SNVAA67 March   2023 LM5012 , LM5012-Q1 , LM5013 , LM5013-Q1

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2Overview of COT
  5. 3Line Regulation
  6. 4Load Regulation
  7. 548 VIN Design Example
  8. 6Summary
  9. 7References

2 Overview of COT

Figure 2-1 produces a regulated output voltage by using its feedback comparator to servo the output by comparing the sampled output voltage, VFB, and the reference voltage, VREF. The feedback comparator issues an on-time pulse, after it trips. This cycle continues at the programmed switching frequency for the set output voltage, dictated by the on-time (RRON) and feedback (RFBT, RFBB) resistors. Equation 1 and Equation 2 illustrate the necessary calculations.

Figure 2-1 LM5013 Functional Block Diagram
Equation 1. R R O N k Ω = V O U T V × 2500 F S W k H z
Equation 2. R F B B = V R E F V O U T - V R E F × R F B T

The feedback comparators terminal connected to the FB node must have sufficient ripple such that during the on-time, the FB node can be charged sufficiently above the reference voltage (VREF). This outcome is achieved with example ripple injection circuit Figure 2-2.

Figure 2-2 LM5013 Design With External Ripple Injection

The integrator (RA, CA) generates a voltage ramp , in-phase with the inductor current and is AC coupled with CB. By following Equation 3, Equation 4, and Equation 5 sufficient ramp amplitude is applied to the feedback node for stability and transient performance is optimized. Note, the recommended minimum ramp amplitude, 12 mV in the case of LM5013, is advised for the device's given hysteresis, along with margin for reduced sensitivity to noise. Please refer to the LM5013-Q1 Automotive 100-V Input, 3.5-A Non-Synchronous Buck DC/DC Converter with Ultra-low IQ data sheet for the recommended value and further clarification of terms. Additional theory on this design is provide in the Stability Analysis and Design of COT Type-3 Ripple Circuit application note.

Equation 3. C A ≥ 10 F S W × R F B T | | R F B B
Equation 4. V R A M P = V I N - V O U T ×   t O N R A C A
Equation 5. C B ≥ t T R 3 × R F B T