UCC2946/3946 Watchdog period

Hi Toshio,

In the Watchdog section of the Electrical Characteristics table we have the minimum Watchdog period for UCC2946 as 0.96s and UCC3946 as 1.12s.  10ms will not work with these parts.

Very Respectfully,

Ryan

Hello Ryan-san,

Thank you for your answer.

I think minimum Watchdog period for UCC2946 as 0.96s and UCC3946 as 1.12s are incase of 64nF capacitor.

TWP can program according to the equation “TWP(ms)=25 x CWP(nF)“ and CWP=TWP(ms)/25=10/25=0.4nF when TWP=10ms.

Please advice using CWP=0.4nF is OK, and acceptable minimum CWP value.

Best Regards.

Hi Toshio-san,

My apologies.  You are correct that the minimum period I stated is for a 64nF capacitor.

I have looked into the part further and have found that we did not characterize either device at such a low CWP capacitor.  I can bench some of the devices and reply to you with my findings next week; however, such a use will not be covered in the datasheet.

Very Respectfully,

Ryan

Hello Ryan-san,

Thank you for your reply.

Customer want to use TWP=10ms.

Q1: Question about TWP equation

I calculate CWP as follows according to the equation “TWP(ms)=25 x CWP(nF)“ to set TWP=10ms.

    CWP(nF)=TWP(ms)/25=10ms/25=0.4nF

According to the internal block diagram of datasheet, CWP is charged by 400nA current source and compare with 1.235V.

Calculated CWP voltage at 10ms is 10V as follows. 10V is around 10 times of 1.235V threshold and it does not much with 1.235V.

     Vcwp=I*t/C=400nA*10ms/0.4nF=10V

Please confirm if datasheet equation of CWP is correct.

Q2: Question about TWP=10ms

Pleas verify if TWP=10ms setting is acceptable by 13th Sep and advice your view.

Best Regards.

Hi Toshio-san,

I was able to bench UCC2946 and UCC3946 at room temperature with a few different CWP.  First I confirmed that the equation in the datasheet is correct.  Second I benched with CWP = 0.39nF and measured a typical value of 10ms for Twp.

Very Respectfully,

Ryan

Hello Ryan-san,

Thank you very much for your cooperation.

Can I hear your opinion to use 10ms setting for UCC2946 and UCC3946 , for example it should be OK because ……, even it can not be guaranteed.

Best Regards.

Hi Toshio-san,

I am basing my opinion on the bench work that I did.  I was able to produce a 10ms period by using a 0.39nF cap.  While the 10ms period is not specifically covered in the datasheet, I had no trouble setting the period.

Very Respectfully,

Ryan