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SMPSx FDBK pin of TPS659037 PMIC

Michi Yama
Guru 10030 points
Other Parts Discussed in Thread: TPS659037, AM5728

Hi,

I have some questions regarding TPS659037 PMIC.

My customer uses TPS659037 with AM5728 processor.

I would like to know regarding SMPSx_FDBK pin of PMIC.

1)
As to my understanding, FDBK inpurt for voltage sense of the processor AM5728.
This is important for keeping proper output voltage from SMPSx. Especially,
this is very important when the customer apply the AVS of the device. PMIC adjusts
the output voltage automatically based on the voltage of FDBK pin.
Is my understanding right?

BTW, according to the data manual of PMIC, SMPS3, 7, 9 are non DVS regulator.
For the SMPS3,7 and 9, should FDBK pin be connected to processor voltage pin?

Or isn't FDBK needed for no DVS regulator?

2)
SMPS1/2 and SMPS4/5 of PMIC has SMPSx_FDBK_GND pin for differential feedback. However,
other SMPS outputs(SMPS3,6,7 and 8) don't have such pins(SMPSx_FDBK_GND).Non differential
line may not sense correct voltage value if GND is not stable. In this condition, should the remote sense be used?

3)
If feedback line of SMPS output is long, device mulfunction of PMIC is concerned by noise.
So I would like to get some guidelines.

For example, pattern width, clearance, pin name of processor that should be connected to PMIC,
and how should the feedback line be shielded?(feedback line should be passed through the internal layer of PCB
and so on)

Please advise me.

I appreciate your quick reply.

Best regards,

Michi

  • 0
    TI__Genius 16410 points
    Hello Michi,

    1) For all SMPS, FDBK is needed to sense the output voltage. If FDBK is left floating the regulator will not work correctly. DVS just means there is a slew-rate controlled transition between voltages for AVS/DVS steps.

    2) For differential remote sense, it's recommended to route the FDBK and GND lines very close to each other. This way any common-mode noise will get cancelled out on the sense lines. These should not be current-carrying traces, and should be routed to the output and ground very near the load. This will compensate for IR drop between SMPS output capacitor and the actual load.

    3) We still recommend to use remote sense with single-ended feedback lines. This should be connected to the output voltage, but physically located near the load. This could be at a decoupling capacitor at the load.
    The feedback line should not be routed near the switching lines (eg. output inductors, clocks, etc.). If possible, it's recommended to route between power/GND planes to improve shielding. Since these do not carry current, these do not need to be wide traces.

    Regards,
    Karl
  • 0 in reply to Karl Wallinger
    Guru 10030 points

    Dear Karl-san,

    Thank you for your quick reply.

    I have some questions regarding your answer.

    >2) For differential remote sense, it's recommended to route the FDBK and GND lines very close to each other. This way any >common-mode noise will get cancelled out on the sense lines. These should not be current-carrying traces, and should be >routed >to the output and ground very near the load. This will compensate for IR drop between SMPS output capacitor and the >actual load.

    I don't know the meaning of red character sentence.  What is the meaning of "current-carrying trace"? What is the meaning of "ground very near the load"?

    >3) We still recommend to use remote sense with single-ended feedback lines. This should be connected to the output voltage, > >but physically located near the load. This could be at a decoupling capacitor at the load.
    >The feedback line should not be routed near the switching lines (eg. output inductors, clocks, etc.). If possible, it's >recommended to route between power/GND planes to improve shielding. Since these do not carry current, these do not need to >be wide traces.

    I don't know the meaning of red character sentence. What is the meaning of "these do not carry current"?

    Also do you have some application note or wiki for FDBK? Should customer refer the AM5728 GP EVM ?

    Please advise me again.

    Best regards,

    Michi

  • 0 in reply to Michi Yama
    TI__Genius 16410 points
    Hello Michi,

    By "do not carry current" I mean that there is no current on the feedback lines. For any traces with high current, you want to minimize the trace resistance by making wide traces. For feedback, this is not as critical because the feedback traces have no current (input is high impedance).

    By "routed to the output and ground very near the load", I mean that instead of connecting the feedback to the output capacitor, you should connect it close to the processor. This is because of the IR drop due to the resistance of the PCB trace, the voltage at the processor will be lower than the voltage at the output capacitor. By sensing at the processor, you can regulate the voltage at the processor.

    I don't know of any application note about feedback lines for remote sense. The closest I found was an application note for SMPS, which has a short section on feedback.

    www.ti.com/.../SNVA021

    Regards,
    Karl