LTC3351 Charging Problem


I'm using the LTC3351 in a product for backup power in case of power failure.

I'm using 4 x 70F supercapacitors for the backups power.

My design is based on the evaluation board with soms alternations.

Input: 18 - 25 V, Max. 5 A.

Backup voltage: 16 V.

Backup Power: about 40 W maximum.

I'm using the schematic below:

The capacitors are on a seperate PCB. I'm connecting the supercapacitors PCB with a 12 cm wire assembly.

I have made and prototype two PCB designs. My problem is that the supercapacitors don't charge if they are below 7 V. If I charge them manually to about 7 V and then connect them to the LTC3351 board they charge okay and backup is working.

If the caps are below 7 V the LTC3351 on my board tries to start charging for about a couple of ms. After that it stops and does nothing.

What can be the problem?

Best Regards,

Rick Meijers

  • 0
    •  Analog Employees 
    on Jan 14, 2020 1:51 PM over 1 year ago

    Hello, Sorry I could not read the schematic even if save as picture and zoomed in. To small and fuzzy. Would you include the schematic using the insert link/insert image/video/file? 

    Are you able to read the registers via I2C? is it possible to obtain the contents of the sys_status_reg and the monitor_status_reg? When the LTC3351 is failing to charge what is the status of the VINGD pin?

  • Hello MartyM,

    Thank you for your fast reply. Yesterday evening I discovered the problem. It was a solder problem. There was a solder bridge I missed due to assessive solder paste. I build a new prototype pcb and it works fine now. Works as expected.

    Though, I have some other questions:

    - Is it possible te put the LTC3351 through the reflow proces (oven) twice? For mounting SMD components on the backside or in a two phase proces.

    - In the evaluation board schematic there is a 0,1 uF capacitor on the VIN pin witch is not populated. Is there any reason to use a 0,1 uF or larger capacitor on the VIN pin for stability?

    - I use a ATMega164PA MCU on the system / board. I coupled the I2C interface of the LTC3351 to the ATMega164PA. On the moment I am not using this interface (I2C) but maybe I will in the future. I used 100K pullup resistors on the I2C pins. Is it good practise to set the relating pins on the ATMega164PA as output and pull them low. Or can I set the pins as input and keep both I2C pins high? What is the best way? I have no pulldown resistors and no room for them on the moment.

    Thank you for your help, I really appreciate it.

    Best Regards,

    Rick Meijers

  • 0
    •  Analog Employees 
    on Jan 15, 2020 2:13 PM over 1 year ago in reply to Rick Meijers

    Hi Rick, I am glad you found the issue. I do not know about the reflow question. I'll have to get someone from packaging to get that info is someone else does not chime in. 

    The DNP cap on VIN was a place holder incase we felt the need to decouple the pin. I do not recall ever needing it for any of the applications or customer boards. 

    If there are pullups on the I2C pins they can be left pulled high as inputs or open drain outputs. Pulling them low will not hurt but will use the tiny bit of current in the pullup resistors. These are high impedance inputs so the main thing is that these pins do not float. 

  • Hello Marty,

    Thank you.

    Okay, It would be really nice to know about the reflow question for production purpose.

    I will try and measure the difference with and without the decouple cap on VIN. For now I left it open and everything seems to be working fine.

    Thank you for replying on my I2C question. I will leave them high than, I was just wondering of this would be a problem on long term stability because they are pulled low on the evaluation board.

    Best Regards,

    Rick Meijers

  • 0
    •  Analog Employees 
    on Jan 22, 2020 1:39 PM over 1 year ago in reply to Rick Meijers


    I finally have an answer on the reflow. We guarantee up to 3 reflows.

    The demo board has weak pull downs on the SDA & SCL line to keep them at a known state when our USB to I2C adaptor board is not used. Our USB to I2C boards have there own pullup and can be selected to different voltages on the DC590B board.

    No long term stability, they just need to be in a known state.