Really low value current sense resistor (50mV peak) will it have noise issues ? - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 04, 2020, 01:51:09 pm
content: Hi,I have a question if I can use this low value sense resistor to get reliable current mode boost without sense amplifier just next to itI want to keep sense resistor as small as possible about 50mV at peak current 5A, fsw is 200kHz so will also make some messQuestion is, can I then trigger by it with basically 1mV or even less resolution, or will noise just mess this low signal and Ill need to get some amplifier ?Also I am little scared of noise in chip itself as it is digitally controlled and some noise can get to analog part of chip

author: TimNJ
author_rank: Super Contributor
timestamp: May 04, 2020, 02:29:06 pm
content: What chip are you using? Anecdotal evidence, but...I'm using LM5122 (75mV over-current threshold) in an application with a peak current of about 25A. I'm using a normal 2512 sense resistor, and an 2R-C filter, as per the datasheet recommendation. The distance between the sense resistor and the chip is around 10mm. I've done pretty extensive testing, and no weird glitching, unexpected shut-downs, instability, etc. If not using a true kelvin package sense resistor, you can still get adequate sensing by routing the traces direct to the pads. Assuming normal high-side current sense method, make sure to route the + and - current-sense traces as close to each other as possible. Route them over a ground-plane, preferably part of the ground plane not carrying heavy current.

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 04, 2020, 04:38:40 pm
content: It is controlled by Atmel/Microchip SAM MCU and its integrated comparator/current mode PWMThat is why I am little nervous about chip noise itself

author: Marco
author_rank: Super Contributor
timestamp: May 04, 2020, 05:11:01 pm
content: Offset for the comparator looks a wee bit high relative to 50 mV.

author: Weston
author_rank: Regular Contributor
timestamp: May 04, 2020, 06:08:47 pm
content: What noise are you worried about? Noise has a source. Due to the low impedance of the current shunt you are mostly going to get voltages induced through other return currents flowing through the ground plane.The most common source of noise is generate by the switching node, converters typically blank the current sense comparator during this period to prevent false triggering.If you have more than one phase its possible to get switching noise coupled from other phases. You can also get voltage offsets due to DC currents flowing through the ground plane.Differential connections to the current shunt resistor would reject most of these nose sources, but I doubt the SAM can be set up like that. You could probably try some star grounding with the central ground connection being the low side of the current shunt. A differential amplifier would make this easier but add to your BOM cost.You might have noise internal to the comparator, but the SAM datasheet possibly has info on that.Also, with low value shunt resistors the series inductance of the shunt can be an issue, leading to higher current readings for higher frequency components of the current waveform. Often its not that much on an issue if you have a blanking period though.

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 04, 2020, 07:42:02 pm
content: I have two phasesBlanking is supportedBut datasheet is silent about noise transfered from CPU to comparators, or even detailed specs of comparatorsLayout can be tricky to avoid inductive coupled on sense line or ground issue as it will be single ended measure

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 04, 2020, 08:06:43 pm
content: I use PIC's in similar situations but preceded the internal comparitor with an external differential op-amp r-r io kelvin connected directly across the sense resistor. Even so I use LEB as the noise in an hard switched SMPS of any power (mine are 1-3Kw) is substantial never mind how much care you take IMOP.

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 05, 2020, 06:40:07 am
content: I also considering use of high side sense likeHV7802it is reasonably cheap and can be placed before chokes to give much cleaner signal and have configurable gain

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 05, 2020, 08:01:49 am
content: I agree with amplifier right next to sense can voltage go much lower

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 05, 2020, 09:57:20 am
content: Quote from: Miyuki on May 04, 2020, 01:51:09 pmThe problem will be wires to that shunt (their lengths) but not the shunt DC resistance because they can be longer than the shunt itself.