Burning 50 mV - Page 1
Forum section: electronics / projects

author: RFenthusiast
author_rank: Newbie
timestamp: November 02, 2021, 02:00:37 pm
content: Hi all,I am new here and hope my poste is correct in this forum area.I'm thinking about my next electronic project which will include several digital ICs. Now I found that one of the ICs demands a voltage of 1V (+-25mV) while another wants 0.95V (+-25mV).As the current @0.95V is 50mA max. I was wondering, if there is a clever way to derive the 0.95V from the 1V rail?Does anyone have an idea how to accurately "burn" the 50mV?Kind regards!

author: wraper
author_rank: Unknown
timestamp: November 02, 2021, 02:20:55 pm
content: If you get them from higher voltage through LDO, the best would be to simply use two separate LDO. There is no some simple part that will drop 50mV at variable current. Also you could slightly undervolt 1V part if you don't run it at full speed (if it's say an MCU core). What ICs in particular you want to use?

author: ConKbot
author_rank: Super Contributor
timestamp: November 03, 2021, 06:54:54 am
content: I've seen ideal diode drivers that operate by keeping the mosfet operating with 20ish mv of forward voltage drop across it. Forward voltage increases, and it increases gate voltage, forward voltage decreases, and it lowers the gate voltage to drive the Vf back up, eventually shutting off the gate when current tries to reverse.If you have other higher voltage rails to work with, a logic level N-FET and an opamp or two to keep it at a precise 50mv drop may do it.

author: daqq
author_rank: Super Contributor
timestamp: November 03, 2021, 07:13:10 am
content: QuoteAt which point the complexity and extra parts are such that he may as well use another LDO or a two channel DC DC converter.

author: moffy
author_rank: Super Contributor
timestamp: November 03, 2021, 07:23:46 am
content: If you can afford 300ma from the 1v rail, then a 0.2 ohm resistor in series with a 4 ohm resistor will give 0.952 volts with an extra 10mv drop at the full 50ma.

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2021, 08:30:09 am
content: What ICs require such tight voltage tolerances? It sounds bizarre to me.Just power them both from 975mV, with a tolerance of 0.5% or under, job done.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 03, 2021, 09:31:10 am
content: Indeed, what part requires 0.95V +/- 0.025V?If the design was mine, such special snowflake with such special requirements would likely get its own regulator, one specified with 1% reference tolerance and FB divider using 0.1% resistors, just in case.Regulators are cheap and small anyway. I have no bad feelings using two regulators if there is any reason.

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2021, 10:56:31 am
content: Quote from: Siwastaja on November 03, 2021, 09:31:10 amIndeed.I think it's likely the original poster is mistaken. He or she has probably mis-understood/read the data sheet.

author: daqq
author_rank: Super Contributor
timestamp: November 03, 2021, 11:17:11 am
content: QuoteFPGAs are bastards in this. Not saying they would explode because of a 50mV difference, but they're specified at those values. See attachement.

author: Gyro
author_rank: Super Contributor
timestamp: November 03, 2021, 11:39:38 am
content: That looks a bit like the parts being binned - those that meet spec at 950mV and the other ones that need an extra 50mV to perform?Edit: Or is it a dissipation limiting measure on larger (/smaller geometry) parts?

author: NorthernWing
author_rank: Contributor
timestamp: November 03, 2021, 02:42:46 pm
content: If I remember correctly, on some of the Xilinx UltraScale+ parts the fabric/core voltage is binned like this to guarantee the device makes timing when considering the difference in rated speeds between the commercial and industrial temperatures. At least that was my understanding of it. If this is an FPGA, I bet you can run that rail way down and the only side-effect would be that you'd have to slow the system clock down to make timing (assuming it's not also powering any PLLs/etc).I'm going to assume that if you're using basically any part modern enough to have these requirements on a separate ~1.0V VCC rail, it's also expensive enough that giving it its own LDO for the core voltage is a pretty trivial expense, and this will make it easier to meet any other power sequencing/startup requirements anyway. There really isn't any reason to try and put together a more convoluted solution that will probably perform worse than an actual precision regulator anyway... at least that's my opinion.

