SLVAE09B July   2018  – August 2021 TPS560430

 

  1.   Trademarks
  2. 1Introduction
  3. 2Peak Current Mode Loop Modeling
    1. 2.1 Overall Control Block Diagram and Transfer Function Derivation
    2. 2.2 Inside Current Loop Model
    3. 2.3 Overall Loop Model
    4. 2.4 Inductor and Output Capacitor Design Limits
    5. 2.5 The Equation to Calculate Bandwidth and Phase Margin
  4. 3Inductor and Output Capacitor Design
    1. 3.1 Inductor Design
    2. 3.2 Output Capacitor Design
    3. 3.3 Simulation and Bench Verification
  5. 4Summary
  6. 5References
  7. 6Revision History

3.1 Inductor Design

Equation 17 calculates the value of the output conductor. KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current of the device. A reasonable value of KIND is 0.2 – 0.4. Since the ripple current increases with the input voltage, the maximum input voltage is used to calculate the minimum inductance LMIN, while KIND = 0.4 is selected. The minimum inductor value is calculated to be 16.3 μH. Choose the nearest standard inductor: L= 18 μH.

Equation 17. GUID-841A315F-3678-4630-8E42-9AB587C83AE4-low.gif

From Equation 12, the maximum inductor value is calculated to get enough phase margin. Three times margin is suggested and the limit is Equation 18 with the TPS560430 internal parameter. If you assume the target crossing over frequency fc is about 20 kHz, then the result is L < 40 μH at the minimum VIN. The selected 18-μH inductor meets the requirement.

Equation 18. GUID-600BCD60-18EC-4B80-9B1C-AA2495D6F6B2-low.gif

The TPS560430 is protected from over-current conditions by the cycle-by-cycle current limit. To prevent inductor saturation in case of short circuit conditions, the inductor saturation current must be greater than the device maximum peak current limit, which is 1.4 A for the TPS560430.