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LMG3422EVM-043: Schematic design questions.

Gautham Venkat Venkateswaran
Prodigy 150 points
Part Number: LMG3422EVM-043

Tool/software:

Hello,

I have 3 Questions:

1. Why are two different isolators used for the High and low side in the daughter card to drive the FETS?

2. Can you please help me understand why these nets are not connected? specifically pins 1, 16. I see the same in the PCB doc when I update teh changes from the schematic to the PCB doc.

3. I am not seeing the source nets for 5V_H and 5V_L. Is there something I am missing? Are we not violating the isolation if the source of the 5V is the same ground reference on primary and secondary side?

  • 0
    TI__Expert 3770 points

    Hello,

    Below are answers for your questions:
    1. The low-side device uses a start-up circuit to control slew-rate during start up when operating with bootstrap supply. This circuit requires input from the high-side FAULT signal which much pass through the low-side digital isolator. Therefore, the low-side digital isolator requires more input/output channels which is why a slightly different part number was chosen.
    For more information about this start-up circuit, please view our LMG341xR050 Datasheet section 9.2.2.1.1 Startup and Slew Rate with Bootstrap High-Side Supply 
    which details this information very clearly

    2. Pins 1 and 16 are No Connect (NC) pins on the corners of the QFN package to help anchor to PCB and improve Board Level Reliability (BLR).
     

    These pins are internally connected to DRAIN which is why they show up as drain pins on schematic symbol, but should be left floating. Here is a description from the datasheet:

    There are other NC pins on the device connected to different nets, please see the datasheet for more information

    3. LMG3422 has an LDO5V output pin to provide a 5V source for digital isolators or other components. The high and low side digital isolators are powered by their respective GaN FET LDOs therefore the isolation is maintained and no additional circuitry is required. Please see the datasheet for more information about how the LDO 5V output works

    Thanks,
    Zach S