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Original question:

LM5163: Type 3 Ripple network

LM5163: Voltage dip at load

vaibhav waman
Intellectual 270 points
Part Number: LM5163

Tool/software:

Hello Team,

I have designed buck converter using LM5163 device. below are the specification and chosen componentes for same. When I load with 04A load current I observed voltage dipping to 9.6V from 9.85V.

Input : 12V

Cin : 4.7uF

Cout : 2 x 10uF

L=18uH

Vout = 10V (Actual 9.85V at no load and 9.61 at 0.4A load)

Feedback resistors are : RFB1 at high side = 24.8K RFB2 at low side 7.87K (this combination created using available resistor so there is tolerance more than 1%)

Used Type 3 ripple network with RA 100K CA 4nF CB 2nF

RON is 30.1K

What could be the reason for voltage dip ? I did not observed input voltage dip.

I observed if i raised input voltage to 14V there is almost no voltage dip at output voltage.

Thank  you.

  • 0
    Intellectual 270 points

    I missed to add :

    During loaded condition I observed feedback voltage is also dip at 1.17V from no load 1.20V.

  • 0 in reply to vaibhav waman
    TI__Mastermind 33140 points

    Hello Vaibhav,

    Can you share the scope shot of the dip you describe.  Also, can you share a shop shot of the switch node voltage at the same time.

    Thanks.

    David.

  • 0 in reply to David Baba
    Intellectual 270 points

    Hi David,

    Please see dip marked as red circle in attached waveform:

    CH1 : 12V input

    CH2 : 10V Output

    CH3 : SW Node

    CH4 : Fb Pin voltage

    I have measured initial no load 9.84V and loaded 9.61V with Multimeter.

    Thanks.

    Also in my previous question as I mentioned this circuit has switchabe feedback so it has additional circuits. In case you want to have review I can send you via email. but this is the behaviour when I put it in 10V output configuration and respective componentes for it. when I put it to 5V configuration this is working fine (12V in 5V out with 0.4A load and no voltage dip).

    Just for information I have build this proto on zero PCB (having manually wires connected parts) board and not with layout.

  • 0 in reply to vaibhav waman
    TI__Mastermind 33140 points

    Hello Vaibhav,

    Can you close up the time base and so I can see the Switch behavior.  The performance looks normal.  the way this device works is as follows.

    the HS MOSFET turns on for a fixed amount of time determined by RON.  these turn off, the output is peaked charged for the period of time the HS MOSFET was on for.  the HS MOSFET does not turn on until the output sensed at the FB node equals the reference voltage.  there will be a ripple voltage presented at the FB node determined by your type 3 ripple injection circuit.  there will also be a ripple voltage determined by the inductor value and output capacitor.  

    please use the calculator spreadsheet to ensure your design is optimal to meet your needs.

    Hope this helps.

    David.

  • 0 in reply to David Baba
    Intellectual 270 points

    Hello David,

    actually at output of our buck converter application there is chances of getting High voltage to avoid this high voltage to enter into buck input rail we have added diode in this path and to avoid voltage drop due to diode we have taken feedback after diode and to protect FB pin we have added another diode and zener diode in parallel. You can see this configuration in below :

    What I observed when my conversion ration goes on decreasing I have more voltage dip like at 13.2V and 10V output everything works fine. and at 10.8V input and 10V output I have larger voltage dip also at 10.8V and 10V (actual 8.60), 400mA output I observed my feedback pin voltage is 1.052 so regulation is not working as expected. I think this is probably due to loss in diode during on and off cycle. Is there any way where I could estimate required diode forward drop without dipping voltage ? as without diode at output everything works as expected. 

    I have tested this circuit with another buck converter LMR14006Y and there it works fine with diode. What do you think could be cause of this in this buck converter(LM5163).

  • 0 in reply to vaibhav waman
    TI__Guru 51911 points

    Hello Vaibhav

    I guess the on-time is not enough to regulate the output voltage. The output voltage drops if the duty cycle is not increased enough when decreasing the input voltage. (Duty cycle =VOUT/VIN)

    I recommend you to purchase https://www.ti.com/tool/LM5164-Q1EVM and check your circuit works well or not. By using the EVM, you can avoid an error which might be caused by the manual wires. 

    Also, please use the calculator spreadsheet to ensure your design is optimal to meet your needs.

    -EL