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I am using this Buck Converter 2A part.

I am giving an input voltage of 3.3V to get an output of 1.2V for a switching frequency of 2.25MHz.

I am giving different load currents of 0A, 0.2A, 0.5A & 1A. But why is my switching frequency like below:

For 0A,

enter image description here

For 0.2A,

enter image description here

For 0.5A,

enter image description here

For 1A,

Why is my switching frequency like that? I am measuring the switching frequency at the SW pin of the IC (before the inductor). enter image description here

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1 Answer 1

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The chip reduces PWM frequency when load gets smaller, and if configured to do so, it switches from PWM mode to PFM mode on very light loads.

See datasheet chapter 9.4.2 for reference.

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  • \$\begingroup\$ Just a comment- You can force fixed-frequency PWM operation by pulling the MODE pin high. Of course that comes at the expense of light-load efficiency. \$\endgroup\$
    – John D
    Aug 20, 2021 at 15:24
  • \$\begingroup\$ Thank you. But I have tied the COMP/FSET to ground to get a required switching frequency of 2.25MHz. So, I should able to see the 2.25MHz at the SW pin of the IC, right? Why am I not able to see it from light load as well as 1A (heavy load) ? \$\endgroup\$
    – user220456
    Aug 20, 2021 at 16:08
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    \$\begingroup\$ The IC only uses the PWM value set on the COMP/FSET pin when it is in forced PWM mode (by pulling the mode pin high). Otherwise it adjusts the switching frequency to maximize efficiency regardless of the value you set. \$\endgroup\$
    – Ryan
    Aug 20, 2021 at 17:42

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