SLUAA12 March   2020 TPS53681

 

  1.   Selection Considerations for Output Capacitors of Multiphase Voltage Regulators Part 1
    1.     Trademarks
    2. 1 Introduction
    3. 2 Current Transfer Function Gii_CL(s) of Load Current to Inductor Current
    4. 3 Output Capacitance Calculation Based on Load Transient
      1. 3.1 Steady-state Output Ripple
      2. 3.2 Load Step Transient Response
        1. 3.2.1 Case 1: Small-signal Transient with Unsaturated Loop
        2. 3.2.2 Case 2: Large-signal Transient with Saturated Loop
        3. 3.2.3 Criteria for Loop Saturation
    5. 4 Analytical Calculations and Experimental Verification
    6. 5 Conclusions
    7. 6 References

5 Conclusions

In Part 1, we conclude that loop saturation is so crucial when calculating load transient response. For a fixed design, it is the nature output response of linear system Gii_CL(s), but not load step itself, will determine loop saturation. In D-CAP+ multiphase solution, maximum slew rate of Isum is limited by voltage, inductance, phase number, switching frequency and blanking time(upwards).

If Isum maximum slew rate can follow the linear system Gii_CL(s) output nature response, loop saturation never comes. Loop bandwidth becomes the most critical factor coming into picture; If not, loop saturation occurs with more undershoot and overshoot expected. Then everything physical comes into picture except load transient slew rate, others like phase number, blanking time and switching frequency(ton related) will make impacts.

Part 2 will describe capacitors selection (including types and value) to meet the requirements of output impendence and high rate repetitive load transient.