High Current Draw on SNVS_IN with imxrt 1062

[bennxp]

I have a custom board with the i.MXRT 1062 processor has VDD_SNVS_IN connected to a rechargeable 3V coin cell battery with a 2k resistor in series. (Per the hardware design guide, there is a diode between VDD_HIGH_IN and VDD_SNVS_IN to pull up SNVS). Everything works fine, and when the device is powered, it charges the battery. However, when I power off the system so the only voltage in the system is the coin cell batter, I see a 225mV drop across the 2k, meaning the battery is discharge over 100uA. This seems high. Looking at AN12094 it seems like the SNVS domain should draw between 14uA and 5uA (not sure if I'm reading the right table).

Is 100uA high for the SNVS line? If so, any ideas on what it could be? All the IO lines from the i.MXRT go to an unpowered FPGA, and I haven't seen any IO lines sag or draw any current. Do I need to run some code / shut-down procedure on  the i.MXRT prior to powering off the system to get it to be more power efficient?

Thanks,

[bennxp]

I've attached the schematic around the i.MXRT Power and SNVS_IN. The whole schematic is large and complicated, is there a section in particular you want to look at?

[jeremyzhou]

Hi,
Thanks for your reply.
Firstly, whether the power off means the 3.3V_D is shut down, if yes,
when power off happens, you find that 225mV drop across the 2k, I'd like to know whether you ever measure the current of the line that is marked in the below figure.

Further, I'd like to know whether measure the current of the same place as the above when the code is running.
Looking forward to your reply.
Have a great day,
TIC

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[bennxp]

Sorry if this is a double-post, my previous submission seems to have not gotten through.

Yes, when power off the device, the 3.3V_D line shuts down. The only power/voltage on the system when power is off is the battery. When powered off, I measure 0.2V on the 3.3V_D line.

Unfortunately, I cannot directly measure the current going into the i.MXRT pad M9 as I have no sense resistor there. The current to it comes from both the battery (which I can measure via R24) and the diode D4. When powered off, D4 is 0.2V on the high side (3.3V_D) and ~3V on the low side, so no current is flowing through diode and thus the R24 current (~100uA) is the current that goes into pad M9.

When the system is powered on, I see a 130mV forward drop across D4, so there is some current flowing through it, but it is hard to measure how much. The datasheet for the diode shows a ~150mV drop at 100uA of current, but doesn't show voltage for lower currents. Additionally, that D4 diode is also charging the the battery via R24, which, depending on the charge of the battery is typically taking 100uA -200uA. Given that low voltage drop and that most of the D4 current is going to the battery, I think the current into pin M9 is less than 50uA, but I don't think I can measure how much less.

Thanks,

Ben

[jeremyzhou]

Hi,
Thanks for your reply and clarification.
Maybe you can remove the R24 resistor and measure the current via the line when the power is off.
TIC

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[bennxp]

Maybe I'm not understanding, but if I remove R24 then when power is off, then the batter is disconnected nothing is powered and so there is no current to measure. Can you clarify what you'd like measured?

Thanks,

[jeremyzhou]

Hi,
Thanks for your reply.
1) Can you clarify what you'd like measured?
-- Sorry for confusing you, I'd like to suggest you use a 0-ohm resistor instead of the R24.
Have a great day,
TIC

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[bennxp]

Thanks for the clarification. If I replace R24 from 2k to 0 Ohms, then I can no longer measure the current flowing through it (as the voltage drop across it will be 0). I'd just be able to see the battery voltage. Maybe I'm missing something, but what can I measure / test with this change?

Thanks,

[jeremyzhou]

Hi，
Thanks for your clarification.
It seems a bit weird and the current exceeds the expected value a lot.
I'd like to suggest you remove the extra components on the condition that the RT1061 can boot up well to check whether the current will change.
Have a great day,
TIC

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[jeremyzhou]

Hi，
Thanks for your reply.
1) What can I measure/test with this change?
-- After removing the R24, you can populate the 0 ohms to replace it, definitely, you also can use a multimeter to connect it to measure the current.
Have a great day,
TIC

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[bennxp]

Ok, I removed R24 and instead placed a current meter across it. I measured 175uA of current when the board was off and the battery was at 3.3V. Any ideas on why the VDD_SNVS_IN line is drawing so much current?

Thanks,

[jeremyzhou]

Hi,
Thanks for your reply.
I'd like to suggest you to measure the current of VDD_SNVS_IN when the customize board enters different low power modes and check whether the results are according to the below figure.

