SNVAA63 March   2023 TPSM365R6

 

  1.   Abstract
  2.   Trademarks
  3. 1Inverting Buck-Boost Topology
    1. 1.1 Concept
    2. 1.2 Output Current Calculations
    3. 1.3 VIN and VOUT Range in Inverting Configuration
  4. 2Design Considerations
    1. 2.1 Additional Bypass Capacitor and Schottky Diode
    2. 2.2 Start-up Behavior and Switching Node Consideration
  5. 3External Components
    1. 3.1 Capacitor Selection
    2. 3.2 System Loop Stability
  6. 4Typical Performance
  7. 5Digital Pin Configurations
    1. 5.1 Digital Input Pin
    2. 5.2 Power-Good Pin
  8. 6Conclusion
  9. 7References

3.2 System Loop Stability

Stability is an important factor in the system when adding more output capacitance. The general rule of thumb for a stable design is a desired phase margin (PM) of at least 45° and under 70°. In some cases, a feed forward capacitor, Cff, is necessary to maintain the stability of the system's open loop conditions. The TPSM365R6 EVM includes a 10-μF feed forward capacitor inherently. As a general rule of thumb, Equation 5 can be used to determine valid values for Cff. Cff is to be placed across the RFBT resistor shown in Figure 1-1.

Equation 5. C f f   =   1 2 π   x   R FBT   x   2 f c _ n o C f f  
  • fc_noCff is the crossover frequency of the device without Cff
Table 3-1 shows the theorized PM and capacitance with respect to VIN, –VOUT, and IOUT. With the 10-μF feed forward capacitor included on the TPSM365R6 EVM, these capacitor values and associated voltage ratings provide the desired phase margins.
Table 3-1 Phase Margin of IBB TPSM365R6
VIN –VOUT COUT Voltage Rating (V) PM (°)
24 –2.5 10 25 56.59
24 –3.3 10 25 63.47
24 –5 22 25 66.45
24 –12 44 25 60.40