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TPS40210-Q1: What is the risk when the Rsense is shorted

Part Number: TPS40210-Q1

Hi team,

My customer shorts Rsense resistor for some reason. It looks the device works normally.

But, I don't understand why the device can work correctly in this case. The inductor current sense voltage is used for the feedback control because this IC is controlled by the peak current mode control, right? The IC integrated the slope compensation. It is only for the compensation for avoiding the sub-harmonic oscillation. I believe if there is only the slope compensation without the current sense voltage, the feedback control is not stable. 

What is the risk in the case that Rsense is shorted?

I didn't get why they wants to short Rsense. I will get that information. But, in advance could you give me the advice the risk of Rsense is shorted.

Regards,

Saito

  • Hello Saito,

    Thanks for your question. The risk of shorting Rsense is that the device is not detecting the fault condition of load overcurrent. The over current is detected when the voltage drop on the Rsense is higher that 150mV  and if you short this resistor you will never have any voltage drop there (which might be dangerous). Rsense helps wrt subharmonic instability only if you work in CCM and with duty cycle greater than 50% , if the customer is not working in this condition, it might be that it looks like the device works normally. I would not recommend shorting Rsense. 

    Let me know if I can help you furthermore.

    Kind regards,

    Elisabetta

  • Hi Elisabetta-san,

    I'm surprised that the device works normally without Rsense.

    Customer doesn't need OCP and if the duty is less than 50%, is it okay to short Rsense? Is there no risk about the stability or load transient response?

    Regards,

    Saito

  • Hello Saito-san,

    I would really not recommend shorting this resistor. During load transient and start up it is not possible to guarantee the duty and in these conditions you need Rsense to protect the circuit. Furthermore, since this is a current mode controller, removing the Rsense means that you are breaking the inner current loop and therefore you might affect the compensation too incurring into instability issues outside steady state.
    I will send you a friend request too, in case you want to continue this discussion privately. 

    Kind regards,

    Elisabetta 

  • Hi Elisabetta-san,

    When the Rsense is shorted, the device operate in the voltage mode, right?  Because it operates internal ramp signal and the voltage feedback signal. In that case, the compensation will be complicated.

    Regards,

    Saito

  • Hello Saito-san,

    No, I would not say that shorting this resistor the converter is operating in conventional voltage mode. Still, I would suggest not to short it, as it might create as well, under certain conditions, problems of instability. Let me know if I can help you furthermore. 

    Kind regards,

    Elisabetta

  • Hi Elisabetta-san,

    Understood. Customer is still moving forward to short Rsense resistor. And, they check the output is stable in the operational temperature range. I would recommend to customer to check the loop stability as well in whole temperature range.

    I got the background information why they are thinking to short Rsense. I also got the schematic from customer, so I will discuss with you through internal email if there are any solution to solve an issue without shorting the Rsense resistor.

    Regards,

    Saito  

  • Hello Saito-san,

    Feel free to reach out for me internally so that we can discuss this topic more in detail. I'm looking forward to your message.

    Kind regards,

    Elisabetta