Have a great day,
TIC

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[bennxp]

I removed D4 so I can measure the current flowing into the VDD_SNVS_IN (M9). I ran the SDK "power_mode_switch" example to set the device in different power modes.

With that I measured the current as:

57.5uA: "Over RUN" ("Press A" in SDK) 600 MHz Processor Clock

55.5uA: "Full RUN" ("Press B" in SDK) 528 MHz Processor Clock

47.1uA: "Low Speed RUN" ("Press C" in SDK) 128 MHz Processor Clock

51.1uA: "Low Power RUN" ("Press D" in SDK) 24 MHz Processor Clock

47.1uA: "System IDLE" ("Press E" in SDK) 

51.1uA: "Low Power IDLE" ("Press F" in SDK) 

43.9uA: "Suspend" ("Press G" in SDK)

45.4uA: "SNVS" (Press H" in SDK)

207uA: No power except for SNVS_IN

I'm not sure what to make of this as all the numbers are much higher than expected ~50uA  when powered instead of ~10uA. Also when not powered, the draw is ~200uA which also seems very high.

Any ideas why could be drawing all this current?

Thanks,

[jeremyzhou]

Hi，
Thanks for your reply.
I think it needs to dig a bit deeper, please follow the below steps to measure the current.
1) Removing the R24;
2) Removing the diode (x mark point to )
3) After that, measure the currents of the same place when MCU enters different low power modes.

Have a great day,
TIC

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[bennxp]

That is the test I did and reported above. I removed D4 and measured the current flowing through the M9 pad to a 3V power supply. The battery was disconnected, which is equivalent to removing R24. 

[jeremyzhou]

Hi，
Thank you for your interest in NXP Semiconductor products and for the opportunity to serve you.
To be prudent, before answering your question, I was wondering if you can upload the schematic.
Have a great day,
TIC

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[bennxp]

Just a follow up. My problem was I was using the PMIC_STBY_REQ pin as GPIO but it is powered off the SNVS power line, so a 50k impedance on that pin causes a 100uA of current flow. When I changed the impedance, I was able to get 15uA of current draw when only SNVS is powered, which is just what is show  in the datasheet. Thanks for all the help.

[mjbcswitzerland]

Hi

Can you specify whether you have the 32kHz oscillator running when you measure 15uA?

This is an extract from the 1060 data sheet which suggests 20uA at 25°C is typical but not guaranteed:

When I measure just the SNVS current on my otherwise un-powered MIMX1060EVK and also my MIMX1064EVK I get around 59uA on both:

Then I have a few custom boards (where I have a battery and it is running the RTC and the SRTC so that time/date is reserved over power down periods ) which measure typically 39uA (with one however at 60uA).

Unfortunately, as the data sheet specifies, the power down (RTC operating) back-up current is not guarantied (with no max. value presently specified), meaning that it is not possible to know what the expected deviation in the current will actually be and what size of battery needs to be used to ensure data retention over a specified period without power.

As I start the first question is whether the EVKs are representative (mine being 3x higher than that which is declared) and whether my custom boards are also representative?
The second question is whether there are more accurate values of the range to be expected (with the max. value being the most important for engineering the correct battery capacity in a particular design)?

If NXP doesn't have these figures (yet) could users with representative results across larger number of custom boards and batches of i.MX RT maybe give some practical values?

Regards

Mark

[bennxp]

My measurement was with the external 32kHz oscillator on. If you look at app note 12245 (and this is again typical at 25C), it has few more decimal places on the current draw than the 0.02mA in the datasheet and shows a current of 15.7uA

I would think that there wouldn't be much variation (<10%?) on this between devices, but I'm afraid I don't have an data or statistics to back that up.

[mjbcswitzerland]

Thanks - I see that they remove two resistors on the EVK, which I haven't done yet - therefore I will compare without these a little later.

However, on my custom board I don't have an external pull-up on POR so my present 37uA (at 25°C) is high still and worrying as I was hoping for less that 20uA in order to reach min. 5 years battery backup with the (non-rechargeable) Lithium battery that is foreseen.

Regards

Mark

[mjbcswitzerland]

Hi

The resistors are so small that I decided not to remove them but instead measure the voltage drop across them. However this only showed about 2uA additional current so that still leaves the EVKs with fairly high SNVS_IN current (about 3x the expected).

Regards

Mark