SM72445: AN-2241 SM72445 Evaluation Board

Hi William,

Sorry for the delayed response

The DNPs are for "Do Not Populate". Those are placeholders for components in case the user needs to change some the configuration of the board but are not used commonly. In the case of U7, PIN16, the OVS function of the SM72295 is not used so it is pulled down through R49. Another example are the decoupling capacitors for pin 14,16,18 and 20 of U8. They are usually not required but a placeholder has been added, just in case.

The main inductor L1 is a custom part for the design. The specifications are 20uH,50mohm DCR, 13A peak. Any inductor with similar performance should do.

I'm not sure about the chokes but they are not necessary for the basic operation of the application.

Best Regards,

Florent

I'm attaching the schematic for the EVM in PDF format and the spec. sheet for L1 as well:

SM72445_EVM_Schematic Prints.pdfPA2965_XXXrevP4.pdf

Thank you so much! This goes a long way in helping me complete my design.
One last quick question in reference to the "Do Not Populate": For connections such as U7, PIN 16, the only connection is R49 to ground. Am I assuming correctly? I just want to make sure I'm terminating fragmented "Do Not Populate" connections correctly.

William,

The pin you refer to (Pin 16 on device U7) is OVS which is the over voltage sense input. It uses a resistor divider (R49 as bottom resistor, R46 as top resistor for the voltage divider) to set the input threshold for over voltage protection. If you remove the R46 aka you "DNP" which means leave it unpopulated aka no component in that spot, the over voltage threshold will be the voltage at Vdd. If you want over voltage to be set at a different value, then you can change R49 and R46 as needed! Hope this helps!

Note: anytime you see "DNP", that leaves you the choice to have no component in that spot. These DNP spots usually are used for altering the thresholds for the protection or adding extra capacitors to help with stabilizing the voltages. Essentially they just add extra flexibility.

Best regards,

Michael

Michael, I appreciate your response but I think there is some confusion. The Eval board is shipped with R49 installed and R46 removed. I'm simply giving you an example of what I'm seeing on the SCH. It looks to me that OVS is pulled to GND if R46 is removed. What am I missing?

Pretty tough to find something exact for the main inductor spec. I reached out to CoilCraft to get somewhere close.  Doesn't match the 50mOhm spec but I think it will work. Can you look at the part number and select something that will work?

William,

You are correct. The OVS pin requires an external resistor divider. The only way the system will work with R46 removed is if VOUT, which is connected directly to OVS pin in the above schematic, is less than VDD. I recommend adding R46 resistor and scaling the resistor divider so that the system trips the over voltage protection when the middle voltage of the resistor divider (the net connected to OVS pin) exceeds VDD. This is the proper configuration for the overvoltage protection but can be modified to fit your application. When the OVS (overvoltage sense input) detects a voltage above VDD, the OVP (overvoltage indicator output) is brought low (due to the internal pull up).

-Michael

William,

It appears that any of those inductors should work. The main things you need to match are the inductor value and to make sure the current rating of the inductor is high enough for the current required for the application. If you are matching a 20uH, 13A peak inductor then any of those inductors should work. The inductor resistance becomes more important when efficiency or power dissipation is a concern or if the system is nearly unstable.

-Michael

I finally have a PCB completed for my design. To mitigate damage to the board I did not install the power FETS or the transformer. I did this so I can complete testing. My design follows the Eval reference design, the only difference is I designed in a separate +5 power supply for the CPU so I can monitor a few things.
I have a few questions:
- When I power up the board should I be seeing any signal activity on the gates of the FETS?
- If yes, what could be causing the lack of activity? The inverter is NOT in reset, I gave an LED on the PCB that will light up if it's attempting to reset the inverter IC's.

Quick question reference your Eval design:
- The Gate of Q7 is connected to RB1 of the PIC microcontroller yet nothing in your firmware controls B1. is this FET circuit doing anything?