+LTS/SOPINE Coin cell rapid discharge
#1
Hi people

For the (probably a minority) who need to use the RTC backup with a coin cell:

The RTC is powered from a 3V supply via a Schottky diode when the Pine64 or Sopine is powered. In theory, a coin cell should be able to back up the RTC when power is off so that a reasonable time reference is available even when there is no network.

Lithium coin cells when new are significantly above 3V, and depending on temperature and newness could be up to 3.3V. This means that if the coin cell is connected via a similar diode, then the RTC will be powered from the coin cell even if the Sopine is fully powered, until the coin cell is partly discharged. 

A possible solution to this would be to increase the supply fed to the diode connected to the RTC pin to 3.3V. 

A solution we are testing for Buffalo Grid, on a custom baseboard for a Sopine, is to use a simple circuit with a P channel MOSFET (connected to conduct in the reverse direction to normal) and a P channel JFET, which is switched off (because it is depletion mode) by the 5V supply connected via a resistor to its gate. When the 5V supply falls below the threshold of the depletion fet, it switches on and that switches on the MOSFET, connecting coin cell to RTC pin.

This only works for cells with a voltage slightly above 3V, because there is still the body diode of the MOSFET which can conduct if there is too large a voltage difference. However this is a junction diode so has a higher forward voltage and lower conduction at low voltages than a Schottky diode. We expect this to make the coin cell last a useful amount longer in its backup role.

It might be a good idea to incorporate a fix for this problem on any new editions of Pine boards.

-Andrew
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#2
(09-05-2018, 12:02 PM)Andrew Armstrong Wrote: Hi people

For the (probably a minority) who need to use the RTC backup with a coin cell:

The RTC is powered from a 3V supply via a Schottky diode when the Pine64 or Sopine is powered. In theory, a coin cell should be able to back up the RTC when power is off so that a reasonable time reference is available even when there is no network.

Lithium coin cells when new are significantly above 3V, and depending on temperature and newness could be up to 3.3V. This means that if the coin cell is connected via a similar diode, then the RTC will be powered from the coin cell even if the Sopine is fully powered, until the coin cell is partly discharged. 

A possible solution to this would be to increase the supply fed to the diode connected to the RTC pin to 3.3V. 

A solution we are testing for Buffalo Grid, on a custom baseboard for a Sopine, is to use a simple circuit with a P channel MOSFET (connected to conduct in the reverse direction to normal) and a P channel JFET, which is switched off (because it is depletion mode) by the 5V supply connected via a resistor to its gate. When the 5V supply falls below the threshold of the depletion fet, it switches on and that switches on the MOSFET, connecting coin cell to RTC pin.

This only works for cells with a voltage slightly above 3V, because there is still the body diode of the MOSFET which can conduct if there is too large a voltage difference. However this is a junction diode so has a higher forward voltage and lower conduction at low voltages than a Schottky diode. We expect this to make the coin cell last a useful amount longer in its backup role.

It might be a good idea to incorporate a fix for this problem on any new editions of Pine boards.

-Andrew

Thanks on the suggestion and already pass your info to hardware engineer to digest.
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