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TPSM84624: Rtt value

Part Number: TPSM84624

Wondering what happens if the Rtt value is lower than the recommended value.  

I have this setup:

Vin: 12V

Vout 1.2V

Cout: 188 uF Ceramic and 220 uF polymer tantalum

I calculate that the Rtt should be at least 3 kOhms.

However, the value used in the design currently is 1.02 kOhms.  It seems from section 7.3.7 that the function of this is to optimize the transient response.  Apart from sub-optimal transient response what are other effects that may happen with a value this low?

Thanks,

Jon

  • Hi Jon,

    Lower Rtt will lead to lower loop bandwidth (slower transient response) in general. With the values you've given, the lower Rtt of 1.02kohm likely has lower phase margin (less stability margin) than if you used the 3kohm value determined by the calculation. Higher Rtt is not always better, as at some point you will also see decreasing phase margin by increasing Rtt further, but for the values you have given I expect the phase margin to decrease going to 1.02kohm versus 3kohm.

    However, if you have tested the design and do not see unacceptable ringing in transient response over your intended system operating temperature range, etc, then using 1kohm is likely fine and no cause for concern.

    Regards,
    Kris