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

1.2 Output Current Calculations

By changing the buck configuration into an IBB configuration, the average inductor current is affected. The output current capability in the IBB topology is less than the buck configuration. As a result, the maximum achievable output current is calculated using Equation 1.

Equation 1. I O U T ( I B B ) = I L _ m a x   ×   ( 1 - D )
  • IL_max is the maximum rated inductor current.
  • The TPSM365R6 has an inductor with an IL_max of 0.65 A.
  • D is the operating duty cycle

The operating duty cycle for an inverting buck-boost converter can be found with Equation 2.

Equation 2. D = V O U T V O U T - V I N × η
  • In Equation 2 VOUT is represented with a negative value.

The efficiency term in Equation 2 adjusts the equations in this section for power conversion losses and yields a more accurate maximum output current result. This efficiency is found by averaging the efficiency of the TPSM365R6 buck in its standard configuration. Use Equation 1 and Equation 2 to calculate the recommended maximum output current. For example, with an input voltage of 24 V, an output voltage of –12 V, and an estimated efficiency of 0.9, the duty cycle will be calculated with Equation 2 as:

Equation 3. D = -   12 -   12   -   24   ×   0.9 = 0.357

The result of Equation 4 is then used to calculate the maximum achievable output current with Equation 1:

Equation 4. I O U T ( I B B ) = 0.65   ×   ( 1 - 0.357 ) = 0.42   A

Table 1-1 provides a general idea of the maximum output current allowed from the TPSM365R6.

Table 1-1 Recommended Maximum Output Current Calculations for TPSM365R6 in IBB Configuration
VOUT (V) VIN (V) IL_max (A) η D IOUT (A)
–1.8 24 0.65 0.6 0.111 0.58
–2.5 24 0.65 0.7 0.130 0.57
–3.3 24 0.65 0.7 0.164 0.54
–5 24 0.65 0.8 0.210 0.52
–12 24 0.65 0.9 0.357 0.42
Figure 1-2 Recommended Maximum Output Current for TPSM365R6