TI TPS61235P boost converter IC question - Page 1
Forum section: electronics / projects

author: aiq25
author_rank: Regular Contributor
timestamp: August 23, 2019, 07:25:08 pm
content: Hello. I'm planning to use a TI TPS61235P boost converter IC for an upcoming project (VOUT 5V/3.5A with a VIN_min of 3V). In the features TI mentions there are two 10A internal power switches for the synchronous function but in the headline it's mentioned as a 8A converter, where does this descripency come from?Also, the minimum value for I_LIM (switch valley current limit) is 6.5A. With an output of 5V at 3.5A, with a VIN of 3V the input current roughly be about (assuming 85% efficiency) 6.86A. This should work, according some of the efficiency plots I think this is doable with this IC but what is confusing me is the minimum value of I_LIM at 6.5A. Does this mean I should only draw 6.5A max at the input (since the IC can have this limit at the temperature extremes it seems)?In short, I'm not understanding how the 10A, 8A and the 6.5A are related.

author: aiq25
author_rank: Regular Contributor
timestamp: September 16, 2019, 01:38:11 pm
content: I wanted to provide an answer, if anyone comes across this post. I posted this question on the TI E2E forum and got an answer.6.5A is the limit. This was quite disappointing for me since these IC's are advertised as 8A in the datasheet title and 10A in the "Features" section.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: September 16, 2019, 04:56:30 pm
content: Marketing != reality.

author: nuclearcat
author_rank: Unknown
timestamp: September 16, 2019, 05:06:43 pm
content: Most likely, these data do not indicate the maximum current of the converter.Maximum current greatly depends on the many parameters, frequency, Vin / Vout, heat dissipation capability, operating temperatures, components around and etc.And good advice, try to not pick anything that will operate near maximum limits of IC, always keep a good safety margin.

author: aiq25
author_rank: Regular Contributor
timestamp: September 17, 2019, 02:45:46 pm
content: I typically add 10% margin to my worst case operating conditions and then try to have 10% margin on component parameters. I have learned quite a few things in designing this boost converter, which is good.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 17, 2019, 02:51:57 pm
content: Quote from: aiq25 on August 23, 2019, 07:25:08 pmSomething fundamental to understand with switching regulators is that the max switching current is NOT the max output current. May seem obvious, but hey.- The switching currrent is the current switched from the input of the regulator, and will obviously depend on the conversion ratio AND overall efficiency of the converter,- The output current is, well, duh, the current drawn at the output of the regulator,- Data/fact sheets typically mention a xxx (in A) regulator in the headlines, which means very little in itself. That's the "marketing" part. It's usually given as the max output current of the regulator in a specified typical condition (for instance, a certain VIN, a certain VOUT, and typical passive components around it that are suggested in the application notes.)

author: aiq25
author_rank: Regular Contributor
timestamp: September 17, 2019, 04:07:21 pm
content: Quote from: SiliconWizard on September 17, 2019, 02:51:57 pmThanks. Yes I’m aware that the output current is different than the input. I was just surprised how much marketing they put into this. Lol.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 17, 2019, 07:39:06 pm
content: Quote from: aiq25 on September 17, 2019, 04:07:21 pmWell, you were asking why the discrepancy... I didn't know whether this was a genuine technical question or more of a rhetorical one, so I took the technical path.Sure this is marketing. OTOH, technical documents and technical marketing are difficult tasks.For such converters, the only real figure that would make sense would be the max switching current IMO (and maybe any additional internal current limiting feature). But it would not make it immediately obvious what to expect in terms of the max output current, which is a figure that most designers are interested in when selecting a switching converter.If you only mention switching current figures in a datasheet, many people will complain that it's even more "marketingly" misleading - given that it will always be greater - than giving a typical max output current (in a typical use case), even if obviously the typical use case chosen may not be your use case at all. And it still kind of helps while roughly selecting models.Guess what, people always complain.And of course, moral is that you should always read datasheets and not simply rely on titles and headlines.

author: aiq25
author_rank: Regular Contributor
timestamp: September 17, 2019, 08:36:11 pm
content: Quote from: SiliconWizard on September 17, 2019, 07:39:06 pmNo, I was asking taking technical path. To be honest in the back of my mind I already knew the answer, lol. But I understood the three currents were switch currents, not output current.QuoteI agree. But I think instead of 3 figures they could save something like "8A (nominal)" in the title or features section, to make it obvious. I worked on a boost application before, so had an idea of look for the max switching current and not the output current.