4 Turning point 3~Fully PFM entering point: Analysis of LM5190 PFM/PWM Hybrid Mode Operation

When the load current is less than the current of Turning point 3, the LM5190 will enter a state of mixed PFM and PWM operation. From the previous content, we know that the logic for turning off the high-side switch in PFM is when the COMP voltage is lower than the clamp value, and the turn-on logic is when the output voltage is lower than a certain threshold. In actual testing, this threshold for the LM5190 is 0.99 times the set output voltage. And the turn-on logic of PWM is the CLK signal high level, and the turn-off logic is when the inductor current reaches the peak.

At this time, the typical working state of LM5190 is shown in Figure 4-1. The load current is 1.407A. It can be clearly seen that there are turn-ons during the output voltage drop process when it is lower than about 12V, such as the turn-on point marked by the white dashed line in the figure. This turn-on logic is typical PFM turn-on, which did not appear before Turning point 3, as shown in Figure 3-2 - Figure 3-5. There are also turn-ons when the output voltage is already in a rising state, which does not meet the PFM turn-on logic. This turn-on logic is typical PWM turn-on, caused by the arrival of the rising edge of CLK.

When the load current is 0.991A, the phenomenon where PWM and PFM control logic work together is more obvious, as shown in Figure 4-2. Observing its output voltage and inductor current, it can be seen that LM5190 clearly has two turn-on logics at this time. As mentioned before, the one marked by the white dashed line in Figure 4-2 is PFM turn-on, and the one marked by the red dashed line is PWM turn-on.