SLVUC22 July   2021 TPS62912 , TPS62913

 

  1.   Trademarks
  2. 1Introduction
    1. 1.1 Performance Specification
    2. 1.2 EVM Features and Modifications
      1. 1.2.1 Input and Output Capacitors
      2. 1.2.2 Enable Level Shifter and Adjustable Threshold Voltage
      3.      Power Good Level Shifter
      4. 1.2.3 NR/SS Capacitor
      5. 1.2.4 Feedforward Capacitor
      6. 1.2.5 S-CONF Resistor
      7. 1.2.6 Loop Response Measurement
      8. 1.2.7 Single LC Filter Operation
  3. 2Setup
    1. 2.1 Input/Output Connector Descriptions
    2. 2.2 Ripple Measurement
  4. 3Test Results
  5. 4Board Layout
  6. 5Schematic and Bill of Materials
    1. 5.1 Schematic
  7. 6Bill of Materials

1.2 EVM Features and Modifications

The printed-circuit board (PCB) for this EVM is designed to accommodate some modifications by the user. Additional output capacitors can be added. The output voltage can be adjusted using feedback resistors, the soft start time and low frequency noise filtering can be changed, a feed-forward capacitor can be added, and the switching frequency, output discharge setting, and spread spectrum setting can be changed. Finally, the loop response can be measured prior to the ferrite bead with some board modifications. See the device data sheet for details of the various settings.

When making modifications for the output voltage, it will affect the average inductor current and output current. For an IBB, the average inductor current is no longer equal to the average output current because the inductor does not always supply the load current. The inductor only feeds the load during the OFF time, which is 1-D of the switching period. Equation 1 can be used to calculate the average inductor current:

Equation 1. IL=IO(1-D)

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

Equation 2. D=VOUTVOUT-VIN×1η

Where η is the efficiency at the operating point.

The efficiency term adjusts the equations in this section for power conversion losses and results in a more accurate maximum output current estimate. The peak to peak inductor ripple current is given by Equation 3.

Equation 3. ∆IL=VIN × DfSW ×L

Where D is the duty cycle, fSW is the switching frequency in Hz, L is the inductance in H, and VIN is the input voltage with respect to ground.

Equation 4 calculates the maximum inductor current.

Equation 4. IL,Max=IL,Avg+ ∆IL2

For example, for an output voltage of -5 V, an input voltage of 3.3 V, 2.2 μH inductor, and 1MHz switching frequency, the following calculations produce the maximum allowable output current based on the TPS62913 minimum current limit value of 3.7A. The efficiency term is estimated to be 85% as a conservative figure.

Equation 5. D=-5-5-3.3×10.85 =0.709
Equation 6. ∆IL=3.3×0.7091 MHz×2.2μH=1.06A

Rearranging Equation 7 setting IL,Max equal to the minimum current limit value specified in the data sheet gives:

Equation 7. IL,Avg=3.7- 1.062=3.17

This result is then used in Equation 8 to find the maximum achievable output current:

Equation 8. IOUT=3.17×1-0.709=922 mA

Figure 1-1 plots the maximum output currents for different output voltages (-1.8 V, -3.6 V, -5 V) and input voltage combinations based on an inductor value of 2.2 uH and a switching frequency of 1MHz.

Figure 1-1 Output Current Limitations by Input Voltage and Output Voltage

The input and output voltage for an IBB topology is limited by the recommended operating voltage of the IC, since the input voltage across the IC is from VIN to VOUT rather than from VIN to GND. When looking at the max output current vs VIN, the values are plotted to 17 V + VOUT, so for a -5 V output, the maximum input voltage is 17 V - 5 V, or 12 V.