MGTAVTT and MGTAVCC power supply in Kintex development board  - Page 1
Forum section: electronics / general

author: joniengr081
author_rank: Regular Contributor
timestamp: January 06, 2023, 04:24:03 pm
content: Hi,I am working on power supply for the FPGA board and found the schematic for MGTAVTT (1.2 V) and MGTAVCC (1.0 V). Both are 5 A power supplies. This look like a complex design in the attached schematic. I am wondering if we can also use other regulator for both MGTAVTT (1.2 V) and MGTAVCC (1.0 V) ?In order to understand these power supplies. I will have several questions. Can someone please help me by explaining the attached schematic with the function of each pin doing ?

author: asmi
author_rank: Super Contributor
timestamp: January 06, 2023, 04:35:19 pm
content: Read UG476, Chapter 5, section "Power supply and Filtering" and below. It's got everything you need to know. Xilinx has excellent documentation, but one need to actually read it.

author: joniengr081
author_rank: Regular Contributor
timestamp: January 06, 2023, 04:50:33 pm
content: Thanks for reply and pointing me to the document. I will have a look but I am interested to understand this circuit if someone can describe configuration and functionality of the main device they used in the circuit that would be very helpful.

author: asmi
author_rank: Super Contributor
timestamp: January 06, 2023, 05:30:58 pm
content: Quote from: joniengr081 on January 06, 2023, 04:50:33 pmThese look like some kind of DC-DC PWM controllers, but I don't feel like digging into their datasheets to find more details. But you can do that, and perhaps should.Hmm, on a closer look it looks like that is schematics of KCU105 devboard, so you will probably need to check the relevant transceiver's user guide (I don't remember off top of my head which MGTs are used in that device).

author: joniengr081
author_rank: Regular Contributor
timestamp: January 06, 2023, 08:28:28 pm
content: Now I have read about the pins of max20751 and most of them are getting clear to me but I am not able understand the followings in the datasheethttps://www.analog.com/media/en/technical-documentation/data-sheets/max20751.pdf28 A3_OUT_PS Phase-Shedding Feature (disabled in the MAX20751). This pin must be connected to A3_OUT_NORM with a short trace or a 0Ω resistor.29 A3_OUT_NORM Phase Current-Loop Amplifier Output for the Regulator. Must be connected to A3_OUT_PS witha short trace or a 0Ω resistor.30 A3_IN Phase Current-Loop Amplifier Negative Input31 A2B_OUT This node has the same value as A2_OUT during normal operation, but has programmablepositive and negative voltage clamps that limit the maximum positive and negative output current.32 A2_OUT Voltage-Loop Amplifier Output33 A2_IN Voltage-Loop Amplifier Negative Input34 A1_OUT Differential Error-Amplifier Output35 SENSE_N Negative Remote-Voltage Sense36 SENSE_P Positive Remote-Voltage SenseA question about MGT power supplies. Do we need just a source or source/sink for MGT power supplies. If it is only source then other choices for 1V @5A and 1.2V @5A can be used instead.

author: Someone
author_rank: Super Contributor
timestamp: January 06, 2023, 09:18:49 pm
content: Continuation from the previous thread:https://www.eevblog.com/forum/fpga/power-supply-for-fpga/

author: asmi
author_rank: Super Contributor
timestamp: January 06, 2023, 09:41:09 pm
content: Quote from: Someone on January 06, 2023, 09:18:49 pmOk that's what I figured.To the OP:It's not our job to read documentation, datasheets and schematics for you. We can try to help with specific questions if we happen to know (and remember) answers, but nobody here is going to spend his/her time studying all of those docs just to respond to your question. It's not StackOverflow, there are no ranks, stars, upvotes and whatever other nonsense SO has in order to insentivize users to waste their time digging out answers to random non-specific questions.

author: joniengr081
author_rank: Regular Contributor
timestamp: January 06, 2023, 09:55:46 pm
content: I understand. I will read datasheet and try to understand how this circuit is working. Thanks for reply anyways.

