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

4 Load Regulation

Load regulation is often a concern as well in buck regulators. Typically, regulators have a light-load mode to make sure the light load efficiency is optimized. The implementations can differ, though, in the case of LM5013 the switching frequency decreases with reduced loading. This point is illustrated in Figure 4-1. The reduced frequency leads to the average feedback voltage reducing. Similarly to the previous discussion, this leads to output variance. Figure 4-2 shows how a 12-V output decreases with reduced switching frequency.

Figure 4-1 Light-load, Frequency Reduction Impact on VFB(AVG)
Figure 4-2 Light-load, Frequency Reduction Impact on VOUT Regulation

This behavior can be avoided by changing the inductor so that the inductor current does not go discontinuous. The point at which the inductor current goes discontinuous is often the entry point for light-load mode in the case of LM5013. Alternatively, a minimum load can be applied at the output to make sure that the inductor current does not go discontinuous. Equation 6 demonstrates the minimum inductance (LOUT) to make sure the inductor current does not go discontinuous at the application's minimum load current (IOUT,min) with the consequence of (potential) reduced efficiency.

Equation 6. L O U T ≥ V O U T 2 × I O U T , m i n × F S W × 1 - V O U T V I N

If it is not possible to manipulate either the inductance, or application's minimum load current, then an alternative device with FPWM needs to be considered such as LMR38020-Q1 4.2-V to 80-V, 2-A Automotive Synchronous SIMPLE SWITCHER® Power Converter with 40-µA IQ data sheet. By operating in FPWM, the device's average feedback voltage will not be impacted as much by loading, as in the case of most devices who employ a light-load mode.