Battery source and multiple rails: to cascade or not? - Page 1
Forum section: electronics / projects

author: bitbanger
author_rank: Regular Contributor
timestamp: December 25, 2016, 06:15:45 pm
content: Hi Folks -I have a 3S LiPo battery I'm planning on supplying a micro board: several rails needed, 5V, 3.3V, 1.8V reference. Each under 1A for sake of argument. The 3S will be supplying some high-current (30-40A peak) brushless motor pwm drivers.Is it common practice, battery or not, to cascade these supplies? In other words, 5V supplied from battery, 3.3V supplied from 5V, etc? If I fed directly off battery I would have to check regulator input range over operating range of battery... which means efficiency would be changing, etc...My immediate thought is that I would be improving noise immunity? Sequence of power up is of course a concern, but assuming this is OK as-is or handled with a BOR/power-on-reset controller, thoughts either way?Best,Jamie

author: KhronX
author_rank: Frequent Contributor
timestamp: December 25, 2016, 06:40:31 pm
content: Assuming both the 5v and 3.3v regulators are linear, you're still burning off the same amount of power. Separating or cascading them would only be determined by how you'd be willing and/or able to redistribute the power dissipations.For what it's worth, at least in audio equipment, i've frequently seen the 3.3v regulators fed from the 5v rail. Haven't really noticed any "rule" for the lower ones (2.5v, 1.8v, 1.25v) - either fed from the 3.3v regulator or the 5v rail.

author: georges80
author_rank: Frequent Contributor
timestamp: December 25, 2016, 08:30:11 pm
content: Given how inexpensive decent quality switcher chips can be purchased, I'd at the very least use a switcher to get down to 5V and then post regulate from there.How about specifying what the current (do a quick datasheet sum) for each voltage domain is (versus a guess)? Then you can decide whether to run several switchers or just one and post regulate the others.If you indeed are drawing ~1A from say the 1.8V rail, you hardly want to be going from 3S li-po down to 1.8V with a linear regulator...Of course this assumes you have the design/layout skills to use switchers versus linear regulators?cheers,george.

author: bitbanger
author_rank: Regular Contributor
timestamp: December 25, 2016, 09:05:30 pm
content: Hi -Thanks for the responses. I was thinking the same George, switcher down to 5V, then linear from there. Reference only (ADC) for the 1.8V. By quick tally, 3.3 practically speaking is only at 200mA, 5V round 500mA.Switcher layout is no problem. I would only be avoiding one for time sake (efficiency is not a concern at this stage).Otherwise any noise on the 3S supply really comes down to supply/ground routing practices I imagine? I have a battery current sense circuit and will be sourcing from that point, branch to motor controllers, another to logic. Thoughts on E/M radiation with this micro board about 30mm above these high current traces (simple 4oz copper, leading to drivers further away), maybe 4" in length? I'm limited to a 2layer board so ground stitching and careful layout are my only defenses so far. Just wondering if I need to go so far as considering under-board shielding. Speed controller are switching in the 20kHz region.Happy Holidays everyone.Jamie