TIDUCV2B April   2017  – January 2023

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Overview
    1. 1.1 System Description
    2. 1.2 Key System Specifications
    3. 1.3 Highlighted Products
      1. 1.3.1 TPS82130
    4. 1.4 Design Considerations
      1. 1.4.1 Inverting Buck-Boost Topology Concept
      2. 1.4.2 VIN and VOUT Range
      3. 1.4.3 Maximum Output Current
        1. 1.4.3.1 Thermal Limits
        2. 1.4.3.2 Stability Limits and Output Capacitor Selection
      4. 1.4.4 Design Precautions
      5. 1.4.5 Enable Pin Configuration
      6. 1.4.6 Power Good Pin Configuration
      7. 1.4.7 Discharging Output Voltage
      8. 1.4.8 Input Capacitor Selection
  7. 2Getting Started Hardware
  8. 3Testing and Results
    1. 3.1 Test Results
  9. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Layout Prints
    4. 4.4 Gerber Files
    5. 4.5 Assembly Drawings
  10. 5Related Documentation
  11. 6Trademarks
  12. 7Revision History

1.4.4 Design Precautions

When using TPS82130 module 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 and EN tied together, 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. Ensuring 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 TPS82130 as 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 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 device's 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 1-4 and another using a schottky diode to clamp the EN pin Figure 1-5. 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 1-4 EN Pin Delay Using an RC Filter and Series Resistor
Figure 1-5 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 EN pin does not exceed the maximum rating of VIN + 0.3 V, which can damage the device. See Section 1.4.5 for more information on level shifting the digital inputs.