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LM25190: The frequency will decrease under near full load

Zhan Ryan
Prodigy 10 points

Part Number: LM25190

Hello,

The design uses an LM25190 to provide a 12 V / 8.3 A power supply, with an output adjustable from 0.9 V to 12 V.

When the load increases to around 7A, the output switching frequency drops to about 20 kHz. What conditions could cause the IC to reduce its switching frequency?

OUTPUT:12V/3A

3A.jpg

OUTPUT:12V/7.5A

7.5A.jpg

 

  • 0
    TI__Genius 12512 points

    Hi Zyan,

    Can you please provide operating conditions and schematic to comment further? One possible case would be drop-out mode.

    Regards

    Arpita

  • 0 in reply to ARPITA NATH
    Prodigy 10 points

    Hi NATH

    The LM25190 has an input voltage of approximately 15.3 Vdc. The circuit diagram and the Excel tool are attached for reference.

    3157.SCH.pdf

    LM5190.xlsx

  • 0 in reply to Zhan Ryan
    TI__Genius 12512 points

    Hi Zhan,

    Few observations on schematic

    1. Vin_min is 13V, but desired start-up is set at 14V. Also, C278 would cause additional delay in enabling the device

    2. COMP network, Rff does not match with Excel at 12V vout. Kindly update the schematic with excel sheet filled for 12V output and test the behavior with schematic updated according to excel sheet.

    3. What is R208A?

    4. recommend removing following components as they might not be needed

    5. Remove C60

    6. Part number of MOSFETs and inductor

    • For debugging the issue, I need to look into SW node (take more number of cycles), Vout, IL (if possible, tilt the inductor from one side and add a loop for current measurement), Io, ILIM/IMON pin (200mV/div )  -->  to check at 12V, 7.5A condition- is the device able to maintain regulation or achieve 7.5A load current? Need to double-check in which mode the device is in.
    • Can you test at higher input let's say ~24V and check if the issue appears.

    Regards

    Arpita

  • 0 in reply to ARPITA NATH
    TI__Genius 12512 points

    Hi Zhan,

    Do you have any update?

    Regards

    Arpita

  • 0 in reply to ARPITA NATH
    Prodigy 10 points

    1. After removing capacitor C264A and replacing Q212 and Q213 with PRM6R0N10N5, the frequency no longer drops. However, when using BSC059N04LS6, the frequency still decreases.

    2. What kind of protection could cause the LM25190 to reduce its switching frequency?
    3. R208 is set to 240 kΩ, and according to the Excel tool, the operating frequency should be 100 kHz. However, the actual measured frequency is close to 150 kHz. Why is that?
  • 0 in reply to Zhan Ryan
    TI__Genius 12512 points

    1. Have you done all other schematic modification (along with removing C264A) suggested above before checking with BSC059N04LS6? Can you please double check if the BSC059N04LS6 MOSFETs are not damaged?

    2. To comment further I need to look into the requested waveforms after schematic modifications as suggested above. As it works with replacing MOSFETs externally, recommend you double check replacing with BSC059N04LS6 MOSFETs new ICs.

    3.  Can you please measure the actual RON (R208) placed in board? However, R208 is RT, I had asked about R208A.

    Regards

    Arpita

  • 0 in reply to ARPITA NATH
    Prodigy 10 points

    1.Yes BSC059N04LS6 MOSFETs are not damaged.R209A, C268, and R209B were modified, but it didn’t help.

    2.A new sample using the BSC059N04LS6 was also made, but the frequency drop still occurred.

    3.R208A is not installed, and the measured frequency differs greatly from the set frequency. This is the other issue I want to ask about.The measured value of R208 is 239.2 kΩ.

    4.The COMP network was adjusted based on the recommended values from TI.

    5.The waveform during the frequency drop.

    CH2: LOW SIDE VDS

    CH3:Vin

    CH1: inductor  Current

    CH4:Vo

  • +1 in reply to Zhan Ryan
    TI__Genius 12512 points

    1. Inductor current waveforms do not look linear to me, what is the saturation current rating for the inductor? As already asked, please provide datasheet of the inductor

    2. It does not look like over current protection has been triggered a

    3. As replacing MOSFETs solves the issue, could you please probe HO and LO?

    4. Kindly try replacing R208 with fresh 240kohms and try connecting RT to AGND and check if the frequency (CCM) is 100kHZ.

    5. As asked earlier, do you see same behavior at Vin=24V?

    Regards

    Arpita