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# TPS51218: Help with Preset Switching Frequency

Part Number: TPS51218

I have a customer asking about using Rrf values different from those shown in Table 1 in the DS.

The DS specifies 4 different resistances to achieve 4 different switching frequency "presets"

Would changing the Rrf to GND resistor to a value not in the table, change the switching frequency to a value not in the table?

IE, Table 1 says 470kΩ = Fsw of 290kHz, then 200kΩ = Fsw of 340kHz, etc...

Would a resistor value of 330kΩ for example, still have a Fsw of 290kHz, or 340kHz? or would it be different?

Also, page 4 of the DS shows for Fsw, that with a 470kΩ resistor, Fsw = 266kHz to 314kHz....But this was measured at 25℃...so is there that much discrepancy in the Fsw setting?

In short, could you clarify what the "preset" definition is, and what happens if a different resistor value is used that those listed in Table 1?

Thanks!

Darren

• Hi, As you may know this is an old part, sorry I checked we have no related data with different RF resistors, can you just recommend customer to follow the table to make it? I think the preset table value should be verified a lot. and also as you know, Dcap mode is with pseudo-constant frequency, which is related to VIN/Vout, so some variation happen is normal.

Yuchang

• Hi Yuchang,

Let me try again.

The datasheet says:

"The first 250 μs calibrates the switching frequency setting resistance attached at RF to GND and stores the switching frequency code in internal registers."

My question is, does the RF input have some kind of comparator setup that looks at voltage at the pin, and stores the comparator result in internal registers (4 options means 2 bits? 00, 01, 10, 11) which is what is used to decide the switching frequency?

If you can't find this info for this device (it is old), many other devices have similar tables where Rrf and Fsw are given. Are these inputs comparators as well, to store the result?

Regards,

Darren

• Darren, I know what you mean, but so far I have no files to confirm this, I will try to check with design team, maybe tomorrow or later I can give some feedback. before this, please just follow the table to do it.

Yuchang

• Darren,

I asked design team help to check the circuit, it is exactly similar like you said, there are only four value setting internally. and from the typical compare reference value, 10% variation of the RRF table should be safe to get the frequency we want, such as if you select 450k 1% resistor, the frequency will be 290k. but if you select 300k, maybe some part will be 340kHz and some part will be 290kHz. so please just recommend to choose the 1% resistors closer to the RRF recommendation value, if a range needed, recommend < +/-10% RRF table.

Yuchang

• Yuchang,

Absolutely perfect. I know it was a little extra effort on your part, but this confirmation is extremely apprecaited.

Thank you so much!

Regards,

Darren