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PF8200 input supply must sink current?

Question asked by adevries on Oct 3, 2019
Latest reply on Oct 21, 2019 by Jose Alberto Reyes Morales

Hello,

 

I noticed that the PF8200 datasheet was recently updated to include this note on Table 9: "Input supply for switching regulators must be capable to sink current to avoid overvoltage condition during the power down sequence of the device". My application has the PF8200 being supplied by a power supply that can only source current, so I'm trying to understand the implications of this note. Here are my initial questions:

 

1. The switching regulators on the PF8200 are all buck regulators, so how is it possible that powering down the PF8200 causes the regulator's input voltage to increase above what the input supply is providing the PF8200?

 

2. Given that my supply for the PF8200 can only source current, how high should I expect the voltage on SWxIN to rise? Higher than 5.5V?

 

3. If I were to install a device that limits this overvoltage condition, how much current would it need to be able to sink?

 

4. Can this voltage rise be mitigated by increasing the input capacitance? If so, how much additional capacitance is recommended? I am currently following the recommendation of 4.7uF per SWxIN pin.

 

5. Does this situation only occur if the PF8200 is commanded to power down the voltage rails? What if the input supply for the PF8200 simply turned off? Would the overvoltage condition happen then?

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