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

3.1 Steady-state Output Ripple

Steady-state output voltage ripple is a function of capacitance, capacitor equivalent series resistance (ESR), and inductor ripple current. In multiphase applications, effective inductor current ripple is used to calculate Vout ripple, not per phase current ripple.

For single phase application, inductor current ripple:

Equation 2. sluaa12-equation-2.gif

In multiphase application, in most of the cases, voltage drop ratio is high and not particular many phases are used for interleaving, as there are no overlaps between any two PWM waveforms of each phase, the effective ripple of Isum:

Equation 3. sluaa12-equation-3.gif

If effective ESR and ESL are neglected, the output voltage ripple is as follow:

Equation 4. sluaa12-equation-4.gif

But in most of the cases, the output capacitance should be decided by load transient specification.