MC34063 low voltage idea - any problems? - Page 1
Forum section: electronics / projects

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 02:16:30 am
content: First, for the "that IC's shit" brigade - yep, it is, at least for this purpose. This is for a quick throw-together one off project I want to finish tomorrow, I'm not going to try to source another chip for it.So, I need 5V from 2-3V-ish (two AA cells), at around 100mA peak. The only reasonable boost controller I have on hand is MC34063, which specifies a minimum voltage of 3V. (I'm away from the workbench now, so I haven't tested this. The SPICE model I have operates down to 2.2V or so. I certainly don't want to push it down to 2V, even if it works right now, under the right phase of the moon and the right temperature...)The project I'm building has an MCU (ATmega168P), which runs down to 1.8V, so that could run right off the battery, even with a Schottky diode drop in series. Here's what I came up with (schematic attached):The MC34063 is powered off theoutput, with a fairly heavy RC filter to prevent noise from feeding back in and making it unstable. Even if it's not running, the output will be at least 1.8V. The microcontroller will twiddle Q3 and Q? (yeah, I added that after annotating the schematic) to run the boost converter for a bit. When U1 starts operating, the drive transistor will take current through R3, triggering Q2 to interrupt the MCU and make it stop, so U1 can take over.Does anybody see any problems with this? Like I said, I haven't done any tests; I'm not at home so I can't breadboard anything.

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 02:34:19 am
content: LTspice is showing the MC34063 failing to take over if there is a heavy load attached from the beginning (the voltage drops back off before it gets going), so add to that schematic either 1) Another MOSFET on the output to switch the load off until the voltage reaches regulation, or 2) an MCU connection to the MC34063's feedback pin, to drive it high and hold off startup until the bootstrap transistors can charge up the output capacitor a bit more.Edit: #1.Edit 2: Or... the MCU just not enabling the load yet. I'm forgetting this doesn't have to be a "generic" power supply...

author: BravoV
author_rank: Super Contributor
timestamp: May 28, 2013, 03:16:07 am
content: OOT, is that the new LTSpice ? The schematic symbols look different.

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 03:17:27 am
content: That's KiCad with a custom library. My LTspice file was littered with ugly equations hanging off behavioral sources to model the behavior of the MCU, so I drew it up quickly in KiCad.

author: C
author_rank: Super Contributor
timestamp: May 28, 2013, 03:39:33 am
content: Probably a dumb idea.you say the mpu can work at 1.8, so what if you just had the mpu do a cap boost in to C2 and not the whole load?You could keep the boost going while the switcher starts.C

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 03:44:38 am
content: Quote from: C on May 28, 2013, 03:39:33 amYes, that should work just fine. Thanks!Seems like I've just been full to the brim with stupidly overcomplicated ideas lately...

author: Harvs
author_rank: Super Contributor
timestamp: May 28, 2013, 03:53:34 am
content: This is probably irrelevant but.. If the 1.8V is important to you, you may want to search around this forum a bit on the ATmega. I just remember someone complaining quite vigorously about Atmel's low voltage specs. Something along the lines of the low voltage limit is a hard limit, and you can't actually expect to run them off a rail at that voltage (i.e. they can fall over if the rail drops to 1.7V.)

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 04:00:41 am
content: I just need it to run off two low-ish alkaline AA cells minus a 1N5819 drop so it can bootstrap the boost converter. Assuming 0.4V drop, that's 1.1V per cell. A bit high, but not too bad.I'll do a bit of breadboarding tomorrow and see how low of a voltage it can start under. I can always use a transistor multivibrator to run the charge pump instead. Hell, I can just rip out the 34063 and implement my own boost converter in transistors if I have to. Doubt I'd have to do that, though!I've never used them under 2.5V, so I didn't know for sure, but I did assume it was an "absolute minimum". I really don't care if I have to throw the batteries away a bit soon because the MCU won't run on them.Thank you!

author: C
author_rank: Super Contributor
timestamp: May 28, 2013, 04:35:45 am
content: I saw a circuit some place and can not remember the full details.The mpu did all the power control and regulation for it's self and rest of circuit.the basics I remember was1 meg or higher resistor charged the mpu's power supply cap which was separate from the rest of the circuit.On power up if it happened. The mpu did the minimum to start shaking a pin to boost the mpu supply.The first stage boot code was shake pin for ___ time.it then setup the adc and started, shake pin for ___ time, read mpu voltage via adc and repeat until __ volts.The whole startup was min power use until more power was available.the circuit was such that if it could start, it could keep going down to some lower voltage.I kick my self for not saving a copy.C

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 04:46:35 am
content: Actually, what I failed to mention (sorry!) is that there is already a charge pump tripler running off the 5V rail, controlled by the MCU. The solution was so obvious! I can just give it its own oscillator (instead of MCU control, just in case the power input is shy of the 1.8V minimum), then use it to bootstrap the MC34063.Sorry for wasting everyone's time. I am having one hell of a "stupid day"

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 03:22:53 pm
content: Looks like no matter how I bootstrap the MC34063, it just does not have the drive capability to directly boost 2V to 5V with any sort of output current. It's either going to need an external transistor, or to just be replaced.

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 06:13:55 pm
content: Yes, I have. What I don't like is that hard 1.8V limit - throw in a diode drop and it's cutting out a bit early. I've got a standalone converter working right now on the bench with six transistors and a TL431 - I'm happy with that. The output power is a bit weak, which I'm trying to tweak now (the drive transistors just aren't pulling enough peak current), but it runs directly on 1.7-ish (that's before the diode drop) without any charge pump shenanigans.I also don't really like the idea of an MCU-driven feedback loop because I don't want one little code screwup to be able to blast all the digital electronics. A boost converter that can do 5V at 100mA can do a good bit more than that at low loads. If it's analog I don't risk breaking it every time I make a silly code change at 2AM. It'd be fine if the MCU wereonlyrunning a boost converter, but when I'm actively writing and debugging other code on it I don't want to mess with that.

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 07:28:18 pm
content: Well, this isn't going to happen today. I just don't have anything that'll pull the inductor low enough. I don't haveanyN-channel MOSFETs with a threshold around 1.8V (though I have two different P-channel ones - I'm thinking maybe I got the wrong type at some point...), and no NPN transistors with a VCE(sat)low enough to do anything useful either. Obviously a P-channel wouldn't be much good without a negative gate drive. I think I'll stock up on both a couple boost converter ICs that work at this voltage, and N MOSFETs with very low thresholds...I suppose I could use a separate low-output charge pump to give a gate drive for a 2N7002, but that's getting stupidly complicated again. I'll just put aside my "this needs to be done today" ambition and go place an order with Mouser...This sure is a big day for "well, it worked in LTspice!"....I suppose since it works so damn well in the simulator I'll keep poking at it for a while.