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TPS923655DMTREVM: Spread spectrum affect on output current

Steveb
Prodigy 50 points
Part Number: TPS923655DMTREVM
Other Parts Discussed in Thread: TPS923655

Tool/software:

Using the EVM module to drive a string of LEDs. Vin = 24V. Vout = ~36V. Iout = 100mA - 500mA. PWM pin high. ADIM pin running a fixed 1Khz PWM sequence, set to give a fixed current.

I can see approx 5mA ripple current through the LEDs caused by the DC converter switching frequency, but there is a larger ~30mA p-p,~2.2KHz triangular current on top of this. The EVM has a TPS923655 fitted which includes spread spectrum. Is the spread spectrum feature causing this larger 30mA p-p triangular ripple?  I have tried with different sources of the ADIM pwm signals so I don't believe that it is caused by the inputs. A larger output current produces a large triangular ripple.

  • 0
    TI__Mastermind 23150 points

    Hi Customer,

    Based on your description, the larger ~30mA triangular current should be caused by the spread spectrum feature of TPS923655, which has a 2-kHz modulation frequency. Please refer to Section 8.3.1.2 Spread Spectrum from the datasheet.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 50 points

    Hi Steven,

    Thanks for your reply.

    You say it "should be caused". Does that mean that the spread spectrum enabled devices WILL cause a ripple like this, or that you guess that it is causing it because the frequencies of the ripple I see matches the spread spectrum frequency? Do you think that using the TPS923652DMTR will not have this issue?

    If the spread spectrum does cause a triangular ripple like this then I think this should be mentioned in the data sheet.

    Regards,

    Steve.

  • +1 in reply to Steveb
    TI__Mastermind 23150 points

    Hi Steve,

    When I said "should be caused", I meant this was my assumption based on your description and information provided - The frequency component shown in your oscilloscope capture is very close to that 2-kHz modulation frequency of spread spectrum.

    An easy way to check if this is indeed caused by spread spectrum is swapping the IC to TPS923654DMTR and see if the ripple disappears.

    Steveb said:
    If the spread spectrum does cause a triangular ripple like this then I think this should be mentioned in the data sheet.

    The spread spectrum normally will introduce an around 2-kHz frequency component to the output, which is a sacrifice for better EMI performance. Thanks for your advice. We will consider this when doing the datasheet revision later.

    Best Regards,

    Steven