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TPS54240: Questions about TPS54240 Webench result.

Part Number: TPS54240

Hello guys,

One of my customers is evaluating TPS54240 for their new products.

They use parts parameters which calculated by TPS54240 Webench.

Their Vin/Vo/Io conditions are the follows.

Vin = 33.2V(Min), 36.9V(Nom), 40.6V(Max)

Vo = 3.3V

Io = 0A(Min) ~ 0.5A(Max)

Webench calculated the following values for compensation parts and output C and L with conditions above.

Ccomp = 100nF

Ccomp2 = 82pF

Rcomp = 5.36kohm

L1 = 82uH

Cout = 47uF

They used these parameter values for their evaluation. But they found that the output transient response response was very slow.

The output voltage dropped to 3.0V after 300us on their own board when output current was changed from 0A to 0.5A.

As the result of Webench load transient simulation (0.05A <-> 0.5A) with parameters above, the output voltage was dropped to 3.15V. 

They have the following questions at this moment. Could you please give me your reply for them?

Q1. I want to reduce the output voltage dropping level at load current increasing (0A -> 500mA).

       Could you please tell me how to achive that? For example, Is it Ccomp, Rcomp, Ccomp2 adjusting?

       Or is it better to add small capacitor between Vo and VSENSE terminal? 

Q2. In case of Ccomp, Rcomp, Ccomp2 adjusting, could you please tell me how to chose the best combination.

       Is it webench transient response and bode plot simulation using with each parameter changing.

 Q3. In case of TPS54240, is there big difference of load transient time between 0A -> 500mA condition and 50mA -> 500mA condition?

 Your reply would be much appreciated.

 Best regards,


  • Hi Kazuya,

    Looking at a sample webench design here

    It looks like you need 2 x 47uF output capacitor. Could you try to increase the output capacitor and try the load transient?

    As for the load transient, yes there would be a difference and it's because the part is operating in PFM at 0A, the drop maybe larger since the frequency have to go up first then responding to load transient. You can try it with a small load for example 100mA to 500mA and see the drop



  • Hi Arief,

    Thank you very much for your suggestions.

    They confirmed that the output voltage drop level is similar with Webench load transient simulation result after changing from 47uF*1 to 47uF*2.

    But the output voltage drop level is about 150mV at the load transient.

    They want to reduce the voltage drop level from 150mV to 50mV~100mV.

    Is there any idea or way?

    Could you please give me your reply for them?

    Thank you again and best regards,


  • Hi,

    You may put a 470pF capacitor in parallel with RFBT of 30.9kohm. 

    Or you may add more output capacitor to see what's the acceptable voltage drop since the output capacitor would help to handle the transient energy.