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LMR16030: "Design value of DCDC switching frequency" in case of output load fluctuation

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Prodigy 40 points
Part Number: LMR16030


Vout (VS1) in the circuit diagram below is used as the voltage to drive the solenoid.
The solenoids themselves will be connected in multiple connections, and they will all be turned on at PWM:20kHz.

  

We checked the waveform of VSW at the maximum load (peak 1.2A), which is currently assumed, and found a part of oscillation.

　1ch = voltage indicating solenoid On
　3ch = Vfb
　4ch = Vsw

    

We thought that the period that looks like oscillation at Vsw of 4ch is about 500kHz.
We tentatively changed the DCDC switching frequency as follows and re-measured it, and the Vsw oscillation disappeared.

　Before correction: DCDC switching frequency = 500kHz / R125=27k , R129=18k
　After correction: DCDC switching frequency = 1.5MHz / R125=15k , R129=0

   

I would like to get advice on the following.

What should I pay attention to in order to avoid oscillation inside the IC?
　When C100 = not mounted, "the ripple of Vfb and the oscillation of Vsw were synchronized".
　Therefore, C100 = 1000pF was used to reduce the ripple of Vfb, and the ripple of Vfb was suppressed, but Vsw oscillated.
　Therefore, we would like to know where we should pay attention.

What should we refer to when deciding the switching frequency of DCDC considering the expected load fluctuation (approximately 20KHz)?

We would like to know how to reduce the overshoot/undershoot of VSW that is expected when the switching frequency of DCDC is increased (in particular, we would like to know how to do this other than changing to a fast recovery diode).

Best regards.

  • +1
    TI__Mastermind 35210 points

    Hello, the oscillation you are seeing in the Vsw measurement appears to me as being normal as the device operating in discontinuous mode. With an increased switching frequency, the inductor ripple current amplitude decreases, so the inductor current doesn't go discontinuous. 

    If having the inductor current go discontinuous is nonideal for the application, I suggest selecting the inductor such that:

    iout(min) + deltaiL/2 = 0

    iout(min) is minimum load current

    deltaiL is equation 9 in the datasheet

    Or you can increase the switching frequency as you have done.

    To ensure the Vsw overshoot/undershoot in minimized, the advise would be to follow the layout guidelines in the datasshet, with primary focuson keeping the high di/dt loop small.