author: joniengr081
author_rank: Regular Contributor
timestamp: January 08, 2023, 03:08:03 pm
content: Now I have a specific question. I have attached two power supplies in the first post, one for MGTAVCC (1.0 V @ 5 A), and one for MGTAVTT (1.2 V @ 5 A).Calculation for MGTAVCC (1.0 V @ 5 A):Number of populated phase = 1, because only ISENSE2 along with PMW2 are connected to slave device which is one-phase according to Table 2. Phase Population Position.RSEL3 = 80.6 Ohm (Fsw = 600 KHz and slew rate = 1.25 V /ms)RSEL2 = 402 Ohm (Vout = 0.030 V)RSEL1 = 576 Ohm (Vout = 0.970 V)RSEL0 = 0 Ohm (RDES = 640 Ohm)Vout = 0.030 + 0.970 = 1 VIoutmax = 0.85 x IocpIocp from Table 5 for RDES = 640 Ohm and one-phase is 25 A that gives Ioutmax = 0.85 x 25 = 21.25 A but in the schematic it says 5 A. How they calculate 5 A ?Calculation for MGTAVTT (1.2 V @ 5 A):Number of populated phase = 1, because only ISENSE2 along with PMW2 are connected to slave device which is one-phase according to Table 2. Phase Population Position.RSEL3 = 1020 Ohm (Fsw = 800 KHz and slew rate = 0.5 V /ms)RSEL2 = 576 Ohm (Vout = 0.030 V)RSEL1 = 1020 Ohm (Vout = 1.170 V)RSEL0 = 0 Ohm (RDES = 640 Ohm)Vout = 0.030 + 1.170 = 1.2 VIoutmax = 0.85 x IocpIocp from Table 5 for RDES = 640 Ohm and one-phase is 25 A that gives Ioutmax = 0.85 x 25 = 21.25 A but in the schematic it says 5 A. How they calculate 5 A ?

author: asmi
author_rank: Super Contributor
timestamp: January 09, 2023, 08:38:50 pm
content: Take a look at the user guide for this board, those values could be just a requirements/expected design max values. You don't want to design your DC-DC converter (or anything really) so that it's just on the edge of capability, so you tend to oversize your PDS to make sure it's got some margins. Also thermal regime must be also taken into account, a lot of compact FPGA boards run fairly hot by design, and as such PDS components must be significantly derated. This is the difference between "on paper" design and the real functioning board - these things are hard to catch without having actual prototypes and subjecting them to rigorous testing which would push them much harder than what a real application calls for so that engineer will get enough data to make a call on whether make any further adjustments or not.

author: joniengr081
author_rank: Regular Contributor
timestamp: January 10, 2023, 03:51:46 pm
content: Yes, that is fine but I am trying to understand the schematic of MGTAVTT and MGTAVCC power supply attached in post 1 because I might need to use that power supply in my design if I am able to get how they found 5 A current with the given resistor values in the schematic. My calculation mentioned in my last post shows that it is much higher than 5 A. Can someone please able to check my calculation in the last post. Thanks in advance.

author: miken
author_rank: Regular Contributor
timestamp: January 10, 2023, 08:57:13 pm
content: There are other requirements on a power supply besides the max current. Read the MGT user guide carefully and you will find ripple requirements. I would guess that the 5A is what is being designed for given ripple.

author: Someone
author_rank: Super Contributor
timestamp: January 10, 2023, 09:36:04 pm
content: Quote from: joniengr081 on January 10, 2023, 03:51:46 pmIs that circuit on the reference/example board the same as the circuit in the regulators application note/datasheet? You can't blindly apply recipes to different situations.This task is clearly way beyond your capability. Wind it back and design around what you can understand.

author: joniengr081
author_rank: Regular Contributor
timestamp: January 14, 2023, 05:15:03 pm
content: In post number 8 above, I describe my question on calculation how they get 5A in the schematic. Please indicate if there is anything wrong in calculation in post 8 ? I have attached datasheet of MAX20751 and the schematic in which is used.