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LM3150: ILIM suitable range

Part Number: LM3150
Other Parts Discussed in Thread: CSD18504Q5A

Hi, team.

Our customer is evaluating LM3150.

Their condition is 28-30Vin, 24Vout, 2A, 150kHz.

They changed Low-side FET to lower Rds(on), so RLIM is changed to lower value.

But current limit is not hit over the setting value, so they think ILIM has minimum value for detecting correct value.

How much is the suitable range of ILIM and RLIM?

BR

  • Kobayashi-San,

    Current limit detection in LM3150  occurs during the off-time by monitoring the current through the low-side switch using an external resistor, RLIM.. If during the off-time the current in the low-side switch exceeds the user defined current limit value, the next on-time cycle is immediately terminated.THe drop across the Rlim resistance (due to internal current source) is compared to the drop across the low side FET for current sensing .

    ie .

    As you can see the limit threshold depends on internal current source which has tolerance in the range of :

    Further it also depends on Rdson of the FET which varies too .

    So for the worst case condition , Choose the min limit to current source as well as  resistance value of the low-side FET at the expected maximum FET junction temperature.

    Ambreesh

  • Hi, Ambreesh
    I understand your explanation already.
    But even double of a setting value doesn't detect over-current.
    How many voltage is the ILIM terminal which can expect correct over-current detect?
    Is it possible to detect several mV?

    BR
  • Hi, team.
    My customer changed RLIM resistance to 75-ohm from 150-ohm.
    Then RLIM resitace burned.
    Please tell me a minimum of the RIM resistance value and necessary electric power rating.

    BR
  • Hi Kobayashi-san,

    The value for the RLIM resistance seems quite small.
    What is your Rdson and current limit? Is it 2A ?

    Thanks
    -Arief
  • FET is CSD18504Q5A, so Rdson is 7.5m ohm.

    The combination, Rdson is 15.6m ohm and RLIN is 150 ohm , is OK and ILIM hit about 3A.

    But the combination, Rdson is 7.5m ohm and RLIN is 75 ohm is NG and ILIM did'nt hit and RLIM resistor burned.

    Do you know the cause?

  • I am not sure why the resistor is burned, the resistor should not get any large current flowing through it.
    Is the resistor burned during current limit event or is it burned at the first time you install it?
    Also do you have schematic for it? Maybe something else is happening

    Thanks
    -Arief