UCD3138ALLCEVM150: Fault Pins Assignment in LLC EVM Firmware

Hello Yedida,

The code is used as a backdoor hardware reset for the UCD3138A to go into ROM mode. Please see the following E2E posts that explain the code in more detail: 

• https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/780146/ccs-ucd3138-what-does-pmbus_read_write_rom_mode-pmbus_write-mean-in-main-c
• https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1011250/ucd3138a-how-to-start-a-project-from-llchbfirmware-1-2 

I do not believe we have a good in-depth walkthrough of the LLC-HB firmware. We do have good documentation of the PFC EVM firmware. The PFC firmware is much more complicated than the LLC-HB firmware. So, if you can understand the PFC firmware, then understanding the LLC-HB firmware will be straight-forward as long as you know the LLC design fundamentals. The EVM User's Guide has a good firmware overview in section 12.4 on page 37 (EVM Firmware - Introduction). Many of the architecture ideas can be transferred to the LLC.

PFC Firmware Documentation:

• Using the UCD3138PFCEVM-026 (https://www.ti.com/lit/ug/sluu885b/sluu885b.pdf) 
• Evaluating UCD3138 in Interleave PFC or Bridgeless PFC Configurations using UCD3138PFCEVM-026 (https://www.ti.com/lit/an/slua677/slua677.pdf) 
• Designing a UCD3138 Controlled Bridgeless PFC (https://www.ti.com/lit/an/slua713/slua713.pdf)
• Designing a UCD3138 Controlled Interleaved PFC (https://www.ti.com/lit/an/slua712/slua712.pdf)
• Designing a UCD3138 Controlled Single Phase PFC (https://www.ti.com/lit/an/slua708/slua708.pdf) 
• UCD3138 PFC Tuning (https://www.ti.com/lit/an/slua709/slua709.pdf) 
• PFC THD Reduction and Efficiency Improvement by ZVS or Valley Switching (https://www.ti.com/lit/an/slua644/slua644.pdf) 

It may also be helpful to look at other Half Bridge LLC reference designs. We have 3. The LLC-HB EVM used to be a reference design before becoming an EVM so there is also a reference design test report available.

• PMP40586 1-kW digital-controlled current mode LLC reference design (https://www.ti.com/tool/PMP40586) 
• PMP20978 High Efficiency and Power Density 1kW Resonant Converter Reference Design with High Voltage GaN FET (https://www.ti.com/tool/PMP20978) 
• TIDA-00512 340W Digital Controlled LLC Resonant Half-Bridge DC-to-DC Power Conversion Reference Design (https://www.ti.com/tool/TIDA-00512)

Regards,

Jonathan Wong

Hello Yedida,

Reposting code below:

//Check to see if FAULT2 is pulled high and FAULT0 is pulled low.
//If they are go to ROM.
if (MiscAnalogRegs.GLBIOREAD.bit.FAULT2_IO_READ && !MiscAnalogRegs.GLBIOREAD.bit.FAULT3_IO_READ)
{
pmbus_read_write_rom_mode(PMBUS_WRITE);
}

Yes, the comment is a typo. The comment should say FAULT3 instead of FAULT0. This code was taken from the Open Loop EVM board and the FAULT0 pin is used for the hardware backdoor. 

Regarding the intention of the FAULT2 pin, including the FAULT2 is meant to be a robust hardware backdoor. If your UCD3138 is locked into flash mode, then you want some sort of hardware backdoor that will switch the UCD3138 back into ROM mode. This will be purposely driving a pin high or low. This hardware backdoor must also be the FIRST thing that is run in the main loop. Thus, as long as you restart the power (power cycle) to the UCD3138, then you know that the hardware backdoor code will run. Notice how the code is one of the first commands in the main loop. 

However, we do not want this backdoor to be set off accidentally. Thus, a robust hardware backdoor must be only triggered in such a way that the user must have intentionally pulled signals high or low. Having two separate pins be required to do this is one way, which is why both the FAULT2 and FAULT3 pins are considered for a backdoor. 

Because this code is the one of the first commands, it will not affect the rest of the code. After the code reads the value of the FAULT2 pin, the firmware will go into the main loop, where the FAULT2 pin may or may not be driving a FET. So, this code doesn't affect the Q5 function. 

FAULT2 was chosen as the second pin in the hardware backdoor because, by the nature of the schematic, you will need to intentionally drive it low. If your UCD is locked up, then you can take a 3.3V external supply, solder a jumper wire and supply voltage to FAULT2 while shorting FAULT3 to ground. This combination of actions is very unlikely to occur just on accident. So, it serves as a robust backdoor. If you have no other option, then this is what you can do to unlock your UCD3138.

Regards,

Jonathan Wong