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PTH08T240W output noise quesetion

jacky lu
Intellectual 340 points
Other Parts Discussed in Thread: PTH08T240W

Sir:

    I use pth08t240w in my board, and the sechmetic is in PIC(1). In my design, I use the TurboTrans Technology, and I choose the input capacitor Ci and output capacitor as follow：

type

Rated Volt

Capacitance

ESR(m ohm)

Ci

6TPF470MAH

6.3

470uF

10

Co

4TPF680MAH

4

680uF

10

But the input and output of pth08t240w is with much more noise than that was described on the datasheet. The output of pth08t240w has a Vo ripple more than of 70mv, while the datasheet saied it should be about 10mv. The test of pth08t240w input and output is in PIC(2) in below.

Is there something wrong for my using with pth08t240w? Why this DC-DC work so terrible in my board?

PIC(1) the schematic of pth08t240w in my design

PIC（2） the output and input voltage of pth08t240w tested with oscilloscope. The yellow signal is the output voltage of 1.0v, and the red signal is the input voltage of 5.0v.

  • 0
    TI__Mastermind 38230 points

    Jacky:

    Schematic is as per the specification . Input and output  capacitors 680uF Vout ESR10mΩ and Turbotrans resistor 22.6k are per the  specification. The Smart sync. is recommended to be grounded if not used.

    1.Oscilloscope  bandwidth should to set tot 20MHz. There are two data waveforms present on each line. The scope probe for accurate low mV measurement should be tightly coupled across a high value ceramic capacitor capacitor (10uF or greater).  

    a.  The yellow  low frequency low amplitude sinusoidal  ripple at about 300KHZ . It appears to be about 8-10mV PP .

    b.The high frequency narrow duration  pulses at the switching frequency are a E-field pick-up from the loosely coupled scope probe ground wire loop. They are not present on output bus, but are present on the scope ground.

    See the attached recommended scope connection with a minimum ground loop wire.

    Tom  

     

    PTH ripple measurement method .pdf
  • 0 in reply to Tom Guerin
    Intellectual 340 points

    Tom:

        Even though I do not test the pth08t240w output according exactly to "PTH RIPPLE MEASUREMENT METHOD", i surely use a probe with the groud close enough.

    Why you so conveiencely said that the narrow duration pulses are a E-field pick-up ?

    Besides, what do you mean by "present on the scope ground"? Dose it mean that  the switch frequency narrow duration pulses was coupled into my ground?

    third, where is the E-field signal from? Can I say that the E-field high frequency  narrow duration pulses are from the pth08t240w switching noise?

    Thanks!

    Regards

    Jacky

  • 0 in reply to jacky lu
    TI__Mastermind 38230 points

    Jacky:

    The standard method to accurately out noise and ripple is to closely couple the  scopte probe directly across a ceramic capacitor.

    The standard high frequency FET switching noise is an air e-field stray field which is always  present on all switching power supplies power Inductor.. This field has no power associated with it, unlike the  sinisoldial lower frequency ripple. The high  frequency switching pulses are not present on the  conductive output bus plane when closely  coupled.  

     The scope probe senstivitiy when set to 10 -20mV /div with  long 2.5 to 3" ground loop  is antenna which  picks  all distributed radiated fields. By simply moving the long  ground scope probe wire position closer and farther away  from the inductor can change its pickup field .

    For ripple measurement the  oscilloscope shall be an analog or digital  measure with  20-MHz bandwidth 10-20mV/div resolution and the scope probe must be closely coupled across a ceramic capacitor as described in the last attached file.

    Tom

     Email is tguerin@ti.com