Dropout voltage of buck converter - Page 1
Forum section: electronics / projects

author: tom66
author_rank: Super Contributor
timestamp: June 08, 2012, 07:58:25 pm
content: I've got an LCD TV to repair (37LC2D) whose power supply has a dead 3.3V output, measures 1.2V. The board is really burnt around the area of a buck converter which produces the 3.3V from the 5V standby.So, I am thinking it might be better to make my own 3.3V power supply instead of repair the badly damaged one, as it's likely to fail again anyway.I was going to use something like an LM2576T, but I'm not sure what the drop out voltage of this device is. I need a 1.8V headroom. (The 5Vstby measures 5.1V.) My problem is, none of the datasheets seem to specify an actual dropout voltage.How can I find this dropout voltage?I was going to assemble the DC-DC on a piece of veroboard/stripboard; can anyone see any potential pitfalls with this?

author: Bored@Work
author_rank: Super Contributor
timestamp: June 08, 2012, 08:23:49 pm
content: Quote from: tom66 on June 08, 2012, 07:58:25 pmOne of the first diagrams in the datasheet.QuoteRead the datasheet more carefully.QuoteMany. A good PCB layout is essential for the performance of a DC-DC converter. Up to the point that the circuit just doesn't work. At least plan your veroboard layout.

author: tom66
author_rank: Super Contributor
timestamp: June 08, 2012, 08:33:22 pm
content: Quote from: Bored@Work on June 08, 2012, 08:23:49 pmRight you are sir - my page down flicking through seems to have missed it. Around 1.6V, which is close, but okay. Worth a shot.Quote from: Bored@Work on June 08, 2012, 08:23:49 pmYes, of course I will plan it, as the layout is important. I will probably use thick solder tracks to enhance current capacity and reduce inductance of traces. I have designed a few buck converters before but only on PCBs, and only up to around 500mA. The converter switches at around 60kHz so the layout isn't as crucial as the 500kHz one I designed... which actually worked.

author: mariush
author_rank: Super Contributor
timestamp: June 08, 2012, 08:53:30 pm
content: You might get by with a simple linear regulator... a lm2941 for example has 0.5v dropout at 1A :http://uk.farnell.com/national-semiconductor/lm2941t-nopb/ic-reg-ldo-adj-1a-to220/dp/1564645No worries about inductors, emi just a couple of capacitors and resistors. I don't think the LCD would use even half of that Amp of power on 3.3v. And from 5v down to 3.3, the heat output would be easy to manage.

author: tom66
author_rank: Super Contributor
timestamp: June 08, 2012, 09:17:23 pm
content: Quote from: mariush on June 08, 2012, 08:53:30 pmI think it would be okay with a linear regulator, even an LM317T will do 3.3V from 5V in at 1 amp out, but the PSU is rated at 5 amps out. I'm going to try it with 3 amps and see if that shows problems. The 3.3V provides power only for the remote control input, side button processor and the audio DSP - all three don't work - surprisingly the rest works, even has a perfect picture!

author: digsys
author_rank: Unknown
timestamp: June 08, 2012, 11:20:19 pm
content: I would seriously consider a drop-in DC-DC module, just 3 pins. You can get them from 1W-6W+All done for you. And they're quite small. For 0.5A OP, cheap as $3-4

author: T4P
author_rank: Super Contributor
timestamp: June 09, 2012, 03:27:51 am
content: Quote from: digsys on June 08, 2012, 11:20:19 pmActually no, even the cheapest ones from Recom costs like 12SGD :\

author: digsys
author_rank: Unknown
timestamp: June 09, 2012, 05:56:13 am
content: QuoteThis is from Farnell in OZ, not the cheapest supplier by far. The $3 price is the 100X price, and only for a smallrange, hence "from". 1W upwards DO become dearer, but if you keep an eye on places like X-ON, sometimesyou can pick up a great deal. I use heaps of these suckers, all brands and types.

author: T4P
author_rank: Super Contributor
timestamp: June 09, 2012, 09:50:11 am
content: Oops, i just found it i thought you were talking about 1amp versions but of course i only have Element14 here and RS