Tips to bypass buck converter / help picking a switcher? - Page 1
Forum section: electronics / projects

author: sporadic
author_rank: Regular Contributor
timestamp: December 31, 2015, 04:36:17 pm
content: I'm working on a project where I need 5v and 3.3v rails, ~400ma total peak. Input voltage however could be anywhere from 5v to 24v. Size is a concern, so I'm looking for single inductor solutions with high switching speeds. I was planning to keep it simple with a LDO off the 5v rail for the 3.3v rail. I've seen buck converters from TI and a few others that offer built-in bypasses, but they're all geared for mobile device usage with very low max Vin. A buck/boost topology seems overkill for what I need and haven't found anything that meets the spec aside from MC34063 based designs, and calculations started falling apart on me with my specs.I was looking at theMPS MP2456for my buck converter as it seems to fit the bill. The datasheet states a Vout of .81V to .9xVin. With a feedback voltage of .812v, I wouldn't be able to get 4.5v from 5v Vin would I?? If so, the 5v rail could drop 4.5 and still work in my design, just need to pick the proper LDO for the 3.3v rail.

author: ConKbot
author_rank: Super Contributor
timestamp: December 31, 2015, 05:41:03 pm
content: What are your current requirements for each rail? Could you buck to 3.3 and boost to 5v from that? Good boost converters can be 85-90% efficient (Ive measured a TPS63020 at 92% efficient at 3.3V in, 600mA @ 5V out, not just data sheet wank). So depending on how the current requirements are divided up you could be just as efficient as bucking to 5v and an LDO to 3.3

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 31, 2015, 05:59:38 pm
content: Setting the duty cycle of the buck switch to 100% will accomplish what you want, but there will still be some drop across the switch and you can't use the typical bootstrap charge pump for gate drive (either a PNP or p-MOSFET and inverted gate drive). Once you factor in those gotchas it becomes clear that you will need to pick a different topology. Most popular for this application would be a flyback - either the single winding inductor non-isolated kind, which inverts polarity, or the muti-winding inductor ("transformer") kind which can be isolated or not and which can invert polarity or not.The flyback can look a little daunting to design at first (what's all this Right Half Plane Zero stuff!?!?) but trust me when I say it won't be harder to get working than a "simple" buck that has to operate all the way up to 100% duty.

author: prasimix
author_rank: Unknown
timestamp: December 31, 2015, 06:40:08 pm
content: At least TI and LTC have buck controllers that drives PMOS that can provide 100% without charging pump. If that is an option check forLM5085,LTC3864orLTC3824. The LTC3864 will be my first choice.

author: sporadic
author_rank: Regular Contributor
timestamp: December 31, 2015, 06:45:02 pm
content: Quote from: ConKbot on December 31, 2015, 05:41:03 pmThe bulk of the current is on the 3.3v rail powering an RF module. The 5v rail is driving a comms buffer for an external component that doesn't like 3.3v logic. I'll look into it, but afraid size may be an issue. Thanks!Quote from: MagicSmoker on December 31, 2015, 05:59:38 pmI'm afraid I wouldn't have the room for a flyback. I may end up going back to making this a configuration option via jumper - "5V" / "8-24V" or something. Just trying to fool-proof it so the magic smoke doesn't get let out so easily.

author: sporadic
author_rank: Regular Contributor
timestamp: December 31, 2015, 06:46:52 pm
content: Quote from: prasimix on December 31, 2015, 06:40:08 pmI'll check these out, thanks!!

author: DanielS
author_rank: Frequent Contributor
timestamp: December 31, 2015, 07:50:58 pm
content: Quote from: sporadic on December 31, 2015, 06:45:02 pmWhy? If you used a semi-isolated flyback (transformer-coupled for power to avoid the inverting flyback, direct-coupled for feedback), the component count is the same except for the secondary winding which may require a slightly larger core. Simpler than SEPIC (two fewer capacitors) and it gives you whatever buck-boost range you might want within reason. You can even have two taps on the secondary to provide group-regulated 5V and 3.3V: if either one voltage is non-critical and the minimum load on both is high enough to dissipate energy from stray inductance, use the critical output for feedback and the non-critical one should remain reasonably close to its nominal value.

author: jt
author_rank: Regular Contributor
timestamp: December 31, 2015, 11:08:48 pm
content: The smallest solution I found is 3x2mm and switches at 1MHz. It integrates the power FETs so the only external components needed are a few RC passives and a smallish power inductor.http://www.ti.com/lit/ds/symlink/tps62175.pdfIt is specified for 100% operation (see page 12); and your use-case is shown in the system design example in section 9.3.3.1 (top of page 25).

author: ConKbot
author_rank: Super Contributor
timestamp: December 31, 2015, 11:43:47 pm
content: Quote from: sporadic on December 31, 2015, 06:45:02 pmSounds pretty low power? You can get ~100mA charge pumps in a 2x2 package, plus a few (3 usually) 0402 ceramics, and you have your 5V rail.

author: sporadic
author_rank: Regular Contributor
timestamp: January 04, 2016, 02:29:05 pm
content: Quote from: jt on December 31, 2015, 11:08:48 pmI think this may just fit the bill, thanks much!!

author: jt
author_rank: Regular Contributor
timestamp: January 04, 2016, 08:16:44 pm
content: Great, glad to be of help!