...
The only problem with this chip, is that the switching frequency is fixed, that may cause some problems when selecting Inductors, luckily, after calculation I can source pretty good inductors based on the fixed frequency, so in the end I chose the MP2491CGQB-Z chip
EN Current Calculation:
R_Pullup +3kOhm (internal pullup) = (24V-7V)/I_EN (7Volt is the internal Zener Diode)
...
In this case, R_Pullup=220kR, so I_EN=62.7uA, smaller than the 100uA, which is good.
Inductor
From the data sheet the design requirements of the inductors is that:
...
I have already set V_FB=0.5V by connecting VSEL_1 and VSEL_2 to VCC
...
Updated (10/3): to compensate for the DCR losses, the voltage aimed to be regulated is changed to 5.1V
In this case, R1/R2=9.2, R2=10kOhms, R1=92kOhms, the library values I found are 10k and 91k.
System Block Diagram
...
Reverse Polarity Protection
...
RPP IC selection:
...
RPP diagram:
...
PCB Layout
Some guidelines in the datasheet of the Buck IC:
...
Placement
Initial placement:
...
Kevin’s feedback are as follows:
...
Some notes:
1. Top priority, input caps and output caps, Buck IC, and RPP circuit, then everything else;
2. the distance between the inductor and the switch node pin should be as close as possible;
3. the path for high current traces should be as large as possible (use polygon rather than traces for high current lines)
4. can move the RPP circuit down to leave more room for the BUCK IC
Updated Placements (From 9-30 to 10-2)
...
Routing
...
PDN Analysis
Goal is to calculate loop impedance for the following:
VBUS: Input connector -> NMOS, NMOS -> Buck IC
SW: Buck IC -> Inductor
5V: Inductor -> Pixhawk Conn, Inductor -> USB-C Conn, also do Inductor to farthest decoupling cap down (basically before the via punch down to L4).
Used the Power Analyzer by Keysight tool, not able to calculate the loop impedance directly, use the voltage drop and current simulated to calculated loop impedance.
...
Pixhawk Wiring
The Pixhawk connector that I find is the 5024430670 Molex Clickmate connector, linked below:
https://www.digikey.ca/en/products/detail/molex/5024430670/2380429
The link below is the pixhawk wiring standard
...
The wiring harness that WARG use is 1:1 harness, as pictured, the red cable indicating pin 1
...
Comparing the cable with the pin placements on my PCB shows that it is correct
...
...