Hi,
as the title implies, i wanted to ask if there is a limit to how high you can set the Current limit resistor or it can be set at any (reasonable) value?
Same goes for the Slow Start capacitor, is there a limit to how long we can set the slow start or is it “free”?
Thanks
Datasheet is here:
https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP5083/document_id/1806/
(Relevant tables at bottom of page 6 for current limit, start of page 7 for slow start capacitor)
Hello,
The realistic limit for the value of the current limit resistance would be close to 100kohms. The accuracy will degrade at low current limit, however reducing the value of the function. If a low current limit is desired (<500mA) then another device may be more appropriate.
The soft start cap would have a much wider range, and large ceramic cap values can be used. Practical values should keep soft start below 100ms, some electric circuits may not function well with very slow power ramp times.
Regards,
Hi, thanks a lot for the answer. I chose this device because there was no other devices with similar features and with a low current control. I need this load switch in series with a tube filament supply. It should be 3v @ 100mA (2 tubes in series). I wanted to prevent overshoot with the slow start (minimum 0.5sec) to allow for the filament to heat up well, hence why I asked about how long the slow start can be, and wanted to set a blocking current of a couple of hunderds of mA. If it loses that much precision I will only be using it as a short circuit protection, and for that I mean if the supply chip shorts out and puts 24v on the output it should trip and save at least one tube right?
Thanks
Sorry, my bad. I should have explained what my application is and what features I need so I will (hopefully) explain all there is to know in order to better choose what IC I need.
What I wanted was to do a supply for the filament of some VFD tubes. I chose the MP1584 as the converter from 24v/12v to 3v, the other supplies have similar needs, just different voltage and less current. What I was searching for was:
- Overvoltage protection: just a volt over and it would cause permanent damage to the IC. I know it’s difficult to find a low voltage protection so I am also happy with a 20v+ limit. That way I protect the tubes from a short in the supply chip.
- Slow ramp up: as the cold filament has less resistance, at start up there is a quite big (enough to lower some life times, but not enough to blow them) inrush current, so I needed a really slow start time, around 0.5sec, even better 2 seconds.
- Current limiting: not fundamental but in case there is still some inrush I should be able to limit the current to max 110mA or so. If that is not possible I’m happy with just a short circuit protection, which would protect in case the supply goes bad and jumps to 24v.
- A power good indicator pin: this will allow me to (in software) delay the start of the anode supply (this is to minimize wearing out and maximize lifespan)
- As the voltage is low I need an external supply, much like the MP5083 does.
- Last but not least: I want this part to be available, on Mouser/TME/MPS. If you look on mouser almost everything is out of stock. That’s why I chose the MP1584 as the supply: there are plenty of modules and this IC is really popular. This is the major limiting factor. there are a lot of cool ICs out there if only were they available for purchase.
The MP5083 is almost perfect for what I need, but I need really long startup times and pretty low current limit. One option is to use some supply IC with an integrated slow start, that would be absolutely perfect. Unfortunately, i haven’t found such supply chip yet.
I would love any suggestion and any help in my quest for finding the perfect IC for my application. Let me now if anyone has any clue on what I could use.
Thanks
You might consider an “inrush current limiter” This is a NTC that is high value when it is cold and as it warms up drops to a low value. Two terminal device, simple to use. They are used a bunch to control inrush in power supplies with large electrolytic caps. Not a perfect solution but not bad.
The MP5083 may be the device with the best fit for most of those requirements. The only other device at MPS that could help would be the MP62551 which includes a precison low current limit down to 100mA and below, but lacks some of the other features, such as adjustable soft start, listed above. Its stocked at Digikey too.
Possibly you combine a couple of devices such as a transient suppressor, NTC, and current limiter to meet the requirements.
Regards,
I’ll have a look at those thanks!
Yeah I imagined it was probably the only option to fit here. It is available on mouser in moderate quantities, but I don’t know about directly buying from MPS (i can select some devices, but no others). Everything could change if I find a DC/DC Buck converter that has a programmable soft start that can reach some seconds, but it seems unlikely.
Anyway, so can I put a higher SS capacitor without limits (up to 1uF or so) or is there a limit? (maybe for stability purposes or something else)
I’ll put the highest resistor possible for the current limiting, hoping that it would prevent some damage.
Thanks
The soft start for the MP5083 should be fine with a large cap/long startup ramp.
There are a few 24V buck DC/DCs (MP2372…) that include external soft start so it is possible to have the rail come up slowly. The actual timing would need to be verified, with an eval board would be best.
Regards,
Thanks, when I have the chance to try the MP5083 I’ll put a big cap and see what it does. It should be just enough I think. This will automatically tule out the need for current limiting (except for short circuit ofc)