I2C leakage current at high temperatures

[edos]

Hi 

I have a LPC11E14 using it over I2C as a slave. 

Now when the micro controller is not powered, I can see a leakage current on the I2C pins PDIO0_4/SCL and PDIO_5/SDA into the controller. This leakage current is influencing our I2C host and lowers the I2C voltage level so much that other devices on the bus are influenced. 

This effect starts at ~40°C and gets worse at >+60°C. 

As soon the controller is powered the bus works fine. 

have ever had a similar effect ? 

BR

[fireman]

Hello XiangJun Rong,

I'm distribution FAE and Mr. Eschenmoser (edos) contacted me, because he wasn't satisfied with the answers that were given.

Today he rised one important point: Not every LPC11E14 behaves like described above:

"Bad" for example are: Z216.104 ZSD18055b

"Good" is for example: Z197.103 ZSB16425B

Regards,

Marno

[xiangjun_rong]

Hi, Edos,

You can not connect the I2C signals to the I2C signals of LPC11E14 while the LPC11E14 is NOT powered, it is risky for the LPC11E14 may be damaged. From our experience, if user wants to connect the signals to a processor while the processor is not powered, we suggest user connect a serial resistor to limit the leakage current. But for the I2C signal, if you connect a serial resistor on the I2C signal, the I2C may dysfunction.

In conclusion, we do not suggest user connect I2C signals to LPC11E14 while the LPC11E14 is NOT powered.

BR

Xiangjun Rong

[edos]

Hi thank you for the fast answer. 

I referred to the datasheet of the LPC11E14  https://www.nxp.com/docs/en/data-sheet/LPC11E1X.pdf 

There is written:

[4] VDD present or not present. Compliant with the I2C-bus standard. 5.5 V can be applied to this pin when VDD is powered down.

So we designed like our system like that. Our System already has serial resistor of 46.4Ohm 

What do you suggest in that case ? Increase the serial resistor or ?

Best Regars 

[xiangjun_rong]

Hi, Edos,

Regarding the Note 4, I suppose it means that the chip is not damaged when the 5.5V I2C signal is applied to the pins while the chip is power down. It does not means that the i2C node can work.

I suggest you increase the serial resistor to suppress the effect from the I2C module which is power down.  But increasing resistor may have impact on the I2C module when it is powered up. so you have to make balance to choose the resistor value.

Hope it can help you

BR

XiangJun Rong

[edos]

Hi XiangJun Rong

Thank you for answer. I checked the I2C specifications which the Datasheet is referring to. There it says that the I2C Bus shouldn't be influenced of the MCU if this one is powered off. 

I have to check if we are able to increase the resistor value.

Otherwise we have to implement a bus switch to cut the I2C signals from the MCU if its not active so that the MCU is not influencing the signal

Best Regards Edos