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

3.2.3 Criteria for Loop Saturation

Judgement for loop saturation depends on the relationship of desired slew rate of Isum rising/falling and maximum supported slew rate for Isum rising/falling, under the assumption of unsaturated loop (linear system response).

The maximum desired slew rate of Isum named kde_max, is decided by the nature response of Gii_CL(s) emulated first-order system, while the maximum supported slew rate for Isum named kmax, is decided by controller limitations and circuit parameters together. Upward and downward load step have different kmax value.

sluaa12-nature-response-of-giic-s-linear-system.gifFigure 11. Nature Response of Gii_CL(s) Linear System

Below tables are used to simply but roughly judge, what status the response should be like, under linear system assumption.

Table 3. Criteria for Slew Rate Relationship

Condition 1 Condition 2 Condition 3
t0 t0<<τ Somewhere in between t0>3~5τ
Response waveform in Figure 11 Right Somewhere in between Left
Desired max slew rate: kde_max A/τ Somewhere in between k

Table 4. Criteria for Loop Saturation

Slew Rate Comparison Loop Status
kmax>kde_max Unsaturated
kmax<kde_max Partially or fully saturated