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TPS546C20A: TPS546C20A:

user1932012
Intellectual 750 points
Part Number: TPS546C20A

1,Master separate test output 5V-35A is OK

2,When the staggered output current reaches 42 A, the 5V output voltage fluctuates greatly. The slave output driver of the product has an exception (duty accounts for a very small driver), and the main output driver is normal, such as attachment.6518.12V-5V D2D.pdftps456c20a-42a.docx

  • 0
    TI__Guru**** 191445 points

    User,

    We will look into this.  Can you share your PCB layout?

  • 0
    TI__Guru 56795 points

    User1932012,

    The TPS546C20A has a high-side Over Current Protection circuit that monitors the voltage drop from AVIN to SW during the switch-node on-time and terminates the switching pulse on SW for the remainder of the switching period if it detects excessive drop from AVIN to SW.

    Check the voltage on AVIN of the slave device, particularly the ripple on AVIN compared with the timing of the SW node and the PVIN node.  It is likely that AVIN at the IC pin is not dropping as much as PVIN is dropping at the IC pin, and that the slave device is triggering a high-side OC protection, terminating the PWM pulse early and causing the output voltage to drop.

    if that is the case, you may need to either adjust your layout so that AVIN and PVIN track closer together during switching, or add a resistor from PVIN to AVIN, sized large enough to create a drop from PVIN to AVIN that is equal to or greater than the ripple on PVIN.

    It should also be noted that, in addition to the main 22uF capacitors from PVIN to PGND, it is recommended that designs include a 2.2-10nF 0402 bypass capacitor from PVIN to PGND to absorb the sudden PVIN peak current draw when the switch node transitions from 0V to VIN with lower inductance than is generally provided with the larger, higher capacitance capacitors.  Lacking this high-frequency bypassing could be contributing to the early termination of the Switching cycles due to the high-side over current protection.

  • 0 in reply to user1932012
    TI__Guru 56795 points

    The layout shows AVIN connecting to PVIN on the top layer before PVIN follows 2 internal layers to 2 bypass capacitors, but it looks like the VIN trace passes through a resistor before the AVIN to ground bypass capacitor.

    This could cause what I described earlier with premature triggering of the high-side short circuit protection.

    A quick check to confirm this would be changing the AVIN R-C network from the current values to a 20-ohm resistor + 1uF capacitor.  That would use the input current into AVIN to create a voltage drop so that AVIN is lower than PVIN and less likely to trigger the short circuit protection.

    However, I am also concerned about there only being 2 PVIN to GND bypass capacitors on a 12V to 5V @ 35A / phase design.  With 5V @ 35A/.phase (175W/phase) from a 12V input, there is going to be a lot of RMS ripple current on the input, and that high RMS ripple current could over-stress the input capacitors.