10-10-2016 03:20 AM
How can I convert the a 3V coin cell to the 1.8V required for the VCCBATT pin. My main concern is that the supply current is only 150nA and I don't want to make the assumption that any LDO regulator will do. What component can I use or how can I implement this? Do you have a reference design?
10-10-2016 03:46 AM
Looing at the Kintex-7 data sheet (DS182) it recommends VCCBATT in the range 1.0v to 1.89v. As such, a 1.5v cell is ideally suited.
A useful reference is the KC705 board which has a 1.5v rechargeable lithium button-type battery.
10-11-2016 06:57 PM - edited 10-11-2016 07:05 PM
On an old design of mine I needed both 3.3V (ish) for an RTC and a 1.8V VCCBATT supply for an FPGA.
I used a single 3.6V cell and a TPS79718 LDO from TI. That part had the lowest Iq of any LDO I could find at the time.
At room temperature I measured total Li cell current of 2.4uA (including the RTC). This current dropped to about 1.2uA when the unit was powered (and the RTC was running from the main +3.3V rail).
That current was ok for my application (which had a 2/3AA Lithium Thionyl Chloride cell) - the Li cell would outlast the expected lifetime of the system by about a decade. Even at that current, the lifetime would be determined more by the temperature than the cell capacity.
EDIT: this part: http://www.ti.com/product/tps797
10-11-2016 09:14 PM - edited 11-23-2016 05:18 PM
I checked for other regulators.
The TPS78318 from TI also seems promising, with 800nA max Iq.
Watch the I SHDN(OUT) parameter though - I think it means that the output voltage could rise (possibly above abs max for the FPGA) if the regulator enable pin is low. You won't need to use the shutdown mode, and can tie the enable pin high.
The MCP1710T-18 from Microchip has a (typical) 20nA Iq but does not guarantee a worst case value. It's also marked as obsolete. The replacement part MCP1711 has a maximum Iq of 1.5uA :(
2016-11-24 EDIT: I just came across the Semtech TS14002 family. It has a (typical) 20nA Iq but does not guarantee a worst case value. The TS14002-C018DFNR is in stock at Mouser at the time of writing.
10-12-2016 12:51 AM
@digieng If you can stick to using a regular 1.5V cell, that would be the way to go. If you need to go the LDO route, the LDO's suggested by allanherriman are a good bet. You could try looking around for other vendors too if you find something with a lower Iq value.
@allanherriman Thanks for the details on the LDO part suggestions and their Iq values.