LM3671TL-1.8EV: PWM to PFM mode transition

Hi Juri,

it's my pleasure to support you.

I think there is a typo in what you calculated. Let's start from the beginning. The datasheet states:

"The device automatically transitions into PFM mode when either of two conditions occurs for a duration of 32 or more clock cycles:

1. The NFET " (LOW SIDE MOSFET) "current reaches zero.

2. The peak PMOS" (HIGH SIDE MOSFET) "switch current drops below the IMODE level, (Typically IMODE < 30 mA + VIN/42 Ω)"

So, depending on the Inductor current's ripple, one might meet the condition 1 or 2 first while the output current decreases. Furthermore IMODE=116 mA circa, not 180mA. On the next page, by the way:

"If the load current should increase during PFM mode (see Figure 31) causing the output voltage to fall below the low2 PFM threshold, the device will automatically transition into fixed-frequency PWM mode."

So  the condition to exit PFM is stated, and it clearly depends not on the current alon, but also on the output capacitor (which determines the voltage drop to kick PWM operation in). As you said, on the same paragraph you can find the load current thresholds estimation:

"when VIN = 3.6 V, PWM to PFM transition happens at approximately 50 mA and PFM to PWM transition happens at approximately 100 mA".

Given the dependency of the threshold currents from the Inductor current's ripple and output capacitor, this estimation is clearly made for a specific application case and reported just to give an idea of how the thresholds can change with the input voltage.