On the mobile part of our system, we need to convert 12V to 5V and 3.3V. Each conversion has its constraints:
- Both need to be able to start slowly enough in order to limit the inrush current from the 12V rail to a minimum.
- The 5V rail needs to withstand the switch of its output current from almost 0A to 9A in a few dozens of nanoseconds, corresponding to the activation of a new LED scan line.
- The 3.3V rail needs to be able to start quickly enough for the FPGA to startup properly.
I looked at the TI modules for our application. I chose the LMZ22005 for the 12V->3.3V@5A conversion, and the LMZ12010 for the 12V->5V@10A conversion because of the low count of external components required to make the design work. TI has an online tool (WEBECH Power Designer) allowing us to select a converter, choose the external components according to our output target voltage and current constraints and simulate its behavior (they have another downloadable program called Switcher Pro, but it is outdated and doesn’t have recent modules). It helped me confirm that the default 1.6ms of soft start time of the modules I chose was enough to limit the inrush current.
The second step is to check if we meet the FPGA requirements. The Cyclone V Device Handbook Volume I indicates the relationship between the rise time of the 3.3V rail and the Power On Reset (POR) delay. The Cyclone V Device Datasheet states that there are two POR delays that we can select: a fast one, between 4 and 12 ms and a standard one, between 100 and 300ms. In conclusion, the soft start time is quick enough for our FPGA to start properly.
Speaking of inrush current, we are in the worst case scenario for the FPGA inrush current at startup because we are powering all the banks with the same power rail (i.e. at the same time). But it translates to at most 2.92A for a maximum duration of 200µs (see table 10-1 of the Cyclone V Device Handbook Volume I), which is inside of our estimated power envelop.
Lastly, we need to be able to have very quick changes in current drawn on our 5V rail. The TI tool is a little buggy and depending on the component I choose in my design, the simulation can have voltage spikes (in the hundreds of volts, unrelated to the current being drawn). So I couldn’t simulate with higher output capacitor than the one selected by default (540µF), which would have probably reduced the output voltage variations (here: -4%, +2%). The current variation has been based on the rise and fall times of our LED drivers (TLC5957), which are respectively 40 and 16ns. The other interest of this simulation is the current actually used from the 12V rail: up to 12A.
However, I wonder if it wouldn’t be easier for the schematics to use the same module twice.