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

3.2 Load Step Transient Response

Each time of load transient can be described with one output current trapezoid wave. Only low repetitive rate load transient will be discussed here, which means we can assume each upward and downward pulse of trapezoid wave will have a long time gap. Each pulse of load transient has enough time to wait for vout transition to a new steady state.

Taking undershoot as example, Io ramps up fast with a certain slew rate k, while Isum will follow as fast as possible, but the difference always happens and comes with electric discharges of output capacitors, causing vout to drop. Overshoot is completely similar with this undershoot situation.

Depending on load step slew rate k and amplitude A, low repetitive load transient can be divided into two cases below.

Table 2. Two Cases of Load Transient

Case Number Loop Status Triggering Conditions
Case 1 Unsaturated loop(small-signal linear operation) Light load step, with small slew rate k and small amplitude A
Case 2 Partially or fully saturated loop(large-signal non-linear operation) Heavy load step, with large slew rate k and large amplitude A

These two cases will cause different corresponding behaviors of the control loop and the controller.