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TPS40090 / TPS40091 Modulator Bode Plot - -20dB/dec between Load Pole & ESR?

sky54664
Prodigy 70 points
Other Parts Discussed in Thread: TPS40090, TPS40091

Hello,

I have a question about the tps40090 datasheet. On the page 15, there is the Modulator bode diagram. I would like to know what is the modulator system. Is it the inductor, capacitor and Rout? But in this case I do not understand how you can have, between load pole and Esr, only a decrease of 20db/dec whereas the load pole is a double pole so it should be -40db/dec.

Thank you for helping me.

 

Regards

  • 0
    TI__Guru 56825 points

    Sky,

     

    The TPS40090 / TPS40091 uses Peak Current Mode Control where the control voltage (COMP) regulates the peak inductor current (IL).  This causes the "Modulator" (PWM Comparator + Half-bridge MOSFET switch pairs) to appear as a transconductance source rather than a voltage source.  This suppresses the pole of the inductor (Inductance in series with a current source) within the bandwidth of the modulator.

    The result is that the "Power Stage" - Or Modulator + Output Filter, approximates a transconductance amplifier feeding the ouptut capacitor (and ESR seres resistiance) in parallal with the dynamic impedance of the load.

    The transconductance of the power stage is equal to 1/( Acs *Rsns) (typically 5.6 x DCR) for the band of frequencies of importance to the control loop and stability.