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TPS56528: About the out of VFB (feedback) specification range

Tao_2199
Genius 5770 points
Part Number: TPS56528
Other Parts Discussed in Thread: TPS54527

Hello,

Regarding to VFB voltage(feedback) on TPS56528, my customer is asking some question.

 (Their conditions)

Input:12V, Output: 3.387V, Iout =2A, Continuous mode operation

Multi-tester: R6452A(ADVANTEST), accuracy: ±0.018%±5d(@2000mV range, input impedance: 1000MΩ

 

Q1:

They confirmed that VFB and output voltage are output higher than D/S specification and design value with multi-tester.

(It is out of VFB specification range .)

･VFB（D/S specification）: min 593mV , typ 600mV , max 607mV(@output 1.05V).

 -> Measurement value is 0,6073mV with multi-tester.

･Output:：3.387V（design value）.　-> Measurement value is 3.445V with multi-tester.

(Please attached file.)

TPS56528_VFB.xlsx


 

When oscilloscope is used for VFB measurement, rise of voltage (VFB, output ) isn’t occurred.

I confirmed same behavior with my EVM (TPS54527). (I don’t have TPS56528 EVM and they don't have it.)

a) Could you please tell me the measurement method of VFB voltage?

b) And what is possible reason of rise voltage?

 

Q2:

They confirmed VFB on output 3.387V. But the conditions of VFB in datasheet is at output 1.05V.

In case their conditions, do you think that there is possible to occur the out of range comparing with D/S specification VFB value range ?

 

Regards,

Tao2199

  • 0
    TI__Expert 3480 points
    1. You can use probe to measure Vfb.
    2. Different condition has different ref value.
    3. The spec written is our datasheet fixes on Vo=1.05V. For different output, ref has different threshold.
    4. How did you measure Vo? Pls attach your layout and label your measure point for Vo.
  • 0 in reply to Gavin Wang (Zejing)
    Genius 5770 points
    Hello Gavin,

    Thank you for reply.
    I’m asking to my customer about how to measure Vo.
    I have additional questions.
    > 1. You can use probe to measure Vfb.
    Q1. Is above meaning that we should use oscilloscope to measure Vfb?
    Q2 About impact of measurement instrument,
    What is it affecting at measuring Vfb?
    For example, high impedance on Vfb? (Vfb input current : 0uA(typ).)
    Q3 Do you have the different threshold data of reference for different output?
    I couldn’t find out that in datasheet.

    Regards,
    Tao2199
  • 0 in reply to Tao_2199
    TI__Expert 3480 points
    Vfb has ripple, you can probe to see its waveforms.
    Why do you want to measure Vfb? I think output voltage is your target.
    We can only provide the data for 1.05V output.
  • 0 in reply to Tao_2199
    TI__Expert 4795 points
    Tao,

    When oscilloscope is used for VFB measurement, rise of voltage (VFB, output ) isn’t occurred.
    => Do you mean if you use the probe，the VFB is within datasheet spec?

    Comment from my side:
    1. Customer must use the IC PIN9 as the GND to measure VFB voltage no matter with Multi-tester or probe. There may be voltage drop between different gnd
    2. 607mV max is the 25C data, at 2A conditon, IC has temp rise, so the FB voltage may exceed 607mV.

    BR, Vincent
  • 0 in reply to Vincent Zhang(BSR)
    Genius 5770 points

    Hello Vincent,

    Sorry for delay.

     

    > Do you mean if you use the probe，the VFB is within datasheet spec?
     => Yes. I confirmed it with TP54527 EVM.

     

    Additional questions

    (1) I’m understanding that VFB threshold is bottom of ripple.(Pleae see below.)

    In this case, what point is output voltage decided on ripple?

    (It is average of output ripple?)

     

          datasheet page 15, Figure 16

    (2) You mentioned that different condition has different ref value and for different output, ref has different threshold.

    In this case, how should I think about output voltage calculate? (datasheet page 13, Equation 2)


     

    I think that VFB threshold range (max/min) will be changed depending to output voltage.

    (When output become large, threshold range will become wide.)

    But, basically typical value (600mV) is not changed.

    My understanding is correct?

     

    Regards,

    Tao2199

  • 0 in reply to Tao_2199
    TI__Expert 4795 points

    Hi Tao,

    (1)Vout is proportional to VFB+1/2 VFB_ripple. With same feedback resistors, the large FB ripple, the larger Vout.

    (2)Yes, we usually use typical VREF to design the output voltage.

    Best Regards,

    Vincent Zhang