SLUU180B January   2022  – February 2022 TPS40070 , TPS40071

 

  1.   Trademarks
  2. 1Introduction
  3. 2Description
  4. 3Schematic
    1. 3.1 Output Filter Components
    2. 3.2 MOSFET Selection
    3. 3.3 Frequency Feedforward Resistor Selection
    4. 3.4 Output Voltage Setpoint
    5. 3.5 Short Circuit Protection Resistor Selection
    6. 3.6 Miscellaneous Parts
    7. 3.7 Control Loop Compensation
  5. 4Test Setup
  6. 5Results
    1. 5.1 Control Loop Characteristics
  7. 6Assembly Drawing and PCB Layout
  8. 7List of Materials
  9. 8Reference
  10. 9Revision History

3.1 Output Filter Components

The power inductor is selected by calculating the range of peak-to-peak ripple current, IPP, which is obtained with various values of inductance over the total input/output voltage range. In previous generations of buck converters, electrolytic capacitors with significant ESR were the normal, and the inductor ripple current would be selected to be 10% to 20% of IOUT to minimize the output voltage ripple. Now, ceramic output capacitors with ESR in the range of 1 mW to3 mW are readily available, so the ripple current can be allowed to be 20% to 50% of the output current. The following equation was used to calculate the ripple current; and complete results are presented for the selected inductor value of 1.6 μH.

Equation 1. ∆ I P P =   T O N   ×   V I N -   V O U T L =   V O U T V I N   ×   f I   ×   V I N -   V O U T L
VIN VOUT IRIPPLE
12 3.3 4.98
12 1.8 3.19
12 1.2 2.25
8 3.3 4.04
8 1.8 2.91
8 1.2 2.12
5 3.3 2.34
5 1.8 2.4
5 1.2 1.9

Ceramic capacitors are selected for the output capacitors. The minimum value is determined by output voltage ripple considerations:

Equation 2. COUTmin⁡= IRIPPLE8 ×f × VRIPPLE= 5 A8 ×300 kHz ×0.018 V=116 μF

Three 47-mF ceramic capacitors are selected to handle the worst case ripple current of 5 A when VIN = 12 V and VOUT = 3.3 V. As the output voltage gets lower, the corresponding ripple current is reduced, so excessive output voltage ripple should not be an issue.