One buck question... easy low voltage protection mod to buck converter? - Page 1
Forum section: electronics / beginners

author: EEVPiobee
author_rank: Contributor
timestamp: October 11, 2014, 07:40:43 am
content: Hello there...I just bought couple of these:http://www.aliexpress.com/item/DC-DC-Buck-Converter-Step-Down-Module-LM2596-Power-Supply-Output-1-23V-30V-YH-S/1991882325.htmlAnd judging by the schematics, the LM2596 has an ON/OFF pin (no 5). Since I will most likely be powering this on Li-Po battery, is there an easy way I could integrate a low input voltage cutoff on this circuit directly using the lm2596 (cost is not critical, but the least parts, the best)Note that I post in the newbie section for a reason...

author: Simon
author_rank: Unknown
timestamp: October 11, 2014, 07:47:14 am
content: read the datasheet, it will tell you the voltage that activates the chip but the looks of it it needs to be connected to 0V to turn on so you need a comparator circuit to put out 0V if the input is good and whatever voltage if the voltage is low so as to turn the regulator off.

author: EEVPiobee
author_rank: Contributor
timestamp: October 11, 2014, 03:53:41 pm
content: So, according to data sheet:http://www.ti.com/lit/ds/symlink/lm2596.pdfIt'll need > +2.0 volts to turn it off, and < +0.5v to turn it ON on Pin 5Pin 5 can tolerate up to +25v, so it ain't a problem with the range I'll be working with, but for safety sake (or dumb end user sake) I'll put some regulator on it...Would that be correct then? (sorry for bad drawing, I really made it fast)Would you absolutely recommend some capacitor at the output of the LM7805 ?

author: Simon
author_rank: Unknown
timestamp: October 11, 2014, 03:58:28 pm
content: What is your input voltage ? many comparators will work up to 36V, depending on what you select as you undervoltage you won't be able to run the comparator at a lower voltage. for example if your using 24v input and want it to cut out at 20V but run the comparator off 12V yyou may have a problem, or you have to use voltage dividers on both inputs to rescale 0-24 to 0-12.Many comparators are open collector so you use a pullup resistor on the output and the most the output will saturate to when the comparator switches to ground is 0.2V

author: EEVPiobee
author_rank: Contributor
timestamp: October 11, 2014, 04:47:28 pm
content: I plan to use 3S Li-Po batteries (referenced as 11.1v) but the actual range is from 12v (fully charged) down to 10v (discharged)--so 10v would be Vmin... any less would shut down the circuit to prevent damage...On the previous schematics I sent, I use Voltage dividers on the bottom so the LM78xx used always has plenty of "headroom" to operate, preventing any case where a voltage drop across the LM78xx in it would create an "always on" situation(for example let's say V+ is not high enough for the regulator value and regulator outputs 0.2v less than V+ Instead of the correct value...)As for the pullup resistor: Is this better?

author: Simon
author_rank: Unknown
timestamp: October 11, 2014, 05:01:17 pm
content: you don't need a regulator for the comparator, a LM339 type will work on at least 30V, it has an open collector type output. The regulator will use power, the opamp will use a few uA

author: EEVPiobee
author_rank: Contributor
timestamp: October 11, 2014, 05:27:12 pm
content: But if I don't use a regulator,How can I get a -fixed- reference value that will never change?Otherwise, that would always equals comparing v+ to v+Zener has its range limitations whereas a regulator is more flexible on that point

author: Simon
author_rank: Unknown
timestamp: October 11, 2014, 05:32:21 pm
content: Thats true, you can use a zener and a potential divider from the input, you set the potential divider up so that at your "drop out" voltage the voltage on the output from the potential divider drops just under the zener voltage, so say you use a 5.1V zener you setup a potential divider that puts out 5.1V when the input is 10V or whatever you want it to be.Of course the zener will use as much as the regulator, it's up to you. Or if it's a battery application that needs to conserve power you can use a voltage reference that will use a small amount of power.

author: EEVPiobee
author_rank: Contributor
timestamp: October 11, 2014, 05:52:53 pm
content: Gotta find a better device than the LM78xx then (at least one than draw a lot less). That it's only for reference anyway...

author: Simon
author_rank: Unknown
timestamp: October 11, 2014, 05:56:03 pm
content: There are voltage references that will draw uA

author: EEVPiobee
author_rank: Contributor
timestamp: October 13, 2014, 08:00:18 pm
content: Seems only LDO regulators will do for this job?? (That's all I'm finding right now) Or you may have one (fixed value) to suggest?(I can always trim resistors values of my bottom voltage divider to fine tune my cut-off input voltage, or put a multi-turn potentiometer)

author: kxenos
author_rank: Frequent Contributor
timestamp: October 13, 2014, 09:24:32 pm
content: Here is a little circuit that will do the job for you. It's a non-inverting comparator with hysteresis.The output will be low for Vin > 2.4(1+(R2/R1)).You will need to select R1 and R2 so that (R1*R2)/(R1+R2) is roughly equal to 10k (the value of R3).The hysteresis will be about 1% of Vin.

