N-Channel JFET in CCG1 Type-C to DisplayPort (DP) Cable Solution - KBA203779

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Question:

Why is a JFET (Q1:TF412S) required in the CCG1 Type-C to DisplayPort (DP) cable solution? What is the replacement part number for the JFET (Q1:TF412S)? Can we replace it with NMOS FET? Can we remove it from the design? 

 

Answer:

 

Why JFET is Required: 
The JFET in Type-C to DP cable solution is for the Ra connect/disconnect function. The primary purpose of the JFET is to limit the current being drawn by Ra when VCONN is detected. Once the JFET detects VCONN , CCG1 drives the RA_DISCONNECT pin LOW, turns OFF the JFET, and disconnects Ra, thus limiting the current consumption. If a JFET is present in a CCG1 Type-C to DisplayPort cable solution, it should be connected on VCONN in initial status, and disconnected after waiting for tCCDebounce (minimum 100 ms, maximum 200 ms) so that a channel JFET will always be enabled in the initial status when VGS =0V, and N channel JFET will be cut off when VGS < VGS (off) or VGS>0. 

Replacement Part: 
Q1 (TF412S) can be replaced with 2SK3796.

Cannot be Replaced with NMOS FET: 
The JFET in CCG1 Type-C to DisplayPort (DP) cable solution cannot be replaced with a NMOS FET because NMOS FETs cannot be cut off when VGS>0. 

Removing the JFET: 
The JFET in CCG1 Type-C-to-DisplayPort (DP) cable solution can be removed if you want to lower the design cost, and power consumption is not a high priority. The JFET (Q1:TF412S) and R1 (100K) can be removed; if you do so, you should change R2 from 806 Ohm to 1K Ohm.