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# LM63635-Q1: Switching frequency variation at Drop out mode

Part Number: LM63635-Q1

Hello Expert,

I have question for LM63635-Q1's switching frequency during drop out mode.

My understanding for the switching frequency during dropout mode @ nominal fsw=2.1MHz is as below.

I think we can divided 3 section from the view point of fsw as follows.

1. 2.1MHz = it is not in drop out mode
2. Variable(130kHz~2.1MHz) = it is in drop out mode but maintain drop out with in 1% during 130kHz~1.85MHz.
3. 130kHz = it is in drop out mode and Vout is almost same as Vin - Iout*RDSon and DCR

For section 2 and 3, Is there risk of happen larger frequency variation and fluctuation?
Our expectation is that the switching frequency is stable as long as Vin and Iout are stable even if device is in drop out mode.

Thank you and best regards,
Kazuki Kuramochi

• Hello

The frequency in drop-out will vary between 2.1MHz and the minimum value of frequency.

The minimum value of frequency will have a variation and can be calculated based on information in

in the data sheet.  The minimum frequency = 1/(Ton-max+Toff-min)

Using data sheet values we have a range of minimum frequency of: 99kHz to 183kHz, with a typical of about 142kHz.

I have attached the drop-out calculator tool for this device to help the customer with their calculations.

Thanks

5516.Drop_out_calc_revC.xlsx

• Hello Frank,