LPC54618 and RTC Battery power consumption

[m_dibiaggio]

Good morning,

I have a problem with power consumption of LPC54618 RTC. As illustrated in attached PDF I have a board with the configuration visible on the schematic. Battery is a rechargeable Panasonic VL-1220 with 7mA/h current capacitance. LPC54618 datasheet reports that, in power off conditions (board off, with only VBAT present), consumption is 340nA. I can't measure precisely the consumption, but after several tests, I see that the current absorbed from RTC is more than that indicated. I tried to left reset pin floating ad suggested in the datasheet, but without significative differences (and how can I left floating a reset pin?!?!). I have 2 different boards with same LPC54618 RTC circuit and same problem.

Is there anyone that can help me to understand how solve this problem?

For information I used same battery on LPC1768 controller (that has similar consumption characteristics) without problems and duration about 6 months.

Many thanks in advance for any information.

Best regards,

Marco Di Biaggio

[xiangjun_rong]

Hi, Marco,

I suggest you use a precise Multi Meter Instrument to test the RTC pin current with VDD=VDDA=VREFP=GND, you can not figure out the RTC current by battery energy consumption for a fixed  time, because the battery has large leakage current, in other words, battery consumes power itself.

Hope it can help you

BR

XiangJun Rong

[m_dibiaggio]

Hi XiangJun Rong,

thanks for your reply.

I measured the current that flows on JP13 (with board not powered) and it is 34.6uA (microampere). 

If I disconnect R33 current absorbtion is about 0.3uA. But, how can i leave floating a RESET pin?? Datasheet says that there is an internal pull-up, but I would feel more comfortable with an external reset supervisor.
So, how could I connect the MCP809 to avoid current consumption?

Thanks in advance for any response.

Best regards,

Marco

[xiangjun_rong]

Hi, Marco,

The Resetn pin floating means that there is not any connection on the pin Resetn or any driven signal.

I have checked your prb_Battery_r0.pdf file, if you connect the MCP809, when the MCP809 is power off, you can not guarantee that the /RST pin of MCP809 is float in the case.

As an alternative, if you replace the R33 with a diode, what is the result? I am not sure if it can solve your issue with a diode, pls have a try.

BR

XiangJun Rong