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I am looking at the datesheet for a buck regulator AP6320x. There is one variant the AP63201 that states,

Pulse Width Modulation (PWM) Regardless of Output Load
o AP63201

Supports Pulse Frequency Modulation (PFM) o AP63200, AP63203, and AP63205
o Up to 80% Efficiency at 1mA Light Load
o Up to 88% Efficiency at 5mA Light Loads

I am wondering in what circumstance you would want to choose the AP63201, given the part price is the same. Why would you want continuous conduction mode at low load, and what are the trade offs/benefits of it vs switching to PFM?

There is the following text on page 10,

... As the load current approaches zero, the AP63200, AP63203, and AP63205 enter Pulse Frequency Modulation (PFM) to increase the converter power efficiency at light load conditions. The AP63201 remains in continuous conduction mode at light load conditions. When the inductor current decreases to zero, zero-cross detection circuitry on the low-side power MOSFET, Q2, forces it off until the beginning of the next switching cycle. The buck converter does not sink current from the output when the output load is light and while the device is in PFM. Because the AP63200, AP63203, and AP63205 work in PFM during light load conditions, they can achieve power efficiency of up to 88% at a 5mA load condition

datasheet

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Your switcher has an energy storage device (an inductor) that you are charging and discharging on every cycle. It has to be large enough to handle your heaviest load at your PWM frequency; each cycle you are storing a small packet of energy and passing it to the load.

When your load becomes light, if you don't change frequency you are switching much more often than is necessary and passing along very small packets of energy compared to the amount of energy that could be stored in your inductor. By reducing the frequency, you are utilizing the full storage capacity of your inductor but not experiencing the inefficiency of switching as often, so efficiency goes up. So in applications where the load can be light and doesn't change suddenly, it might be a choice. However, if you have a rapidly varying load or variation in input voltage, the low frequency switching combined with the control loop that gets you in and out of PWM mode will lead to variations when you suddenly pick up or lose a heavy load or drop or increase your input.

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    \$\begingroup\$ Also some applications don't want the switching frequency to fall into the audio band where you can get piezoelectric "singing" caps or inductors making audible noise. There are sometimes even EMI reasons for not wanting the switching frequency to fall into a particular band. \$\endgroup\$
    – John D
    Apr 28, 2021 at 17:39
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    \$\begingroup\$ Sometimes there's even EMI reasons for wanting the switching frequency to be dead steady, or to vary in a predictable way. Having it suddenly sweeping up and down would Not be Good if there's sensitive receiving equipment involved. \$\endgroup\$
    – TimWescott
    Apr 28, 2021 at 17:57

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