General Category > Discussion - MCU
RTC on FT90x revision C
Minomano:
We would like to use a supercap to power the VBAT pin when the CPU FT90x C is without power. I need the characteristics of the VBAT pin and the impedance when the chip is turned off as minimum maximum voltage and current. Can you help me? Do you have an application note for this?
Many thanks
scoprioprise:
Hi, Minomano, I'm following the discussion.
A supercap is indeed a good energy storage, but how long would you like this storage to last?
I understand that you are asking this to know, instead, how much autonomy you will have given the supercap you are selecting.
BRT Community:
Hello,
VBAT is the +1.5V RTC supply voltage. An external battery should be connected to the VBAT pin.
There is no application note or characteristics available.
The MM900EV1B schematics show an example of the connection required:
Also the RTC needs to be calibrated for an accurate time. See the following documents:
BRT_AN_004 FT93X RTC External Power Switch Circuit
BRT_AN_009 RTC Calibration in FT93X
If you have any other questions, please Contact Us via email.
Best Regards,
BRT Community
Minomano:
Hello
thanks for the replies.
My goal is to keep RTC alive for at least 20 days without power.
The capacitor has some advantages over the battery. Simple and fast charging circuit and above all, it has no discharge problems during storage after the board has been assembled inside the product container. For this I would like more information on the mysterious PIN VBAT for CPU FT90x C. From BRT's answer I understand that the VBAT of the FT90x works in the same way as FT93x. Is this so? In this case, is VBAT pin the only RTC power source? In the AN_004 solution is declared the current when power on is under 1 uA and backup current 9 uA. This seems unlikely because in the TI datasheet (http://www.ti.com/lit/ds/symlink/sn74auc2g53.pdf) because the VCC consumption of the chip alone is 10uA at 2.7V
In your previous version B chip and MM900EV1A (see attachment) you put MCP7940N. You can see the backup current in the figure in attachment. Don't you have a similar test for VBAT pin of FT90x?
I found a good comparison between different backup power source in this related AN:
http://ww1.microchip.com/downloads/en/Appnotes/00002027A.pdf
If we want to seriously use the FT90x also as RTC can you offer the details to design the VBAT circuit? or do you advise me to continue using an external RTC?
scoprioprise:
In our environment, as we needed a 20year appliance, the battery needed to be replaced, so we used an external RTC + battery assembly. So I'm intrested if we could use an enchancement. Which supercap are you using?
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