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TPS53318:

Part Number: TPS53318

Design parameters: Vin : 5V ,Vout : 4.2V and Iout 3A.

Observation:

At 0.3A Load, the output voltage obtained as 4.13V, Duty Cycle - 87.33% , Vfb - 0.62V

At 3A Load , The output voltage obtained as 3.86V, Duty Cycle - 78% , Vfb-0.575V

When the load is getting increasing the output voltage is started reducing.Kindly help us to resolve this.

  • Hi

        Did you create the design using a design tool such as WEBENCH - because it is DCAP and if you are using ripple injection circuit, you need to take care of certain things as part of the design.

    Can you take your design and run a quick Pspice simulation on your design? This will help clarify if it is a schematic issue or a board layout issue.

    Regards,

    Gerold

  • There is a maximum off time of 260ns typ. which may limit the switching frequency in high duty cycle applications. 

    What is the Rf resistor value in your application.  The Rf set the switching frequency of the device

  • Attached the simulation , the changes we have done were output capacitors used are 1Qty of 100uF-125mE and 1 Qty of 22uF  and on input side 2 Qty of 4.7uF.

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=A7D8030A9FDEA31A

  • We checked the design now by changing the Auto skip mode into FCCM .

    The output voltage at 0.6A load was 4.12V and at 5A Load was 4.03V.

    Can you please help us to find the root cause of this behaviour?

  • The webench design you attached has output capacitors: 2x22uF and 150uF with 35mohm esr. 

    You are using the 1x22uF and 100uF with 125mohm esr, if I read your post correctly. 

    Can you check the LL node with an oscilloscope under the load conditions of 0.6A and 5A with FCCM setting.   

    Is the LL node voltage periodic, similar to Figure 13 in TPS53318 datasheet.   If not, I think you need more output capacitance and with lower esr. 

    A simple troubleshooting tactic would be to add another 100uF/125mohm in parallel and check LL node. 

    I believe the output voltage ripple is high and causing the converter to be unstable.

  • The output voltage obtained in FCCM mode is nearly 4.08V at full load and 4.1V at 0.3A load but our expected output is 4.2V.

    Our main query now is ,Why the same is not able to acheive in Auto skip mode ,Please help us to find the root cause .

    Note : We have tried by adding one more 100uF but the same is obtaining and the voltage output on LL pin is same as in the datasheet fig.13.

  • If Rtrip is 44.2kOhm and you are drawing 5A.  You are mostly experiencing a high peak current and the device is current limiting. 

    Check the LL node frequency at 0.3A in SKIP and in FCCM mode.    Are the frequencies different?     

    The SKIP operation frequency  is lower and the peak current is higher.  Also, the higher ripple

    current and the higher esr (125mOHm vs the 35m from webench)  result in the output voltage offset.