running from 3v3?

Questions and discussion about The WiPy 1.0 board and CC3200 boards.
Target audience: Users with a WiPy 1.0 or CC3200 board.
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phsdv
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running from 3v3?

Post by phsdv » Thu Oct 08, 2015 9:23 am

Hi

I have a LED driver that I want to control via the Wipy to make a smart LED lamp. The driver has a 3.3 to 3.5V aux supply which I want to use to power the WiPy. If I connect this to the 3V3 pin and leave VIN open, will this work? The CC3200 should be fine with these voltages, however I do not know if the TPS79601 will like it, when a 3v3 is connected to its output and nothing on its input? Or would it be better to connect the supply to both the Vin and 3V3?

Paul

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danicampora
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Re: running from 3v3?

Post by danicampora » Thu Oct 08, 2015 9:43 am

Hi Paul,

Yes, that's perfectly fine, you can power the WiPy directly via the 3V3 pin, but make sure to leave VIN disconnected!

Cheers,
Daniel

phsdv
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Re: running from 3v3?

Post by phsdv » Fri Oct 09, 2015 8:14 am

HI Daniel,

Yes works indeed with 3V3 only. Now I need to learn the WiPy and get all the network, PWM and power savings working

thanks
Paul

Damien
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Re: running from 3v3?

Post by Damien » Fri Oct 09, 2015 9:46 am

I would have thought that it's ok (and preferable) to connect both VIN and 3V3 to your 3.3v source. This essentially disables the voltage regulator.

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danicampora
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Re: running from 3v3?

Post by danicampora » Fri Oct 09, 2015 10:37 am

Yes, Damien is right, connecting it to both Vin and 3v3 also works and might be preferable depending on the internals of the regulator. We really have to test both options to see if there's any difference. I have tried with 3v3 only and it works without issues, that's why I gave that answer.

phsdv
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Re: running from 3v3?

Post by phsdv » Fri Oct 09, 2015 11:08 am

To be sure I studied the datasheet:
The TPS796xx PMOS-pass transistor has a built-in back diode that conducts reverse current when the input
voltage drops below the output voltage (for example, during power-down). Current is conducted from the output
to the input and is not internally limited. If extended reverse voltage operation is anticipated, external limiting
might be appropriate.
Which means that when putting a voltage on the output and nothing on the input, there will be a voltage on the input too. To be sure, I measured Vin while powering via the 3V3. With 3.5V on the 3V3 pin I measure 3.06V on the Vin pin. This is above the threshold of the EN pin, so the LDO will be enabled (and consuming power).
I have connected my supply to both Vin and 3V3 pin and that works too. I could not measure any difference in power consumption, as the current was changing between 80 mA and 85 mA in both cases. I probably need to switch off the heartbeat LED first to get a more stable situation.

I also checked connecting the supply to VIN instead of 3V3. With both 3.5V in and 3.3V in this also works, with a slightly lower consumption, < 80 mA). So looks like the preferred method. This is called the Dropout Operation and has some limitations:
7.4.2 Dropout Operation
If the input voltage is lower than the nominal output voltage plus the specified dropout voltage, but all other
conditions are met for normal operation, the device operates in dropout mode. In this mode, the output voltage
tracks the input voltage. During this mode, the transient performance of the device becomes significantly
degraded because the pass device is in a triode state and no longer controls the current through the LDO. Line
or load transients in dropout can result in large output-voltage deviations.
However if you supply on the 3V3 pin you also have no regulation, so not a big issue

Conclusion:
1) best to connect the external supply to the VIN pin, even when it is only at 3.3V!
2) You can connect a 3.5 V supply to 3V3 only or to both 3V3 and Vin at the same time.
3) Never connect Vin to GND when supplying on the 3V3 pin!

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danicampora
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Re: running from 3v3?

Post by danicampora » Fri Oct 09, 2015 1:13 pm

Hi phsdv!

Thanks for the tests!

The regulator is very low power, Iq is on the uA range so, it will be hard to tell the difference. Anyway, you can disable heartbeat like this:

Code: Select all

from machine import HeartBeat
HeartBeat().disable()

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