Reverse Current Protection of MCP5515

DC-DC Power Converters
1

Hi,
We are evaluating MP5515 for one of our application.
In MP5515 datasheet, There is a section on RCP on page 19,
“The ISOFET circuit applies reverse-current protection (RCP) when energy is released from the storage capacitors to VB. Typically, a
250mA reverse current from VB to VIN shuts down the ISOFET.”

I have few doubts here,

  1. Unless there is a Reverse current of 250mA flowing from VB to VIN, whether ISOFET won’t be OFF, i.e. the RCP functionality won’t kick in.
    Because, if it is the case, then I need to allow 250mA of current on VIN side, for RCP to kick in.

  2. ISOFET looks like Back-to-Back MOSFET so, when it is OFF there won’t be Forward/Reverse current flow. Why MP5515 will wait until it see 250mA Back current flow to Turn OFF the ISOFET. Am i missing something on this?

Thanks.

2

Can anybody check this question and answer it.

3

Well if the FET is ON but no current is flowing through it, what exactly is the problem?
I tend to agree on some level, if they are discharging the storage cap they have enough reason already to turn prevent reverse current into Vin. So it would seem a harmless move to disable the input fet in the power fail condition. What if you had a 200mA load on the Vin? That would shorten the backup time for no good reason. In the end that can all be compensated for with a larger storage cap.

4

As datasheet description “The ISOFET circuit applies reverse-current protection (RCP) when energy is released from the storage capacitors to VB. Typically, a 250mA reverse current from VB to VIN shuts down the ISOFET.” It happens when PFI = LOW. PFI is pulled down if SAS, OTP, or EN is off. Once PFI = LOW, a typical 250mA current from VB to VIN can trigger reverse-current protection. Besides, IC owns the second protection named as reverse fast off, which can limit the current boundary.