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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

## Top Replies

• Hi,

See snip below from the datasheet on page 7. The resistor would be between INTVCC and FREQ pins to reduce the frequency to lower than 600 kHz (which is the internal switching frequency when no external resistor used). The Rfset formula is when the resistor is between FREQ and SGND pins. Your interpretation about the synchronization is correct.

Regards,

Jhun

• Thanks, Jhun - the formula for reducing the frequency is not part of the datasheet. Is it possible to describe it here (and ideally in the next revision of the datasheet too?)

best,
Graeme

• Hi Graeme,

Please download the LTpowerCAD model of LTM4638: LTM4638 Datasheet and Product Info | Analog Devices

On the model, you can set your desired frequency and powerCAD will give suggested resistor values for RFSET1 and RFSET2. 