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TPS544C20: Circuit and PCB design for TPS544C20 and TPS544C25 dual application

JH Shin
Mastermind 9415 points
Part Number: TPS544C20
Other Parts Discussed in Thread: TPS544C25

Hello,

My customer is considering designing a board that can use either the TPS544C20 or the TPS544C25 with only the BOM change.

Q1. TPS544C20 : BPEXT and TPS544C25 : VSET The function and connection of PIN30 are different.

        Please review whether there will be any problems when designing the circuit as shown below.

             - The PMBUS is not used

Q2. The GND/AGND separation design of the two devices is slightly different(Pin 13, 29, 38).

        Could you please provide a PCB design guide so they can use both devices?

Thanks,

JH

  • 0
    TI__Guru 53975 points

      

    The TPS544x20 and TPS544x25 devices use different control topologies that require difference error amplifier compensation circuitry.  Specifically the TPS544x20 uses D-CAP2 Constant On-time Control which needs type-one or type-2 compensation with the COMP pin connected to AGND through a capacitor or Capacitor plus series Resistor - Capacitor.  The TPS544x25 uses fixed-frequency voltage mode control with input voltage feed-forward, which needs type 3 compensation with a series R-C across the DIFFO to FB resistor and a capacitor plus series R-C between FB and COMP.

    I do not see those added components in the attached schematic.

    JH Shin said:

    Q1. TPS544C20 : BPEXT and TPS544C25 : VSET The function and connection of PIN30 are different.

            Please review whether there will be any problems when designing the circuit as shown below.

    The BP_EXT feature of the TPS544x20 family switches the BP voltage from the internal VDD to BP6 LDO to the BPEXT supply pin at 4.2V, so tying BPEXT to BP3 with a resistor is ok, but if you are not using PMBus, I would recommend connecting VSEL of the TPS544x25 family to ground with a resistor.  I can't read the output voltage of the existing converter, but as long as it's greater than 0.8V, the VSEL pin programming plus an external divider can achieve the desired output voltage.

    JH Shin said:

    Q2. The GND/AGND separation design of the two devices is slightly different(Pin 13, 29, 38).

            Could you please provide a PCB design guide so they can use both devices?

    I would recommend using two resistors from the AGND net close to the AGND pin to connect AGND to the thermal pad on the TPS544x25 and to the AGNDSNS on the TPS544x20.  Failure to follow the recommended connections could result in inaccurate current sense and early over-current shut-down of the converter.

  • 0 in reply to Peter James Miller
    Mastermind 9415 points

    Dear Peter James Miller,

    Thanks for your reply.

    The customer has additional questions.

    1. Please check if my understanding of the AGND connection using two resistors is correct.

        Please review the circuit in the red box and layout design below.

           - For TPS544C20, connect PIN13(AGNDSNT) to AGND

           - For TPS544C25, connect PIN13(GND) to GND

    2. The two pins below also have different recommended GND connections. Is there any problem with the TPS544C20 when connected to AGND?

    Thanks,

    JH

  • 0 in reply to JH Shin
    TI__Guru 53975 points

     

    No, you are not placing the resistors in the correct location.

    Pin 13 in both the TPS544x20 and the TPS544x25 is internally connected to the GND area.  The TPS544x25 does not need Pin 13 to be connected to GND.

    The TPS544x20 needs AGND to be connected to GND at the AGNDSNS (Pin 13) location for accurate current sense.  Connecting AGND to GND at the thermal pad will introduce significant current sense error and could result in low triggering of over current shutdown.

    The two resistors should be placed between the AGND pin (Pin 38) and the GND exposed thermal pad (TPS544x25) and the trace routed to AGNDSNS (TPS544x20)