Primary battery for RTC for Jetson TX2


I’m working with Jetson Tx2 and I put a 3V primary battery in series with diode instead of superCap, but it seems like the battery always drains some current, and it doesn’t matter if I enable or disable the BBCEN bit on max77620, nor any other bit in CNFGBBC register ( except of BBCVS - then it drains more than usual )

My question is, how can I put the primary battery with diode instead of supercap and not have current drains when the VIN is on?
Is there any other register I need to configure besides CNFGBBC?

Best regards

Hi, did you measure the voltage of BBAT pin output without battery? The charging voltage of Backup Battery in MAX77620 can be set, you can try a value =< 3V.

Dear Trumany,

Yes, we measured it with different voltage levels and EN bit enabled and disabled, and it is pretty accurate.

We tried with an empty battery and had the same effect as above(current leakage).
It seems that whenever the battery voltage is higher than BBAT it drains 25uA current,
and when it’s lower it just drains 10-60nA, not depending on EN bit.

Do you have another suggestion?

I’m confused by the diode. What’s it for and how is it placed? There should be no current on some direction since diode exists. And it is impossible to drain current from lower level side to higher side, then why there is 10-60nA still?

Is your battery chargeable or not? Did you compare to a capacitor to see if any leakage current after fully charged?

Dear Trumany,

The battery is not-chargeable and the diode is in series between battery and Jetson so Jetson can’t charge the battery.

We haven’t measured the capacitor, but my guess is it wouldn’t have leakages of 25uA because its voltage is dependable of Jetson, and Vcoin is fixed.
And all of these measures are when the power is on. When the power is off, battery drains 2uA just as it should.

Our first concern was the Jetson charging the battery but with the diode, that current is minimalized so it’s not the problem.
But we are worried about those 10-60nA drains as well as 25uA current drain when BBAT < Vcoin.
For example, if the coin battery is full (3v) and the BBAT is set to 2v5 there is that leakage of 25uA.
Do you have any suggestions for that?

Thank you in advance.

Got what you mean.

If you set BBCVS to >= 3V, in theory there should be no such current from battery to BBATT pin. The 25uA is about the low-jitter mode Oscillator Supply Current.

Dear Trumany,

Thank you for your answer,

Since we use L4T 28.1 can we set that BBAT voltage in the bootloader cfg file using bootrom structures?

Setting BBAT voltage is available on the device tree from 28.2.1.

Best regards

BBCVS[1:0] bits can be set using the pmic cfg file(tegra186-mb1-bct-pmic-quill-p3310-1000-cXX.cfg) as follows,

pmic.generic.1.block[0].count = 2;

pmic.generic.1.block[0].commands[0].0x00.0x40 = 0x40;

pmic.generic.1.block[0].commands[1].0x04.0x18 = 0x08; #For selecting 3V

Thank you very much for the information, we will try this!

Best regards