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

2 Loop Response of Peak Current Mode Converter

Figure 2-1 shows the schematic of a PCM buck converter. A type 2 compensation is used to ensure the stability of the converter.

Figure 2-1 Simplified Schematic of PCM Buck Converter

The loop response model of PCM buck converters is introduced in the application report(2). Figure 2-2 shows the bode plot.

Figure 2-2 Bode Plot of PCM Buck Converter Open Loop Response

In the loop response of PCM converters, the DC gain is affected by output current.

  • fP1_EA, fP2_EA and fZ_EA are the frequencies of poles and zero generated by the type2 compensation
  • fP1_EA is the initial pole at low frequency to boost DC gain for improving output voltage accuracy
  • fZ_EA is a zero to enlarge bandwidth and boost phase margin
  • fP2_EA is a high-frequency pole to increase gain margin and attenuate high frequency noise

In TPS62933, the DC gain, fP1_EA, fP2_EA, and fZ_EA are all determined by the device internal compensation circuit, which is expressed as Equation 1 and Equation 2.

Equation 1.

where

  • IOUT is the output current of the converter
Equation 2.
  • fZ_OUT and fP_OUT are zero and pole introduced by the output capacitor and load

For the application with all MLCC or small ESR output capacitors, fZ_OUT is at high-frequency range and has limited effects on converter stability.

Equation 3.
Equation 4.

where

  • CO is the output capacitance
  • RESR is the ESR of output capacitors
  • RO is the output resistor, which equals to VOUT / IOUT.

fP_ci is a pole introduced by the inside current loop and related with device slope compensation. Its expression for TPS62933 is shown as Equation 5.

Equation 5.