Can a buck input UVLO be used for hi-speed hysteresis? - Page 1
Forum section: electronics / general

author: Faranight
author_rank: Unknown
timestamp: October 13, 2022, 10:27:11 am
content: Hello, a bit of a dilemma here.I'm designing a small PSU for a project of mine, so I'm looking for a buck PMIC withinputvoltage regulation/hysteresis. The goal is to charge a supercap from ahigh impedance power source, and the input voltage should be regulated near a preset value i.e. by adjusting the current draw accordingly. I noticed many people suggest to use an IC with adjustableUVLO(undervoltage lockout) function.I just want to confirm something as I browse various PMIC datasheets. It is claimed that the UVLO function serves to disable the PMIC's when the voltage on the VIN (UVLO) pin falls below a certain threshold. But as soon as the voltage on that pin rises above this value (plus some margin) then the IC will restart and go through a powerup cycle (soft-start or whatever its called). Now suppose I want to use this function to regulate the input voltage. I would need to have the PMIC turn on and off very fast (depending on the size of the input capacitor) as the input voltage oscillates around the preset value.My question is:Can I expect any negative effects (such as reduced efficiency) when the UVLO function (with things like soft-start) is used to turn the PMIC on and off with very high frequency (think a few kHz)?Thanks in advance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2022, 01:33:50 pm
content: That's fine, but keep in mind:1. You need to limit the minimum input resistance. The regulator needs to be able to pull down its input, to be used this way. (If it's okay to run full-throttle, say because of its own internal current limiting, when ample supply is available, then that's fine, too.)1a. Corollary, you will spend at least as much power in that resistance, as the hysteresis band of the input. This limits maximum efficiency.1b. And additionally, you can only operate at input voltages where this is true. So instead of say a 5-30V range the regulator might be capable of, you can only use, say... the 7.2-8.4V range of a 2S lithium battery, for example? (Again, if this mechanism is required for limiting purposes.)2. Regulation is on-off hysteretic, so expect high ripple obviously, and any additional efficiency loss due to that, or the frequent restarting. The regulator itself should be fine, but the fact that it's always going 0/100% throttle, say, means it's running much harder than if it were doing a smooth 20-50%, say. Again, this impacts maximum efficiency.3. Don't expect as much operating range. That is, if it's a 1A regulator, but it's spending half its time starting up (internally programmed ramp?), you aren't going to get 1A average.The biggest problem I suppose, is if you aren't battery operated (which effectively internally covers this aspect), you will need a large capacitance to program the ramp amplitude/rate of #1. Which even if it's going at some ~kHz, will need to be a pretty big capacitor. (A battery typically has considerable capacitance, so this may come for free internally in that case, as I said. You will still want to check that ESR and C are in useful ranges, so it's not being pulled down to dangerous levels before it realizes it should shut off, or so that it can draw useful power before shutting off.)Consider choosing an externally compensated type regulator. Typically, gm-amp types (i.e., the COMP pin uses an R+C to GND, as opposed to a feedback network between two pins) expose the error amp output node, which can be overdriven by an external error amp which can servo to input voltage, or be wired-OR with diodes, to the same end (in which case you can regulate output as normal, while also regulating input voltage or current).If the internal error amp is voltage-mode (R+C compensation between COMP and VFB pins), it can be strapped as a unity-gain inverter with a pair of resistors, and then driven as a control voltage (you'll then have to solve the output voltage regulation separately). Often, the COMP pin can also be overdriven (internal circuitry normally reads the COMP pin voltage, not an internal node separated from it), but its output current range is often poorly specified, if at all, so I don't recommend this. (For example, TL494 and UC3843 have COMP pins capable of fractional to just a couple mA, so are fairly easy to overdrive by an external op-amp; both however have internal error amps that are easily wired as unity-gain inverter (or follower(s), in TL494's case), making this unnecessary for the most part.)Tim

author: Faranight
author_rank: Unknown
timestamp: October 14, 2022, 08:05:52 am
content: Thank you for the comprehensive answer.The high impedance input source I was talking about is actually a small 18Voc 300 mAscsolar panel, and the input voltage limitation serves to set it near the panel's maximum power point. I wasn't sure, if I should originally mention this or not. Usually when I ask a question like this and mention "solar panel" people will automatically reply with "you need a MPPT IC" while completely ignoring the requests listed in the original post.I'm opting for a simpler solution. Analog Devices call this solutionMPPC(maximum power point control) which tries to fix the input voltage around a preset value. This value is chosen close to the panel's voltage at its maximum power point since this voltage changes little with illumination. A hysteretic buck regulator was suggested to me at some point, but I wasn't sure, if UVLO should be used to control the hysteresis. Thanks for suggesting the external loop compensation. I do admit that my knowledge in analog design is rather lacking, but I also agree that controlling the pulses directly is a better way than having the regulator periodically turn on and off.Do you perhaps have a reference design or some schematics for these externally compensated regulators I could learn from?Thanks in advance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2022, 09:01:28 am
content: Yeah I don't care about that, CV is close to MPP as you've noticed. Maybe worst at low light, but you aren't getting much power under low light anyway so maybe who cares.Of course, if you wanted to run an MPPT algorithm, such can be married to a converter in exactly this way, without having to do full control all-in-one.Don't have any references I'm afraid.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: October 15, 2022, 10:31:59 pm
content: You need a buck converter with external error amp.....and make it regulate the Vin to your wanted vin.So if you think of the usual buck....which regulates vout......just make it regulate vin instead....ie, have the divider off vin to the error amp.You can put an output current clamp on aswell if you want.And an output voltage clamp aswell if you want.Please find LTspice

author: timenutgoblin
author_rank: Regular Contributor
timestamp: October 19, 2022, 01:22:48 am
content: Just posting this here for the O.P.This is a YouTube video on The Signal Path channel (TSP #21 - Tutorial and Experiments on Energy Harvesting ICs) demonstrating how a low-powered solar panel and a TEG (Thermo-Electric Generator) can be used to charge up a supercapacitor which can then be used to power a smartphone using DC-DC step-up/boost converter modules.https://youtu.be/lrELRVFEx_s