SLUP413A May   2024  – April 2026 TPS53689T

 

  1.   1
  2.   Abstract
  3. 1 Introduction
  4. 2 Converter Transient Response
  5. 3 Magnetics
  6. 4 TLVR Topology Operating Principles
    1. 4.1 Steady-State Operation
    2. 4.2 Load Transient Step-Up
    3. 4.3 Load Transient Step-Down
    4. 4.4 LC Inductor Selection
    5. 4.5 Steady-State Ripple
  7. 5 Power Loss and Efficiency
  8. 6 Phase Multiplication
  9. 7 PCB Layout
  10. 8 TLVR-Optimized Components
  11. 9 Example Side-by-Side Design
  12. 10Summary
  13. 11Additional Resources

4.4 LC Inductor Selection

The LC has somewhat unique requirements compared to other inductors in a typical DC/DC design. The inductance of LC is a trade-off between current ripple and transient response benefits. Typically, start with LC = LM as a balanced trade-off. Values between 0.8 to 1.5 times LM are common with discrete designs. Lower values may be more common in highly integrated designs, such as power modules.

At steady state, LC carries no DC current – only a small AC current ripple – because it is switching at a high frequency (at least NTOTAL × fSW when there is no pulse overlap). Its current ripple dominates its RMS current at steady state, described in Equation 21. Consider low core-loss materials, such as ferrite cores, because of the high fSW. Another option to further improve transient response may be soft-saturating cores.

Equation 21. I r m s ( L c ) ≈ Δ I L c 12

However, LC can continue to build large amounts of current during transient events, as expressed by Equation 22, where tRESP is the response time of the controller, as highlighted in Figure 15 and Figure 16. Therefore, size the LC with a high saturation current, similar to the coupled inductors used in each phase.

Equation 22. I S A T ( L c ) ≫ t R E S P   ×   N O N s t e p   ×   V I N - N T O T A L   ×   V O U T L c

After building up a large current, the LC current naturally decays to zero, with a relatively high time constant, τLC, as described in Equation 23, formed by the LC and the resistances in the LC loop. During high-frequency repetitive transients, ILC may not settle fully but will not saturate, as load steps up and down push ILC in different directions. Figure 17 and Figure 18 show a simulation of this behavior:

Equation 23. τ L c = L C R D C R , L c + N t o t a l   ×   R D C R , s e c o n d a r y + R r o u t i n g
Low-frequency transient event.
fSW < 1 kHz
Figure 17 Low-frequency transient event.
High-frequency transient event.
fSW = 65 kHz
Figure 18 High-frequency transient event.

The voltage across the LC, ΔVLC, can exceed the input voltage, VIN, during a load step response. Assuming that a controller turns on NON phases in response to the load step, Equation 24 calculates ΔVLC:

Equation 24. Δ V L C m a x = N O N s t e p   ×   V I N - N T O T A L   ×   V O U T

Creepage is not generally a concern, as the high voltage is not sustained for a long period of time. But the high transient voltage across LC may be important to know for application safety and component reliability in some cases.