LTC3119 Adjustable Output Voltage

Hello,

I am using the LTC3119 as a high current, adjustable, buck-boost regulator for a robotics application. The input to this device is the output from a LTC4417 PowerPath controller. I am using a potentiometer to adjust the resistor ratio on the output of the LTC3119, but I am unable to achieve the advertised output voltage. The voltage always seems to have trouble rising above 3-4 Volts regardless of the input voltage (given that it is within the 2.5-18V limit). The input to the LTC4417 is straight from a DC power supply in my testing.

A pdf of my circuit schematic is attached for reference. Please let me know if I have misunderstood something about this chip, or if there are any things that I should change given my application to achieve the advertised operation of the chip.

Thank you.

PDF

  • +1
    •  Analog Employees 
    on Aug 30, 2019 5:51 PM

    Hi,

     

    In the schematics, I see pin 21 (SW1) and pin 8 (SW2) are left unconnected. If that is the case it is not correct. Pin 21 and 24 should be connected together to one side of the inductor and similarly pin 5 and 8 should be connected together to the other side. If the two of pins Sw1/Sw2 are not connected together respectively, functionalities such as current sensing , current sharing etc will not work as desired and might also lead to IC failure or some other undesired performance.

    I would recommend you to connect the pins and check for the performance again. It is better to replace the IC as well with a new one as there is a good chance that the IC might have got damaged. Also check if you are not overloading the output with higher load current than its current capability. 

    Thanks

  • Thank you for your input. I have replaced the IC with a new one and tried your suggestion of connecting those pins. It seems like it has helped out, but the problem still exists in some capacity. The voltage is usually regulated well up until some point, depending on the input voltage, but it always reaches a point well below 18V that it drops the voltage down and the current draw spikes up to 40mA (There is no load attached to the IC during this testing). Once this point is reached, I can adjust the potentiometer back down and it will eventually work again, but it cannot seem to steadily regulate the voltage between the 0.7V and 18V range properly. I have attached an updated schematic that shows the new pins tied together. Are there any other suggestions to help with this?

    I have tried modifying the RT and VC resistors to no avail.

    Thank you for any help with this.

    PDF

  • +1
    •  Analog Employees 
    on Sep 3, 2019 9:54 PM in reply to AdamF227

    Hi,

    The issue now seems to be of loop instability. I see now you have just 10uF as the output cap which seems to be very low and would change the power stage bode plot and the total loop gain as well. I would suggest you to follow the steps in the datasheet for component selection and do the loop compensation redesign (as given on page 22 of the datasheet) for Min Vin and Max Vout as it is usually the worst case condition with highest boost ratio. However also check the loop gain for other operating regions such as Min Vout/Max Vin for stability.

    Also RT pin should be configured less than 2Mhz of switching freq.  At higher switching freq there are higher switching losses ,thus lowering the efficiency and maximum current capability. So depending on your application requirement you should select your operating switching frequency accordingly.

    Thanks

  • Hello Anisha,

    Thank you for all of your input and advice. So far, I have been testing the chip with a breakout board on a breadboard, so I am now going to move the chip back to my prototype PCB to eliminate any unnecessary issues that might be arising from the breadboarding. I will take the advice on capacitor and resistor selection from the database into consideration. I think I will now be able to solve my issues.

    Thank you again for the support.