Other Parts Discussed in Thread: TPS55288, UCC28782, LMG2610
Hello:
In the EVM mentioned above, there is a series pass MOSFET controlled by WT6636 from Weltrend.
For many reasons, our configuration is different- and my proposed approach to connecting the USB C PD sink is NOT to use that series pass MOSFET- because it is superfluous when you are using TPS55288 as the interface. This assumes different programming of WT6636 of course.
I do fear that I might not have understood all the needs of the series pass although I do not see anything arising out of the USB C PD Specifications.
Here are the scenarios one can postulate with TPS55288 connection controlled by WT6636 with or without series pass MOSFET
#1 Opon AC connection, UCC28782 module in our application, turns on for an output of 24V. That is how UCC28782 is set up in our application.
#2 TPS5588 remains OFF since I2C is not active. VREF is 0, VBUS is 0, which is VSafe0 on the USB C PD sink side.
If a sink device is connected, there is no conflict even if a series pass was present. In this state, WT6636 is not turning the switch ON anyways. If the sink has a pre-existing voltage, the series pass body diode will make the switch state ON/OFF IRRELEVANT.
#3 WT6636 does not control our UCC28782 module. . But, Vout of ACF is connected to bias the WT6636.
WT6636 is programmed to run through C1-C2 upon being biased up, its algorithm runs through USB C requirements of the Startup dialog.
# 4 VBUS may be changed to Vsafe5V if the connection dialog allows it to do so. WT6636 controls TPS5588 : so it can be programmed to put VBUS at Vsafe5. Again, the extra switch contributes nothing in this state that TPS55288 output cannot totally satisfy. No matter what the sink device is.
When WT6636 would have decided to turn on the switch to apply VBUS on the device, the same function is quite eminently being done by its control of the TPS55288.
#5 On the other hand, WT6636 has no control if the sink device is higher than 5V upon connetion per clause #2. With TPS55288 connected, this poses no problem whatsoever: WT6636 in fact can detect VBUS condition before setting TPS55288 to output any voltage, yet it can make VBUS be at any sink device pre-existing value by keeping TP55288 OFF. VOUT pin can have any value: 24V at the input keepd upper TPS55288 switch body diode isolated the two. WT6636 cannot: if it was controlling UCC28782.
#6 Upon Hard-reset- which is essentially described as a software-related reset although there are state changes: this change the series pass switch does not help at all in any manner given its body diode. Whatever software indicates the state should be, controlling it by TPS55288 is far more effective including "discharge" because a "0" at VOUT of TPS55288 will discharge the sink capacitor as effectively as the switch.
#7 Finally, this setup with TPS55288 offers far better and more effective ramping of VBUS than control of the series pass switch which may be destroyed as a linear device in this mode.
SO WHY THIS SWITCH? What specific item in the USB C Spec implies its use?
It is a question of the cost of having TPS55288 and a less efficient module, prone to failures vs. one that is robust by design. Especially if you are going for higher power level applications of 65W and up.
We do require a different programmed WT6636 of course. Or prefer one that we can program ...one from TI as a replacement.
Using LMG2610 instead of ~20 parts discrete GaN required, is a significant reason to go for our approach: ours are expected not to be fail-prone, repairable..consistent with current trends.
Any comments will be highly appreciated.
-r