author: Simon
author_rank: Unknown
timestamp: October 14, 2014, 07:00:21 am
content: Quote from: EEVPiobee on October 13, 2014, 08:00:18 pmWhy ? basically you need a voltage reference that won't drain your battery, having said that depending on your overall current draw if you switch off power to your undervoltage circuit as well and it's consumption is negligible compared to your load you will avoid it discharging the battery further and won't need hysteresis as once it cuts out it cuts out the only way to put it back on would be with a bypass switch to "reboot" it

author: salbayeng
author_rank: Frequent Contributor
timestamp: October 14, 2014, 08:12:51 am
content: An alternative approach would be to use a TL431 + 2 resistors ,the "reference" terminal will pull the "cathode" = "positive terminal" low when it is above 2.5v ,so 2 resistors e.g. 7.5k and 2.5k will make it switch at 10v .The problem with this approach , and many of the simple comparators, is the circuit will likely turn back on again as the battery recovers its voltage, so the 7596 will go on-off-on-off . The 7596 on/off input has a vaguely defined threshold and no hysteresis.You can add some positive feedback to make it stay switched off, for example by coupling a larger resistor from the output to the reference node, just waving my arms around, I would say 75k (being 10 times the 7.5k , should give 10% hysteresis or 1v in 12v) (150k for 0.5v hysteresis) , this approach might work with the comparator approach too. You will probably need a pull up resistor , the circuit layout would be similar to fig34 in the datasheet. (The TL431 behaves like a NPN transistor/MOSFET but with a 2.5000V threshold )The fig34 circuit would do what you want anyway, so what's the problem with that? , you can add some hysteresis by attaching a 100k resistor from output to the top of the zener (220k for smaller hysteresis). Getting the zener right for 10v cutoff would be tricky , possibly 9.1V? (The big advantage of the TL431, is you use a trimpot to make an adjustable zener).Have a look at the TL431 it's a pretty cool part!.

author: Seekonk
author_rank: Super Contributor
timestamp: October 14, 2014, 06:03:40 pm
content: I've used that pin to turn on and off that regulator quite fast and operate a solar panel at near the power point. It works quite well as a regulator for a small wattage solar panel.

author: EEVPiobee
author_rank: Contributor
timestamp: December 28, 2014, 07:36:09 pm
content: Quote from: Simon on October 14, 2014, 07:00:21 amWell, mostly for sake of cost ... They work really well (I've successfully assembeled the circuit with a TPS7250 and an LM393P) but i can't really find an LDO under 1.40 USD each...Plus, according to the datasheet, I'm overdriving the TPS7250 by feeding it 12v instead of its maximum recommended 10v

author: Christopher
author_rank: Frequent Contributor
timestamp: December 29, 2014, 12:40:08 am
content: Can you not use a battery protection icYou will still get quiescent current thru these extra chips wjen the battery is under voltage

author: salbayeng
author_rank: Frequent Contributor
timestamp: November 22, 2016, 10:34:04 am
content: There are some SMPS converters that require a positive voltage on the enable, and some even it label it "UV" pin, and it has hysteresis.Just saw them a few weeks back. Maybe they were used for LED's?You just need a voltage divider with some megohm resistors and thats it.

author: Seekonk
author_rank: Super Contributor
timestamp: November 22, 2016, 12:56:59 pm
content: I do exactly this in solar to operate solar panels at power point. From my experience this pin is really fast. I just use a NPN transistor with a resistor pullup. If more than 25V on input, use a second resistor to form a divider. In the input I use a zener and resistor to limit current, it doesn't take much drive current to the base. An additional pot makes it adjustable

author: salbayeng
author_rank: Frequent Contributor
timestamp: November 27, 2016, 11:12:31 pm
content: OK, I've just designed this part into a circuit.TPS54202 28v LDO, sync buck 500kHz , 2A out , sot23-6. $1.30 in qtyAlso there is TPS54302 same but 3A $1.50 in qty.These are extremely efficient, even at low load, so you will get an extra 10-15% out of your battery, and it will be smaller and cooler design. You typically use an 8x8mm or 10x10mm inductor with these. And it works with ceramic capacitors, so about half the size of the module you are looking at. See alsohttps://www.pololu.com/category/131/step-down-voltage-regulatorsThe enable input has hyster esis , and switches at 2.19 / 2.22 volts, and the data sheet shows how to use this for UVLO. The standby current is miniscule, so the undervolt battery drain is determined by the UVLO divider so ~10uA?I'm using this in conjunction with a CD4013 to make a LED torch running off a 9v battery , the UVLO features means the torch will flash when the battery is low. The cost for the TPS54202 is 30c more than I targeted, but I needed another 5v supply elsewhere in the box, so rather than add an odd part to the Bom, I just doubled up .