author: Just_another_Dave
author_rank: Regular Contributor
timestamp: November 03, 2021, 02:54:14 pm
content: Quote from: Siwastaja on November 03, 2021, 09:31:10 amThose requirements sound like a reference voltage instead of the input that should provide power to the device. However, it is impossible to say if that’s the reason for such tight tolerances without knowing the components that are going to be used (to access their datasheet)

author: Just_another_Dave
author_rank: Regular Contributor
timestamp: November 03, 2021, 02:58:54 pm
content: Quote from: daqq on November 03, 2021, 11:17:11 am50mA seems a bit too small for a Xilinx FPGA (unless it is a really small one or almost unused). In any case, powering it from a lower voltage than the specified one is not going to damage the device

author: ace1903
author_rank: Regular Contributor
timestamp: November 03, 2021, 03:00:42 pm
content: 50mA is quite low consumption for large FPGA fabric and fast IO.Is this current measured or calculated?I expected to see fast switching regulator per rail instead of LDO.Maybe average current is 50mA but with decoupling capacitors I wouldn't try solutions like resistor, diode or transistor to drop several mV.Such solutions can make voltages with improper ramping times and cause weird behaviors.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 03, 2021, 04:11:49 pm
content: FPGA was just an example of a tight tolerance device. OP has not commented about the part being an FPGA, it can be something else.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 03, 2021, 07:17:43 pm
content: Quote from: Zero999 on November 03, 2021, 08:30:09 amNever seen that, especially for digital ICs. I'd be curious to know more about them. Hopefully the OP will answer and give references.

author: coppercone2
author_rank: Super Contributor
timestamp: November 03, 2021, 08:31:03 pm
content: I recall one data sheet about something FPGA or otherwise semi programmable specialty processor (DSP), I forgot exactly what, but I do recall there being some voltage values and tolerances that struck me as odd, but this was quite a few years ago. I do believe maybe some LT/TI/AD (which one?) datasheets for power regulators made reference to that part.. high current low voltage precision regulators that stuck out from the crowd. I think it was something along the lines of a part with a 1.1V, 1.05V and possibly 0.9V rail... real strangeIf I remember details I will post.also, maybe it was a multi load sequencer switch document that presented this 'scenario' on a graph with a description on it pointing out that red = 0.9, green =1.05.. and so forth, for a part that had some kind of strange requirement like that.I also know while whatever power system for the part I saw was strange/interesting, the part application was so mundane or irrelevant that i totally forgot about it until I read this thread. I think they were showing off the load switches ability to precisely stabilize really sensitive digital loads. Maybe it was something related to internet infrastructure (cisco-like stuff).If I recall I was prowling around in load switch/switching converter documents that were on the organizers when I ran into whatever lead me to the document.

author: langwadt
author_rank: Super Contributor
timestamp: November 03, 2021, 08:57:24 pm
content: Quote from: NorthernWing on November 03, 2021, 02:42:46 pmI seem to remember a comment from a Xilinx guy that said with modern processes wasn't so simple anymore.Changing the voltage up/down could change the speed of different parts of the chip at different rates sothe relative timing between those parts got worse

author: ejeffrey
author_rank: Super Contributor
timestamp: November 04, 2021, 02:59:34 pm
content: The Intel Arria V requires something like 1.1 V +/- 30 mV for speed grades -4, -5, and -6 but 1.15 V +/- 30 mV for speed grade -3. Probably if you run speed grade -3 at 1.1 V it will work as a -4? Maybe if you are not power/temperature limited the slower grades will work fine at 1.15 V? Maybe, but the multi-corner timing analysis is done at the voltages on the datasheet so that is what you use. If specifying slightly absurd tolerance voltage rails lets them get slightly more performance out of a multi-hundred dollar FPGA it makes total sense to do so.Assuming the parts in question are exotic or specialty high performance parts I wouldn't hesitate to use a separate regulator.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 04, 2021, 04:05:37 pm
content: Its not unusual to see CPU core power suplies sub 1V. are usually (amost always) buck switchers. Unless you want a heater for the winter chill. For instance seehttps://www.ti.com/power-management/non-isolated-dc-dc-switching-regulators/step-down-buck/buck-converter-integrated-switch/products.html#sort=p634min;ascAs you can see you can get regulators to go down as far as 200mV.A TPS62000 might be the answer to your prayer. Regulation is better than 1% (~ 10mV in 1V). Maybe play withhttps://www.ti.com/design-resources/design-tools-simulation/webench-power-designer.html

author: Siwastaja
author_rank: Super Contributor
timestamp: November 05, 2021, 04:05:06 pm
content: With switchers & tight tolerances, remember to use switchers that do not use pulse skip modes (or those can be disabled) to keep predictable and low voltage ripple.Also with switchers, larger output capacitance may be needed for transient response than with some very high BW linear regulators.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 05, 2021, 07:01:47 pm
content: Yes, and yes separate regulators would be the way to go here.But I'm still curious about the parts the OP is using.