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[SOLVED] TPS73801 enable pin voltage requirements for 6V input voltage

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d123

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Hi,

In the below circuit, after reviewing the TPS73801 regulator datasheet, again, I am having doubts as to whether the enable pin operates (on/off) proportional to input voltage, or will function with 2V to 3V with a 6V input to the regulator. I have no problem with upping the battery voltage to 5V, if necessary, but beyond that the OPA376 may not be too happy, as max. operating voltage is 7V, and would possibly need to consider an alternative higher voltage op amp. If it says 7V absolute max. could 6V be an acceptable operating voltage for it, as it has to supply virtually no current whatsoever to the regulator input pin (1 to 30uA)?

Also, is the only simple way to forego the battery and power the reference and op amp from the panels directly by doing so with a 3V Zener - to ensure they are powered up before the regulator, even if only by microseconds? Or is there any other method?

This circuit is curiosity/theory, so if it's a needlessly complex design to turn on an LDO when input is over a specific minimum, or unworkable in some way I'd appreciate input, thanks.

pv and regulator schematic values.JPG
 

I believe the datasheet is pretty clear when specifying 2V max EN threshold, independent of the input voltage.

Your comparator circuit has wrong polarity, disabling the LDO above a certain input voltage. Finally there are power supply supervisory ICs, implementing reference and comparator in a single package.

By the way, what's the exact reason to disable the LDO?
 
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    d123

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Hi,

Thanks, that clears up two out of four questions, great.

Where in the datasheet does it say 2V max? I take it you mean the field that says EN High-Level Input Voltage 2V min, 20V max. That's where the doubt came from: 2V threshold for fully enabled or 2V starting threshold, and graphs tail off around 0.7V; I normally tie it to V+/same voltage level so never had to consider that before.

Oops, about the schematic, thanks for pointing it out, I copied layout from my initial rough plan drawing on paper, not what is the partial circuit on breadboard/in simulator... Couldn't simulate this specific LDO as it has no Spice/Tina model

Good call about supervisor ICs, thanks.

Reason for disabling LDO is a) for a circuit only used in daylight hours, and b) to ensure panels are producing over minimum needed 5.3V for LDO 5V output. ...I dimly suspect reason b) is unnecessary as panels are a basically on or off r.e. voltage, not slowly going from 0V at night to 6V by midday - that is so, isn't it?

Is it okay to push an op amp 1V beyond recommended operating condidtions for 30uA or so current, if that is still 1V below max. supply ratings?
 

Hi,

Where in the datasheet does it say 2V max?
--> 6.5 Electrical Characteristics.
Parameter: VEN Shutdown threshold

Klaus
 
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    d123

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--> 6.5 Electrical Characteristics.
Parameter: VEN Shutdown threshold

...Grrrrr... That'll teach me to skim read the specifications only looking for the words EN voltage in the middle of the parameter field... Thanks.

"Academic" question: What about the op amp - if it's recommended up to 5.5V supply, and absolute max. is 7V, then for hobby circuit and 30uA output 6V supply would be fine, would it?

Related, I looked at TL431 datasheet earlier for use as a comparator, I hadn't realised the output LOW is mid-supply/2V at 5V supply, so I can't use it for this.
 

TL431 has wrong polarity. LM385-ADJ or TPS3801 can be e.g. used.

Reason for disabling LDO is a) for a circuit only used in daylight hours, and b) to ensure panels are producing over minimum needed 5.3V for LDO 5V output. ...I dimly suspect reason b) is unnecessary as panels are a basically on or off r.e. voltage, not slowly going from 0V at night to 6V by midday - that is so, isn't it?

Depends on the kind of circuit if this makes sense at all. Consuming some power from the solar panel during night time doesn't actually wear it. If the 5V supplied circuit contains e.g. a microcontroller, it can well decide by itself to activate or deactivate circuit functions.
 
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    d123

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Hi,

Thanks for the device suggestions. Last night read a little about panels and Isc and Voc, and V related to temp... A few calculations to have clear first, it seems.

Regarding microcontroller, which I wouldn't use to learn about this kind of circuit at present, I'm guessing that overcurrent and overvoltage protection are both necessities for anything downstream of panels, and brown-out useful too.

I saw nice Lithium and Ni-Mh PCB mount coin cells (3V, 100mA, or a 3V 50mA) whilst looking for sensible solutions to battery backup - that would mean a battery charger IC too, but as I would like to get an idea of a simple solar circuit that would work I won't get ahead of myself yet because I'm seeing just the panels alone need a degree of thought and calculation to avoid excess current or exceeding voltages for downstream devices.

Thanks.

I know it seems like a stupid question about the op amp, because I know it shouldn't be done, and wouldn't do so myself unless it were an issue of e.g. survival post-apocalypse (joke), but I'm curious what people with many years of designing have to say on how unreliable operation could really be for an op amp 1V below max. ratings but 0.5 to 1V above recommended Vsupply rating.
 

I quoted a datasheet definition of the term "absolute maximum ratings" in a recent thread
These are stress ratings only and functional operation of the device at these or any other conditions above those indicated in the operation sections of the specifications is not implied. Exposure to Absolute Maximum Rating conditions for extended periods may affect device reliability.

Reading the definition strictly, you shouldn't do it. I would however ask in a first place what is limiting the supply voltage to 6V or whatever, how reliable is the voltage limiting. Also, referring to the "extended periods" point, how long is it applied?
 

Hi,

Okay... Let's say you had an ideal voltage source of 6V, extended period: used continuously, but the op amp current output would sporadically be 100uA/500mV - presumably that's actually unlikely to damage the part...then I'm guessing perhaps the issue of max. ratings is perhaps about not burning, or slowly cooking the the IC via excessive PD, which modifies/"misshapes" and weakens internal structures, so with the above conditions it could be done, but under very (unrealistic) controlled conditions, even so I still wouldn't do so.
 

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