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

Parents Reply
  • Hello Marty,

    If any cap reaches the shunt voltad, default about 2.7V, the charger will slow down to make sure the cap does not exceed the shunt voltage and the high voltage cap will be shorted across with the 2x shunt resistors and internal FET RDSON. That will make the resistors get hot if shunting for a long time.

    I think this is happening. The cap voltage in the current design is 10,8 V, so 2,7 V each cap.. Later today I will measure the voltage across the casp and the voltage at the LTC3351 and let you know. I checked the connections, they are okay. I have about 5 of the same modules in test on the moment. My problem occurs about once in two weeks and it is not predictable. So it could be that in some cases, maybe because of moving the machine the connections get bad. But it is almost impossible to test this..

    RETRYB is a high impedance input. It does not need to connect to SGND directly in this case as long as its connected to PGND and can not couple in noise to the input. RETRYB is not floating is it? I do not see how it is connected on the layout.

    RETRYB is connected to the EPAD of the LTC3351 directly, so SGND. It is because there was not much room for additional traces. I am using 0603 components.

    What are the part numbers for the FETs? Are they logic level FETs?

    Texas Instruments CSD18514Q5A with a VGS(th) of 1,8 V so I don't think this is the problem? The total gate charge is about 29 nC. Is it better to look for another FET?

    Because we are experiencing EMC problems while testing in the EMC room, see picture, I have decided to push the supercaps to the main PCB. The electromagnetic emissions in the red oval are radiated by the cable between the LTC PCB and the supercap PCB. Because there is not much space I dind't try it before, but because I can leave some output caps I seem to get it fit now.

    So while we are testing I have been redesigned the PCB and have tried to implement all your suggestions of the first post. To prevent the LTC3351 keeps shunting I have lowered the voltage to about 9 V, I will have just enough energy for our application.

    Can you give me some feedback on the new design? It is not ready yet but the part concerning the layout of the LTC3351 components is ready. I still have to make the ground planes. But Layer 2 (Mid) and Layer 4 (Bottom) are goning to be only ground layers.

    FET: CSD18514Q5A (TI)

    Diode D17: SD0603S040S0R2 (AVX)

    Is D17 okay? it has a reverse voltage of 40 V..


    Layer 1 Top

    Layer 3 Mid

    Thank you very much for helping out.

    Best Regards,
    Rick Meijers