SNVAA61 February   2023 LM5168 , LM5168-Q1 , LM5169 , LM5169-Q1

 

  1.   Inverting Buck-Boost Application for LM5168
  2.   Trademarks
  3. 1Inverting Buck-Boost Topology
    1. 1.1 Concept
    2. 1.2 Output Current Calculations
    3. 1.3 Voltage Range of Inverting Buck-Boost Configuration
  4. 2Design Considerations
    1. 2.1 Bypass Capacitor and Optional Schottky Diode
  5. 3External Components
    1. 3.1 Capacitor Selection
    2. 3.2 Inductor Selection
  6. 4Digital Pin Configurations
    1. 4.1 Optional Enable (EN) Level Shifter
    2. 4.2 Power-Good (PG) Pin
  7. 5Typical Performance
    1. 5.1 VOUT = -12 V Typical Performance
    2. 5.2 VOUT = -24 V Typical Performance
    3. 5.3 VOUT = -48 V Typical Performance
  8. 6Conclusion
  9. 7References

1.2 Output Current Calculations

Changing to an inverting buck-boost topology will have an impact on the maximum possible output current. For an inverting buck-boost converter, the inductor current will always be larger than the output current. As a result, the maximum possible output current is calculated by:

Equation 1. I O U T ( I B B ) = I O U T _ B u c k × ( 1 - D )

where:

  • IOUT_Buck is the maximum buck converter DC output current
  • D is the duty cycle

The duty cycle can be calculated as:

Equation 2. D = V o u t V o u t - V i n × η

VOUT is represented as the negative output voltage of the inverting buck-boost converter. The efficiency term in the duty cycle equation helps account for power losses to provide a more accurate calculation of the output current. For example, if the output voltage is -24 V, the input voltage is 12 V, and the efficiency is assumed to be 85%, then the duty cycle is:

Equation 3. D = - 24 - 24 - 12 × 0.85 = 0.702

In the case of the LM5168, which has a maximum current of 300 mA, the resulting maximum output current of the inverting buck-boost converter would be:

Equation 4. I O U T I B B = 300   m A × 1 - 0.702 = 89   m A
Table 1-1 Maximum Output Current Calculations for the LM5168
VOUT (V) VIN (V) IOUT_Buck (A) η D IOUT (A)
–12 24 0.3 0.85 0.370 0.189
–24 24 0.3 0.85 0.541 0.138
–36 24 0.3 0.85 0.638 0.109
–48 24 0.3 0.85 0.702 0.089
Figure 1-2 Maximum Output Current Calculations for the LM5168 with Assumed 85% Efficiency