Other Parts Discussed in Thread: UCD9222, UCD7242
Here is a description of the problem we’re facing on our board.
- The board (reference 01BRD20209-105 for the latest revision) is quite complex but the issue we’re talking is located on the “DSP” of the board, based on a TI TMS320C6655.
- This DSP requires a specific powering scheme for its core with two separated rail: CVDD and 1.0V. CVDD must be dynamically set to a specific value between 0.9V and 1.1V suing 4 signals coming from the DSP itself. TI requires this behavior in order to adapt the powering to each specific part.
- In order to achieve this behavior, we almost copied the reference design from TI (C6657EVMLite) using UCD9222RGZ + UCD7242RSJ chips.
- The UCD9222 chip is a programmable controller on which we used the same programming file as TI does on the EVMLite. We just had to make an update, as told by TI support, to enable the automatic voltage setting since the original file provided by TI disabled the feature on the EVMLite due to an hardware bug (it’s a bit surprising!).
- Remark: we used this reference design for CC6657 which is a dual core. So the power supplies are over-rated for a single core device such as C6655.
- These controllers are enabled/disabled by a PLD on the board that acts as the power sequencing master for all the processors on the board. This component, upon power-up, starts all the DC/DC converters of the board (a lot) and check for the Power-Good signals coming from all rails. If any of them fail the PD start the power-down sequence of all rails and then try to start again.
- During all the prototyping tests of the board we never had issues with the design. The problems appeared later during production with a few percent of the boards failing (out of several thousands).
- Description of the failure:
- The board is cycling continuously ON and OFF because a power-good signal is not OK.
- When investigating, the power-good signal is always on the two signals from the UCD9222 (it’s a dual controller for each rail of the DSP’s core).
- When further investigating (by connecting an I2C probe on the chip for interrogation during power-up), it seems that the UC9222 is triggering an “medium overcurrent” on CVDD rail fault when trying to ramp-up its voltage and we can see at the scope that the voltage ramp up to 0.85V then stop. If we try to make a “hot” restart of the chip using the I2C interface, the fault flag is triggered in the same way.
- We can’t measure a true short-circuit on the failing rail.
- The problem is always triggered at power-up, never on a running board.
- Temperature and time between ON and OFF events have no influence on the problem.
- We can see, on failing boards, that the static impedance on both rails seems to be lower:
- On a good board, R(1.0V) is about 64 ohms. On a failed board its lower than 40 ohms
- On a good board, R(CVDD) is about 20 ohms. On a failed board its lower than 10 ohms
- When replacing the power part (UCD7242) sometimes it solves the problem and sometimes not!
- On failed boards, PSI Control made some X-Ray analysis and we never detect soldering issues
- I checked recently the status of all documentation from TI (errata sheets, datasheets, manuals, application notes, ..) and nothing has changed since the beginning of the design (2015).
As you can understand, it’s a tricky issue where we try a lot of things. Everything seems located between the DSP itself and the two power chips.
We can share more information (schematic and PCB design)