of the output voltage, so they suggest to damp the ripple (see also above).