Datasheet lacking crucial information - Page 1
Forum section: electronics / projects

author: Fulcrum
author_rank: Regular Contributor
timestamp: November 24, 2020, 11:17:09 am
content: Hi all,I am designing a 12V to 5V voltage regulator using the MAX20008 step-down converter.Datasheet:https://media.digikey.com/pdf/Data%20Sheets/Maxim%20PDFs/MAX20004_06_08_3-24-19.pdfOn page 16 there is an equation for choosing the compensation resistor RC, see attached image.The problem is that the parameter VREFis not defined anywhere else in the datasheet. Am I missing something crucial, or is this a mistake by Maxim?

author: MIS42N
author_rank: Frequent Contributor
timestamp: November 24, 2020, 12:03:58 pm
content: I looked at the datasheet and it shows a diagram on page 9 with Vref coming from the voltage reference. Elsewhere it says the voltage reference is either 3.3V or 5V, depends on the part. e.g MAX20004AFOA/VY+ is 5V but MAX20004AFOB/VY+ is 3.3V. Hope that helps.

author: Fulcrum
author_rank: Regular Contributor
timestamp: November 24, 2020, 12:26:44 pm
content: I must be blind, I have been reading this datasheet for nearly an hour trying to find where the value of VREFis defined. Where exactly did you see it as being defined as 3.3V or 5V?

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 24, 2020, 01:17:23 pm
content: I wasted about 15 minutes and can't find Vref too.

author: magic
author_rank: Super Contributor
timestamp: November 24, 2020, 04:15:16 pm
content: Block diagram and figures 2,3 suggest it's the voltage which Vfb is regulated to, so 1V.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 24, 2020, 04:30:47 pm
content: Yes, the DS is a bit of a mess, I agree, and especially this Vref signal which is not clear.You may need to understand the subtleties of Vref vs. Vfb if you're using the part in a very specific way, but what matters to set the output voltage is Vfb. (The link between the two is shown on p.14...)Vfb is defined as 1V typ. (p.3) and the formula for computing the output voltage is on p. 12 only mentioning Vfb.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 24, 2020, 06:32:49 pm
content: Quote from: magic on November 24, 2020, 04:15:16 pmYes, the Vfb is 1 V, but the question is about Vref.

author: MIS42N
author_rank: Frequent Contributor
timestamp: November 24, 2020, 11:06:20 pm
content: Quote from: Fulcrum on November 24, 2020, 12:26:44 pmI think I jumped to a conclusion. It is a confusing document. It says Vbias is 5V, connect Vbias to Vfb (feedback) for fixed output but elsewhere it says Vfb is 1.0V. I tend to agree with others that Vref is 1.0V but (like you) wouldn't be putting money on it. Time to talk to Maxim?

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 25, 2020, 12:48:44 am
content: Quote from: MIS42N on November 24, 2020, 11:06:20 pmYes. Or possibly somewhere is an application manual with more explanation but I couldn't find it fast.

author: wraper
author_rank: Unknown
timestamp: November 25, 2020, 12:56:37 am
content: Quote from: Vovk_Z on November 24, 2020, 06:32:49 pmIf you bothered to check what he mentioned, it would be obvious that Vfb = Vref as in most of cases.

author: KE5FX
author_rank: Super Contributor
timestamp: November 25, 2020, 02:34:13 am
content: I'd look at the LT8640S, maybe. Better data sheet, less Laplace.

author: Fulcrum
author_rank: Regular Contributor
timestamp: November 25, 2020, 08:03:27 pm
content: Thank you to everyone who helped. I am going to work off the assumption that VREF= VFB. What an annoying datasheet.Quote from: KE5FX on November 25, 2020, 02:34:13 amThanks for the tip, but my design requires 8A output current, and the part you mention is 6A.

author: Mr. Scram
author_rank: Super Contributor
timestamp: November 25, 2020, 08:06:57 pm
content: Friends don't let friends use Maxim. Use another supplier and avoid supply and datasheet issues.

author: Fulcrum
author_rank: Regular Contributor
timestamp: November 25, 2020, 09:10:57 pm
content: I thought I understood, but this datasheet still makes no bloody sense.First of all, it recommends connecting the BIAS pin directly to the FB pin for 5V operation. That is how the typical application circuit right on page 1 is set up. It also says on page 9 :QuoteBut look at the block diagram on page 14. The feedback blockFdoes not exist in this case. VFBwill just be 5V, regardless of what the output voltage is. How is the control system expected to function in this case?And why on earth does the power output come from a block called "FEEDBACK SELECT" on the internal block diagram on page 9??

author: wraper
author_rank: Unknown
timestamp: November 25, 2020, 09:24:15 pm
content: Quote from: Fulcrum on November 25, 2020, 09:10:57 pmMost likely it uses internal divider from OUT pin instead of FB pin, if you do so. And FB is multifunction pin also used to trigger operation in this mode.

author: magic
author_rank: Super Contributor
timestamp: November 25, 2020, 09:37:37 pm
content: This exactly. I don't have this datasheet anymore but I'm pretty sure that it contained "feedback switching logic" or something similar, which turned it into a fixed voltage regulator (5V or 3.3V depending on part number - somethnig like that).

author: Fulcrum
author_rank: Regular Contributor
timestamp: November 25, 2020, 11:02:50 pm
content: Quote from: wraper on November 25, 2020, 09:24:15 pmThank you, that makes sense, and I appreciate your insight. Although it's still only a guess (based in experience, I assume), it makes sense.I am very frustrated with this datasheet. In lieu of explicit explanations of how to use the part, we instead get pages of transfer functions. I have never had so much trouble with a DS before. Is Maxim notorious for this or something?

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 26, 2020, 01:47:59 am
content: Quote from: Fulcrum on November 25, 2020, 11:02:50 pm+1. I don't understand why they think we are scientists here.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 26, 2020, 02:19:52 am
content: Quote from: Fulcrum on November 25, 2020, 11:02:50 pmYes.