None preferred values in buck converter circuit question. - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 20, 2013, 07:04:41 pm
content: I am building a 5 volt bias supply for a solid state RF linear amp. It needs to drop the 48v supply to 5v at about 3 amps. I have an LT1074 buck converter, but Linear's suggested schematic has some odd value components, like 200uF and 500uF electrolytics. I realize the divider resistors need to be exact, but how critical can the electrolytics be? Thanks. The schematic is athttp://www.chriswilson.tv/lt1074.jpgif it is not attached in a clear way.The full spec .pdf is athttp://cds.linear.com/docs/en/datasheet/1074fds.pdf

author: wraper
author_rank: Unknown
timestamp: October 20, 2013, 07:13:36 pm
content: Capacitance is not critical. With bigger capacitance and lower ESR you will get less ripple.

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 21, 2013, 06:19:07 am
content: Thanks for looking and explaining. Just out of curiosity, why would they suggest are values of capacitance rather than common or garden ones? Thanks again.

author: Paul Price
author_rank: Super Contributor
timestamp: October 23, 2013, 01:59:22 pm
content: Cut and try..start with a quite small filter cap and make then thee bigger to decide what flavor of ripple you find acceptable. A scope will tell all.

author: Dave
author_rank: Super Contributor
timestamp: October 23, 2013, 02:07:06 pm
content: Just use a 470µF cap. Most electrolytics are rated +/- 20% anyway.

author: Alex
author_rank: Regular Contributor
timestamp: October 23, 2013, 02:08:08 pm
content: Not critical enough to prevent you from choosing the closest standard value. Remember the large tolerance on many types. The type and quality of capacitor will have a more significant impact. Having said that, in one occasion the values shown are the summation of a bank of smaller caps.Alex.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 23, 2013, 02:11:03 pm
content: Linear always seem to use oddball values in their datasheets & appnotes - maybe their dev dept has a massive E96 resistor kit...

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 23, 2013, 02:41:36 pm
content: OK, I will either parallel up two physically smaller value caps or if space permits use some physically large 470 uF 450V electrolytics I have knocking about. I am now wondering about heat. Would a smallish heat sink suffice for the regulator, and none at all for the fast switching, low on time Schottky? Cheers.

author: Paul Price
author_rank: Super Contributor
timestamp: October 23, 2013, 03:08:54 pm
content: Use your digital wattmeter (fingers) to determine heatsink requirements. Start small and test the waters.

author: wraper
author_rank: Unknown
timestamp: October 23, 2013, 03:11:58 pm
content: Quote from: Chris Wilson on October 23, 2013, 02:41:36 pm

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 23, 2013, 03:23:22 pm
content: Quote from: wraper on October 23, 2013, 03:11:58 pmHmnm, maybe not then, but I am not known for buying stuff needlessly, and they were in the spares box...... Sorry for hurt sensibilities

author: wraper
author_rank: Unknown
timestamp: October 23, 2013, 04:15:11 pm
content: I would use 1000+ uF 16V or 10V, low esr type on the output. That also depends of how much load you are going to connect. With higher load larger capacitor must be used to maintain acceptable ripple and capacitor life.