10MEG resistor to set switching frequency of Buck?

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cupoftea

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Hi,
We are doing a 214kHz Buck with 24vin and 15vout at 3a(out).(as attached in LTspice)
Using the LTC3878.
-Though it needs a 10 Meg resistor into its RON pin in order to give the 214kHz.
That means a current of just 2.3uA into the RON pin of the LTC3878.
Surely using a 10 MEG resistor to set the switching frequency is asking for trouble? (with noise)

LTC3878 datasheet
 

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Thanks, i also doubt that there is anything in that chip that can detect the 2.3uA.
 

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Hi,

I don´t see a problem in detecting 2.3uA. To keep noise low, just make the node as small as possible.
--> place R9 close to the IC pin.

In the datasheet there is a chart "On-Time vs I_ON Current" ... which shows I_ON go down to 1uA


Klaus
 
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I see you haven't mastered how to simulate parasitics.

My advice was a result of verifying my advice in LTspice. The consequences increase 50 mVpp output ripple on Vout-15 or worse , overshoot, freq. and damping factors by changing loop gains from +ve or -ve x pF feedback to Ion from each nearby signal.

If you can imagine it, you can simulate it.
Tony
 
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