Going against v-reg datasheet recommendation for more robust design - Page 1
Forum section: electronics / projects

author: sebmadgwick
author_rank: Regular Contributor
timestamp: December 16, 2019, 08:44:53 pm
content: I am using theTPS63000to generate 2.2V and 3.3V supply rails. The TPS63000 output voltage is set by feedback from a potential divider as shown in the attached schematic.The datasheet (page 11) recommends the use of high resistor values, "...keep the value for this resistor in the range of 200 kΩ" and "...a 1-MΩ resistor should be chosen...". TI'sWEBENCH Designerrecommends the specific values shown in the attached schematic.I am uncomfortable using such high resistances because of the risk of contaminants or even handling of the PCB resulting in destructively high voltages. My preference would be to use resistor values one or two orders of magnitude lower. The total device current consumption is 100-300 mA so the increased quiescent loss is not of concern.Is there any reason I should stick to TI's recommendation here? The recommendation seem specific without justification.

author: iMo
author_rank: Super Contributor
timestamp: December 16, 2019, 08:51:09 pm
content: The divider is at the output, therefore no need to stick with such values.

author: wraper
author_rank: Unknown
timestamp: December 16, 2019, 08:59:27 pm
content: Quote from: sebmadgwick on December 16, 2019, 08:44:53 pmJustification is simple, quiescent current and intended operation from battery in portable devices. By using low resistance divider, it can easily consume more than IC itself.

author: iMo
author_rank: Super Contributor
timestamp: December 16, 2019, 09:05:19 pm
content: The DS says the current through the divider should be at least 100x higher than the current into the FB pin (10nA). As wraper wrote they want to show high efficiency and low quiescent current therefore they recommend the values at higher side.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: December 17, 2019, 12:37:30 am
content: The quiescent current of the IC is ~5 mA without Power Save and it is rated for up to 1.5 A output. It therefore seems strange that recommendations would optimise to save a few uA without qualifying this. The datasheets sates, "...keep the value for this resistor in the range of 200 kΩ", unconditionally.Anyway, it looks like I was reading too much into this. Thank you for your comments.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2019, 04:16:53 am
content: Conversely, I've seen datasheets with:- ca. 10k dividers recommended,- no specification for leakage current of FB pin,- no discussion of how to choose those resistors (the 10k range is simply assumed as "good enough")Had to contact TI via E2E forum to get an answer; turns out it was ~nA, as you expect for CMOS inputs. With the usual fluff about "we don't measure it because it takes too long", of course they didn't have anything to say about "why not at least 'max 1uA' or such?"Probably the DS/AN author of the part I was looking at, should've swapped roles with the one you're looking at!Generally, you can expect CMOS chips to use CMOS input structures (i.e., ESD diodes and gates), having very little leakage (say, 10 or 100nA for a bad part on a hot day); likewise, bipolar has to supply base current, so tends to pull up (or down) with some ~uA. But when youneedto know -- better off measuring it, get a sample or dev board and check. Second best, asking the manufacturer...Tim