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LM5117-Q1: What will be CM output of LM5117 in DCM mode, with diode emulation enabled?

Dhananjay Yadav
Intellectual 290 points
Part Number: LM5117-Q1
Other Parts Discussed in Thread: LM5117

Hello,

 

I have one technical query related to LM5117. We are using this PWM controller in one of the 28V to 13,5V/20A DC-DC converter. This is in production. At assembly plant, at EOL station, some of ECUs fail as they do not give regulated output of 13.5V +/-0.5V. We are testing output at 500mA load current at EOL station.

 

In design, we have implemented a comparator circuit. Current monitor output of PWM controller is given to comparator and it is compared against the reference voltage. Output of comparator controls the UVLO pin of IC. Reference voltage is set such that, comparator output is high for load current > 400mA.  But it is observed that UVLO pin is switching continuously between high and low. As a result, output is not regulated.

 

Query regarding this failure is, we are using CM output for comparison. Now at 500mA load current, converter will work in DCM mode and as per datasheet of IC, current sense amplifier samples the voltage across current sense resistor just before turning on high side switch. But when converter is operating in DCM as in our case at 500mA, voltage across current sense will be 0V, before turning on high side switch. So this will result in 0V CM voltage. (We are using diode emulation) and we suspect, that is causing output of comparator to low at 500mA.

 

Please answer to below queries considering above scenario:

 

  1. Is it reliable to consider CM voltage in DCM mode operation?

 

  1. What will be CM output voltage in DCM mode?

 

  1. Does current sense amplifier sample voltage across current sense resistor just before turning off of LS switch or just before turning on HS switch (As explained earlier, diode emulation mode is used)?

 

Please let us know your response to above queries asap as this ECU is in production.

  • 0
    TI__Guru 74052 points

    Hi Dhananjay,

    The CM output does not reflect the output current when operating in DCM.

    More specifically, when operating in CCM, both the peak and valley are sampled and the average is then found to derive Iout. However, this does not hold true in DCM where the peak can be much higher than the average current and the valley is effectively at zero. Maybe you an operate your circuit in FPWM so that CCM operation always occurs and the CM output is valid.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 290 points

    Thanks for response.

     

    Please find attached waveforms of VCM pin voltage with respect to load current. This measurement is done on one OK board and NOK (failed) board. On NOK board, VCM is unstable at 500mA load current hence comparator is switching continuously and output is not regulated. But when load current is increased to ~ 600mA, VCM is found to be stable, output is also regulated.

    While on OK board, VCM is stable even at 500mA load current and output is also regulated.

     

    Can you please explain why such different behavior of CM pin of PWM controller with the same operating conditions.

     This is in production so now we can not implement FPWM. 

     

    Thanks in advance.

      Best Regards

    Dhananjay

  • 0 in reply to Dhananjay Yadav
    TI__Guru 74052 points

    The RC filter at the CM pin should remove any high-frequency noise.

    It seems you're operating near the CCM-DCM boundary, which depends on Vin, Vout, L, the inductor current at zero-cross detection, etc.

    If the peak inductor current increases in DCM (based on a longer off time), then that may affect the CM output.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 290 points

    With input voltage of 28V, output of 13.5V, 200KHz switching frequency, 6.8uH inductor, load shall be  ~2.5A to operate at boundary of CCM & DCM. Below 2.5A, converter will operate in DCM. So at 500mA load current it is sure that, converter is operating in DCM. 

    So can you please elaborate, from above waveforms, why one IC is giving switching output at CM pin at 500mA load while other IC is giving DC signal (If HF noise is ignored)?

    The IC which is giving switching CM output at 500mA, stops switching at CM pin and gives DC output ~600mA load onwards. Please explain this observation also- Please refer above waveforms. 

    Regards

    Dhananjay

  • 0 in reply to Dhananjay Yadav
    TI__Guru 74052 points

    Have you got waveforms of the inductor current?