SLVA469D June   2013  – January 2023 TLV62130 , TLV62130A , TLV62150 , TLV62150A , TPS61175 , TPS61175-Q1 , TPS62130 , TPS62130A , TPS62131 , TPS62132 , TPS62133 , TPS62135 , TPS62136 , TPS62140 , TPS62140A , TPS62141 , TPS62142 , TPS62143 , TPS62150 , TPS62150A , TPS62151 , TPS62152 , TPS62153 , TPS62160 , TPS62161 , TPS62162 , TPS62163 , TPS62170 , TPS62171 , TPS62172 , TPS62173

 

  1.   Using the TPS6215x in an Inverting Buck-Boost Topology
  2.   Trademarks
  3. 1Inverting Buck-Boost Topology
    1. 1.1 Concept
    2. 1.2 Output Current Calculations
    3. 1.3 VIN and VOUT Range
  4. 2Design Considerations
    1. 2.1 Design Precautions
    2. 2.2 Additional Input Capacitor
    3. 2.3 Digital Pin Configurations
      1. 2.3.1 Digital Input Pins (EN, FSW, DEF)
      2. 2.3.2 Power Good Pin
    4. 2.4 Startup Behavior and Switching Node Consideration
  5. 3External Component Selection
    1. 3.1 Inductor Selection
    2. 3.2 Capacitor Selection
  6. 4Typical Performance
  7. 5Conclusion
  8. 6References
  9. 7Revision History

2.1 Design Precautions

When using any of the TPS6213x/4x/5x/6x/7x converters for an inverting buck-boost application, there is a risk if the VIN and EN pins are connected together directly.

The inverting buck-boost is commonly used to power the negative side of a differential rail. If the positive rail is applied to this differential rail first, the downstream devices activate and their Iq charges a positive prebias voltage on the negative rail. Input brownout and quick power cycles are other scenarios that can prebias the negative rail and cause startup issues. With VIN/EN tied together and having a positive prebias on the output of inverting buck-boost, the device may enable before it has time to initialize internal circuitry which can cause unexpected startup behavior or cause the device to get stuck. Ensure that the EN pin is asserted after VIN is powered on would eliminate this issue robustly.

There are three proposed workarounds to avoid this issue:

The first suggestion is to adjust the system power up sequence to prevent the unintended voltage buildup on the negative rail. This means enabling the negative rail first so that it is able to start up correctly, then enabling the positive rail. Adjusting the power on sequence this way will ensure that the inverting buck-boost converter has a correct startup.

If the application relies on the device enabling with VIN, then an RC filter is required to add a delay between VIN and the EN pin. This ensures that the device has enough time to initialize the internal circuitry before the device is enabled to start regulating the output. The 100 kΩ and 1 μF RC filter provides the necessary delay between the VIN and EN pins for devices' initialization. An example of this schematic modification is shown in two different options, one using a series resistor to limit the current into the EN pin Figure 2-1 and another using a schottky diode to clamp the EN pin Figure 2-2. When using the Schottky diode option, the forward voltage drop should be selected less than 0.3 V so that the device doesn't exceed the absolute maximum rating on the EN pin.

Figure 2-1 EN Pin Delay Using an RC Filter and Series Resistor
Figure 2-2 EN Pin Delay Using an RC Filter and Schottky Diode

Lastly, if the application requires enabling and disabling of the device from an external control signal, like a microcontroller or PG pin from an upstream device, then the order of power sequence is important. During power up, VIN must be applied before the EN signal and during power down, the EN pin should go low before VIN is removed. This ensures that the EN pin does not exceed the maximum rating of VIN + 0.3 V, which can damage the device. See Figure 2-4 for more information on level shifting the digital inputs.