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

4.1 Optional Enable (EN) Level Shifter

Since the ground of the buck converter IC is now referenced to the negative output voltage, a level shifter is required if a control signal is to be used on the enable pin. An example circuit is shown in #ID-FC8B6C24-397D-430E-BA46-DE5304603482 that can be used to level shift an incoming enable signal. While the circuit requires two transistors, it has no hysteresis and requires no current from the control signal. If the enable pin is not rated for the full input voltage range, then a Zener diode must be used to clamp the enable pin below its maximum voltage. The enable pin needs to be configured properly even without a control signal, and the buck converter data sheet can be referenced for the proper connection of the EN pin.

When the enable signal is pulled low, then the NMOS switch is turned off, pulling the gate of the PMOS to VIN. The PMOS then turns off, pulling the enable pin below the high-level threshold.

When the enable signal is pulled high, the NMOS switch is turned on, pulling the gate of the PMOS low. The PMOS then turns on, pulling the enable pin above the high level threshold from VIN.

-VOUT is the negative output voltage of the inverting buck-boost converter
Figure 4-1 EN Pin Level Shifter