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Observer digieng
Observer
4,116 Views
Registered: ‎02-26-2016

Ibatt Supply Current from 3V Coin Cell

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?

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5 Replies
Xilinx Employee
Xilinx Employee
4,106 Views
Registered: ‎09-05-2007

Re: Ibatt Supply Current from 3V Coin Cell

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.

 

Ken Chapman
Principal Engineer, Xilinx UK
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4,060 Views
Registered: ‎01-08-2012

Re: Ibatt Supply Current from 3V Coin Cell

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.

 

Allan

 

EDIT: this part: http://www.ti.com/product/tps797

4,051 Views
Registered: ‎01-08-2012

Re: Ibatt Supply Current from 3V Coin Cell

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.

Moderator
Moderator
4,037 Views
Registered: ‎07-23-2015

Re: Ibatt Supply Current from 3V Coin Cell

@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. 

- Giri
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266 Views
Registered: ‎01-08-2012

Re: Ibatt Supply Current from 3V Coin Cell

TI have released the TPS7A0218, a 1.8V LDO that features 25nA (typical), 120nA (max) ground current.  No prices yet.  X2SON package.