MC34063 - Stucks with too much parallel devices - Page 1
Forum section: electronics / beginners

author: Yaroooo
author_rank: Regular Contributor
timestamp: May 29, 2019, 09:54:00 am
content: I've a circuit schematic:I power my circuit using only PWR+ and PWR- at 24V.When my power supply powers only one circuit or two, in parallel mode, I don't have any problem.When I attach more like 3 circuits, sometimes at power ON all this circuits doesn't goes on.I've measured voltage on PWR+ and PWR- when they stucks and all of them have 4.8V between PWR+ and PWR-. I've removed FUSE, D3 and V4 but nothing. Also reduced 100uF to 2.2uF and nothing.I've supposed that if the voltage is too slow to rise on this device, it may stuck at certain point with a voltage at 4.8V (and output at 2v).How I can solve or made immune this device to this behavior?Also I can't understand how this device reduce my Vin from 24v to 4.8v, since I've 24V on my power supply instrumentation.

author: mikerj
author_rank: Super Contributor
timestamp: May 29, 2019, 11:48:49 am
content: Where does you 24v supply come from? It sounds like it has foldback current limiting which is causing your problem.

author: mariush
author_rank: Super Contributor
timestamp: May 29, 2019, 12:15:59 pm
content: Any particular reason you don't add some decoupling capacitors (0.1uF ceramic near the Vcc and GND pins at the very minimum on each IC)?You should check the voltage ripple on the output of your MC34063... may be surprised to see +/- 0.1-0.2v fluctuations ... and you could have less than 4.5v on output which may cause some chips to reset due to too low voltage.The LT1615 should benefit from a 4.7-10uF ceramic close to its input.Also, careful about output capacitor on the LDO... your schematic shows a 4.7uF 16v electrolytic ... keep in mind the datasheet recommends less than 1 ohm ESR and not all electrolytic capacitors have ESR below 1 ohm at such capacitance/voltage combo. I'd suggest going with ceramic capacitor on output, or using a 47-100uF 25-50v electrolytic.You could do much better than MC34063 if you're designing this yourself ... for example seeAP63201(500kHz sync , 32v max input, 2A) :https://www.diodes.com/assets/Datasheets/AP63200-AP63201-AP63203-AP63205.pdfIt's more expensive in small quantities, but you're saving money on the other parts (fewer passives, less pcb space used, no diode, smaller and cheaper inductor, higher efficiency - above 85% at more than 0.1A output - )

author: Yaroooo
author_rank: Regular Contributor
timestamp: May 30, 2019, 08:10:33 am
content: Quote from: mikerj on May 29, 2019, 11:48:49 amFrom a laboratory power supply. I have, with a single circuit, about 0.03A at 24V.I've also through about this, by instrumentation doesn't say nothing. Where voltage from power supply is 24V. Measured on Circuit input is 4.8V on all of them. That is strange.

author: Yaroooo
author_rank: Regular Contributor
timestamp: May 30, 2019, 08:35:20 am
content: Quote from: mariush on May 29, 2019, 12:15:59 pmUnfortunately this is not my circuit. Yes, several things can be improved, but it's not something related to me. As said, what is strange to me is that voltage between PWR+ and PWR- stucks at 4.8v, while I've 24V in parallel between this 3 devices. I'm sure 99.9% that it's something related to this regulator, but don't know why.

author: iMo
author_rank: Super Contributor
timestamp: May 30, 2019, 10:41:16 am
content: What is the current limit set on your lab PSU (the 24V one)?