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

4.2 Load Transient Step-Up

Figure 13 and Figure 14 show a simulated comparison between a multiphase buck converter and a TLVR design under the same load step-up condition. Table 2 summarizes the simulation parameters. These are closed-loop simulations using the TI TPS536C9T DCAP+™ constant on-time controller.

A few observations about Figure 13 and Figure 14:

  • The TLVR design responds to the transient (ISUM catches up to ILOAD) much more quickly because the ISUM rises at a faster rate. As a consequence, the output voltage deviation is significantly lower.
  • During the transient response, the multiphase buck converter design required many more pulses to respond than the TLVR design, meaning that the TLVR design delivers more energy per pulse during the transient event.
  • Given the nature of constant-on-time control, pulses overlapped during the transient response. The LC voltage increased to a level significantly higher than the input voltage during pulse overlap operation, then returned to normal operation at steady state.
Multiphase buck converter.Figure 13 Multiphase buck converter.
TLVR.Figure 14 TLVR.
Table 2 Simulation parameters for transient load step-up and step-down examples.
Parameter Description Value
VIN Input voltage 12 V
VOUT Output voltage 0.8 V
NTOTAL Total operating phase number 4 phases
fSW Switching frequency per phase 600 kHz
ISTEP Load step size 25 A to 325 A, instantaneous
LM/LBUCK Magnetizing inductance LM for TLVR, filter inductor LBUCK for buck 150 nH/150 nH
LC LC value for TLVR 180 nH
COUT Output capacitance 5.0 mF, idealized

Following the relationships described in the Steady-State Operation section, it is evident why the TLVR is able to ramp its ISUM up more quickly than the buck converter, and why its transient response was superior.

ISUM for the buck converter is simply the sum of its individual inductor currents, as shown in Equation 13. For the TLVR design, ILC gets added once for each phase, in addition to each magnetizing current (ILM), as shown in Equation 14:

Equation 13. I S U M ( b u c k ) = I L 1 + I L 2 + …
Equation 14. I S U M T L V R = I P R I 1 + I P R I 2 + … = I L m 1 + I L c + I L m 2 + I L c + …

All inductors in the system follow the fundamental inductor relationship. During the transient response to the load step-up, the converter turns on NON phases simultaneously. For various reasons, it may not be possible to turn on all phases at once, so also consider that NOFF phases remain off at any one time. Equation 15 and Equation 16 show the rising ISUM slope for the multiphase buck converter. These equations do not account for the controller response time, but show only the limitation from the converter topology.

Equation 15. ↑ S l o p e ( b u c k ) = Δ V L 1 L + Δ V L 2 L + …
Equation 16. ↑ S l o p e ( b u c k ) ≅ N O N V I N - V O U T L - N O F F V O U T L

Equation 17 and Equation 18 show the rising ISUM slope for the TLVR design, assuming that the TLVR magnetizing inductance LM was equal to the buck filter inductor L for comparison purposes:

Equation 17. ↑ S l o p e ( T L V R ) = Δ V L 1 L M + Δ V L C L C + Δ V L 2 L M + Δ V L c L C + …
Equation 18. ↑ S l o p e T L V R ≅ ↑ S l o p e b u c k + N T O T A L   × N O N   ×   V I N - N T O T A L   ×   V O U T L C

Written in this way, the additional terms clearly show the influence of ILC in enabling the TLVR design to respond more quickly to transients than a traditional multiphase buck design.