Constant off time control chips for SMPS are not available...why? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: September 02, 2017, 07:07:01 pm
content: Hello,We wish to design a constant off time controlled SMPS. There are virtually no constant off time control ICs on the market, so therefore we have to make our own. Unfortunately, all of the components shown here are required, and it’s a lot of components.Do you know of any Constant off time controllers for SMPS?

author: Benta
author_rank: Super Contributor
timestamp: September 02, 2017, 07:24:35 pm
content: Check with ON Semi or TI.Here's an example:http://www.onsemi.com/pub/Collateral/NCL30105-D.PDF

author: ocset
author_rank: Super Contributor
timestamp: September 03, 2017, 12:22:24 am
content: Thanks, but its a shame the Current sense input limit is 1V...that can mean much loss in the sense resistors, unless current is low.There are no COT chips with a low sense voltage such as 100-200mV.

author: jbb
author_rank: Super Contributor
timestamp: September 03, 2017, 01:05:56 am
content: Quote from: treez on September 03, 2017, 12:22:24 amIt's a bit fiddly, but you could drop in an x10 sense amplifier.Alternatively, if you're feelingextrakeen, you could deploy a micro controller like a TI Piccolo or Cyress PSoC (or there's probably a cheaper ARM option) with features as follows:Internal oscillatorADC (use for voltage feedback path)DAC for ADC settingComparatorPWM stage with connection to comparatorSure, you'll need to program it and use an external gate driver, but you will actually have quite a small solution. Additionally, you get some options to add features like cunning soft start modes, status indication, digital trimming of output voltage etc.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 03, 2017, 02:20:12 am
content: Good riddance, there are better methods!Tim

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2017, 02:36:40 am
content: Quote from: treez on September 03, 2017, 12:22:24 amLinear Technology makesa few. Their smaller ones implement the current sensing internally.http://www.linear.com/product/LT1615http://www.linear.com/product/LTC1265

author: ocset
author_rank: Super Contributor
timestamp: September 03, 2017, 11:07:48 am
content: Thanks, Constant off time control is by far the best methiod when using offline, single stage power factor corrected LED drivers up to several 100W and without using electrolytics.The lack of need for slope compensation allows use of DCM/CCM boundary with enormous step up ratios highly possible with great ease.Thus it is worth doing it in discrete components but we would prefer to have them all on an IC if possible.Still the SMPS commercial fraternity dont want anything to do with the street light market because of the need to give a 10 year warantee, and the rough outdoor environment, plus the £60 charge whenever one fails from a big mains transient etc, because somebody has to get the cherry picker out.

author: mikerj
author_rank: Super Contributor
timestamp: September 04, 2017, 12:57:56 pm
content: ST have anapplication notefor using their L6564 in fixed off-time operation.