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# LTM4638 - Minimum switching frequency, free-running and with external sync

I am looking at pg. 12 of the LTM4638 datasheet:

There is a formula relating RFSET to free-running switching frequency. The claimed range (sentence below) is 400 kHz to 3 MHz. I think this is incorrect:

• The lowest possible switching frequency, according to the formula, uses an open (infinite) RFSET resistor. However, when I calculate the resulting switching frequency, it is a minimum of 609 kHz. (It is physically impossible to produce a higher parallel resistance than 274k!)
• The paragraph above states that the default operating frequency is 600 kHz.

Assuming, then, that 400 kHz is a typo - I think the lowest free-running switching frequency is 609 kHz. Can you confirm this interpretation?

If correct, then: assume we are operating with a 609 kHz oscillator and are now supplying an external clock at CLKIN. This clock must satisfy the +/-30% constraint described in the datasheet - hence, the part cannot be synchronized below about 0.7 * 609 kHz = 425 kHz. Can you also confirm this interpretation?

thanks,
Graeme