Dear Sirs,
in our radar sensor, we are currently experiencing some failures of your integrated DCDC converter, the LMZ10503.
The fact. Each radar has two digital boards; each of them mounts a LMZ10503 to produce a 3.3V rail for the internal peripherals.
The board #1 has a load of 340mA, and the board #2 has a load of 130mA; the boards are identical (I mean they have the same BOM), but the brd#1 provides also the 3.3V for an external module, so this explains the extra load current.
We have produced 384 pieces of digital boards, 192 mounted as brd #1, 192 mounted as brd#2 and we have experienced 15 failures only on the brd #2, due to the damage of LMZ10503.
The LMZ10503 mounted on board #2 (the one with 130mA) works in FCCM (Forced Continous Conduction Mode).
Although the synchronous buck converter allows the FCCM, my idea is the LMZ10503 with a lighter load is more stressed, perhaps due to the inductor current flowing in both directions (assuming as forward the direction of load current): to reinforce the correctness of this hypothesis, the LMZ mounted on board #1 has never failed yet.
Operating conditions. The LMZ10503 derives the 3.3V from a 5V input dc voltage (supplied from an external AC/DC converter); so the buck ripple current, with a duty cycle of 0.66, L=2.2µH, and fsw=1MHz, is:
From the equation above, the board #2 is surely functioning in FCCM, while the board #1 is weakly or not in FCCM.
Simulation. To demontrate the FCCM mode, I also performed a PSPICE simulation using the TI LMZ Pspice model; see the schematics and plot below:
Boards workaround. To limit the reworking intervention on hundreds of boards, I suggested -as praticable workaround- the introduction of a 15Ω resistor (a dummy load) on 3.3V rail, to port the overall load to 342mA, so equalizing the load on each board. Further, on brd#2, I have also increased the output capacitor value to 100µF to improve the filtering.
In the simulation, I implemented the workaround and I introduced a variable load to test the loop stability: see below:
Testing the workaround. As further step, to test the solution described, we applied the workaround on a batch of 10 radar, leaving them powered without interruption; after more of one month of continuous working the results was no LMZ has failed.
Here my questions for you:
- Could be the FCCM a reasonable explanation for the LMZ failures?
- Do you have any other explanation?
Thank you very much for any help
Marco Fogli