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LTC3869 Current Limit Behavior

Hi,

I encounter a issue about current limit hysteresis too large

While 12Vin->7.9Vo/14A Full load is normal, the ocp point is around 19A.

However the recovery point is around 10A back to normal operation.

Is there any possible to this ?

Dual Phase single output operation

• Tie all of the ITH pins together.
• Tie all of the VFB pins together.
• Tie all of the TK/SS pins together.
• Tie all of the RUN pins together.

freq=300kHz

L=3.3uH

Rsense=5mR//3mR

RF=100x2

CF=22nF

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  • Hi,

    Before I answer the questions you have, could I ask you several questions about the test condition you have?

    (1) What is the current limit level you set on the ILIM pin? As we know, we have 3 grades for ILIM. They are 30mV, 50mV and 75mV respectively. Which one do you pick up for your application?

    (2) As you mentioned, Rsense=5mR//3mR. Do you apply a 5mOhm and a 3mOhm sensing resistors in parallel for each channel?

    Meanwhile, based on the information you provided, I have some clues for your reference:
    (1) You are using R sensing for the output current. In ideal case, we will get pure triangular Iout signal by measuring the voltage across the Rsense. However, there is a parasitic inductor for Rsense and it will influence the voltage drop across the Rsense. We need to pay attention to it and apply additional filter to get the pure triangular waveform for Isensing signal. It will impact the threshold we want since it is instantaneous peak current limit. You can refer to the Page 15~17 of LTC3869 datasheet for details.

    (2) We have a current foldback scheme for current limit of LTC3869. If the output falls below 50% of its nominal output level, then the maximum sense voltage is progressively lowered from its maximum programmed value to 1/3 of the maximum value. You can refert the Page 24 of LTC3869 datasheet for details.

    Those 2 factors will impact the ILIM threshold. We need to pay attention to them.

    Thanks,

Reply
  • Hi,

    Before I answer the questions you have, could I ask you several questions about the test condition you have?

    (1) What is the current limit level you set on the ILIM pin? As we know, we have 3 grades for ILIM. They are 30mV, 50mV and 75mV respectively. Which one do you pick up for your application?

    (2) As you mentioned, Rsense=5mR//3mR. Do you apply a 5mOhm and a 3mOhm sensing resistors in parallel for each channel?

    Meanwhile, based on the information you provided, I have some clues for your reference:
    (1) You are using R sensing for the output current. In ideal case, we will get pure triangular Iout signal by measuring the voltage across the Rsense. However, there is a parasitic inductor for Rsense and it will influence the voltage drop across the Rsense. We need to pay attention to it and apply additional filter to get the pure triangular waveform for Isensing signal. It will impact the threshold we want since it is instantaneous peak current limit. You can refer to the Page 15~17 of LTC3869 datasheet for details.

    (2) We have a current foldback scheme for current limit of LTC3869. If the output falls below 50% of its nominal output level, then the maximum sense voltage is progressively lowered from its maximum programmed value to 1/3 of the maximum value. You can refert the Page 24 of LTC3869 datasheet for details.

    Those 2 factors will impact the ILIM threshold. We need to pay attention to them.

    Thanks,

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