SLUAAG4 February   2022 TPS62933

 

  1.   Trademarks
  2. 1Introduction
  3. 2Loop Response of Peak Current Mode Converter
  4. 3Output Capacitance Upper Limit for Internally-Compensated PCM Buck Converter
  5. 4Output Capacitance Lower Limit for Internally-Compensated PCM Buck Converter
  6. 5Design Example and Experimental Validation for TPS62933
  7. 6Summary
  8. 7References
  9.   A Validation and Calculating the Output Capacitance Upper Limit

4 Output Capacitance Lower Limit for Internally-Compensated PCM Buck Converter

The chosen capacitance, CO, cannot be too small. An output capacitance that is too small could cause worse load-transient performance and loop instability at the same time. Normally for a peak current-mode converter, the lower limit with loop stability is much smaller than the limit with the load-transient performance restriction. So the only method to derive the lower limit through load-transient restriction is introduced in this section.

To calculate the lower limit for output capacitance for a specified VOUT overshoot and undershoot in load transient, see the Output Capacitor Selection section in the TPS62933 3.8-V to 30-V, 3-A Synchronous Buck Converter in SOT583 Package data sheet(1).

Equation 20.

where

  • D = VOUT / VIN, duty cycle of steady state
  • ΔVOUT is targeted output voltage change for load transient
  • ΔIOUT is output current change in load transient
  • K is the ripple ratio of the inductor current (ΔIL / IOUT_MAX)

If the CO lower limit calculated from Equation 20 is bigger than the upper limit from Equation 12, there is no available output capacitance range with limitation. A feedforward capacitor must be used to increase phase margin at this condition, which is introduced in the Stability Analysis and Design of Internally-Compensated Peak Current Mode TPS62933 - Part II: How to Select the Feedforward Capacitor application note.