Critize my constant current switch circuit? - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: December 15, 2023, 08:22:17 pm
content: MP5035 datasheet:https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP5035GJ/document_id/10530/Circuit: Attached image #1Reason for the choice of MP5035: Low cost, high current range support (50ma-2a), simplistic, wide input voltage (2.9v-22v)D1 + D2 = protects against extreme voltage scenariosD3 = Reverse input polarity protectionC4 + C5 = Reduce input/output voltage noiseR1 + RV1 = Adjustable (fine-tuning) current trigger (from ~50ma-1.5a)C1 + C3 + R2 = Recommended by datasheetQuestion: Does anyone have any criticisms for my circuit? Things to change/consider?Thanks!

author: Slh
author_rank: Regular Contributor
timestamp: December 15, 2023, 09:58:07 pm
content: What's the purpose of the circuit? If it's just for transient protection then it may be fine. If you're hoping for something like a lab power supply current limit then the chip won't take it for long.I wouldn't like to comment on the TVS without knowing part numbers and required non operating conditions. Also, why the bidirectional one? If you put a unidirectional one then you could get rid of the Schottky. I know the datasheet says -0.3V max but I suspect it will take the brief reverse voltage before the fuse goes and chances are the Schottky voltage will be greater than -0.3V anyway. Do check datasheet for TVS for forward voltage before removing Schottky.Do you care what happens if the potentiometer wiper becomes disconnected? You might want to tie the other end to 0V. The input isn't linear anyway. It's worth bearing in mind that, as it is, you'll have around 200mA to 2000mA in 10% of the potentiometer travel.The 5A fuse sounds a little larger than you need. 5A fuses can sustain 5A indefinitely. Why not use a 2A? The inrush on those caps won't be enough to pop the fuse.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: December 16, 2023, 08:38:06 pm
content: Quote from: Slh on December 15, 2023, 09:58:07 pmIt'll be used to charge 5v USB devices at lower currents, such as phones and bluetooth devices (smart watches, earbuds, etc.). It's a similar design to () video but based around a different I.C.I did try to find an I.C that allows for adjusting an ILIM pin to control an external FET for easy adjustable constant-current (slightly similar to the LT3081https://www.analog.com/media/en/technical-documentation/data-sheets/3081fc.pdf) but I couldn't find any.Quote from: Slh on December 15, 2023, 09:58:07 pmCould a undirectional TVS get rid of the schottky for reverse-polarity protection??? I had no idea, thanks!Quote from: Slh on December 15, 2023, 09:58:07 pm"When ILIMIT is floating, the current limit is fixed at 300mA." per datasheet. I do understand that the ILIM current is non-linear, but I think that'll be fine with me. Maybe using an exponential pot (instead of a linear pot) will help?Quote from: Slh on December 15, 2023, 09:58:07 pmThis is another good idea that I'll implement, thanks!

author: Terry Bites
author_rank: Super Contributor
timestamp: December 17, 2023, 12:08:16 pm
content: If you have transient protection on the input you dont really need it on the output as well. Use the dv/dt for that.You've got to hope the fuse blows before the input Schottky melts down. A series Schottky would be better.I've become a fan of PMOS reverse polarity protection and polyfuses lately- zero maintainence and highly relaible.