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LM7480-Q1: LM7480-Q1 inrush current issue

Chaitanya Satuluri
Prodigy 185 points
Part Number: LM7480-Q1
Other Parts Discussed in Thread: LM74800-Q1, TDA4VM, , LM5060

Hi TI team,

We are using LM74800-Q1 part for Reverse voltage protection and load dump protection in our custom TDA4VM board. we have connected the circuit in common Source configuration as shown in below. we are seeing a issue with this device whenever we power on the board we see a short duration of high inrush current in both input and output side of this circuit.

we did following experiments to see the issue is due to the firmware on the board is causing this inrush current.

1. We have erased the software on the board and measured the inrush current with current probe at output of LM7480-q1 we see the following scope output

2. Secondly we did is add a snubber on the HGATE pin of the device with 100 ohm + 10nf capacitor to increase the slewrate of the devices, however we dont see any improvement on the inrush current.

could you please provide more suggestions and feedback on the same.

Regards,

Chaitanya

  • 0
    TI__Guru 57737 points

    Hi Chaitanya,

    Please help with the following information,

    1. What is the total output capacitance ?
    2. Is there any load during startup ? What is the load profile ?
      1. Have you tried starting up by disconnecting the output load ?
    3. What is the FET part number ?
    4. Please share waveform captured with the following signals,
      1. Vin, Vcap, HGATE, Iin
      2. Vin, VS, VCAP-VS, DGATE
  • 0 in reply to Praveen GD
    Prodigy 185 points

    Hi Praveen,

    1. What is the total output capacitance ?

    [Chaitanya]: The total capacitance is 103uf

    1. Is there any load during startup ? What is the load profile ?
      1. Have you tried starting up by disconnecting the output load ?

    [Chaitanya]: we have removed the all the firmware flashed into the chip and the load should not be morethan 250mA.

    1. What is the FET part number ?

    [Chaitanya]: The first FET part number is #SQM60N20-35 and second one is BUK7Y4R8-60EX

    1. Please share waveform captured with the following signals,
      1. Vin, Vcap, HGATE, Iin
      2. Vin, VS, VCAP-VS, DGATE

    [Chaitanya]: I will capture the waveform and provide you more details.

    Regards,

    Chaitanya

  • 0 in reply to Chaitanya Satuluri
    TI__Guru 57737 points

    Hi Chaitanya,

    Thanks for the information. I will wait for you to share waveforms. 

  • 0 in reply to Praveen GD
    Prodigy 185 points

    Hi Praveen,

    Please see the Captured waveforms as requested.

    a. Vin, Vcap, HGATE, Iin

    b .Vin, VS, VCAP-VS, DGATE

    Regards,

    Chaitanya

  • +1 in reply to Chaitanya Satuluri
    TI__Guru 57737 points

    Hi Chaitanya,

    From your waveforms, it is clear that the input voltage is dropping during startup. This is probably due to

    1. High impedance in between input power supply and the evaluation board (may be long cables)
    2. lack of input decoupling capacitance 
    3. Current limit of the input power supply

    To reduce the inrush current during startup, increase the Cdvdt capacitance to say 47nF or 100nF. For more understanding, please refer to section '9.3.2.2 Load Disconnect Switch Control (HGATE, OUT)' and '10.2.5 MOSFET Selection: Hot-Swap MOSFET Q2' of datasheet. 

  • 0 in reply to Praveen GD
    Prodigy 185 points

    Hi Praveen,

    Thanks for the reply,

    we will try to modify the our application board as per your inputs.

    i have added a quickly 100nf capacitor between HGATE and Ground and also at the startup i have disconnected the load and here is the input current waveform.

    We still see the around 8A pulse at startup of LM74800.

    Regards,

    Chaitanya

  • 0 in reply to Chaitanya Satuluri
    Prodigy 185 points

    also forgot to add that added a 100uf input capacitor in the above experiment.

    Regards,

    Chaitanya

  • 0 in reply to Chaitanya Satuluri
    TI__Guru 57737 points

    Hi Chaitanya,

    Do let me know if you have any further questions. 

  • 0 in reply to Praveen GD
    Prodigy 185 points

    Hi Praveen,

    As per your feedback we have done following changes in our custom application board

    1. Added a input capacitance of 100uf 

    2. Modified the HGATE of LM74800 to 100 ohm + 100nf.

    3. Disconnected the load.

    4. Power supply we are using a 12V, 7Ah battery with shorter power cables.

    After above modifications we still see closely around 8A inrush current as below. Do you have any further feedback on this.

    I suspect Large load capacitance are contributing this sudden spikes at input current. 

    Regards,

    Chaitanya

  • 0 in reply to Chaitanya Satuluri
    TI__Guru 57737 points

    Hi Chaitanya,

    As earlier, please share waveform captured with the following signals,

    1. Vin, Vcap, HGATE, Iin
    2. Vin, VS, VCAP-VS, DGATE

    In the schematics, I see only 0.6uF capacitance at the input. Is there any additional capacitance at the input which is not shown in the schematics ?

  • 0 in reply to Praveen GD
    Prodigy 185 points

    Hi Praveen,

    In the schematics, I see only 0.6uF capacitance at the input. Is there any additional capacitance at the input which is not shown in the schematics ?

    [Chaitanya]: I have added manually at the input meaning modified physically on the board to add 1x 100uf electrolyte capacitor.

    Rest all the inputs you have requested i will provide the details.

    Regards,

    Chaitanya

  • 0 in reply to Chaitanya Satuluri
    TI__Guru 57737 points

    Hi Chaitanya,

    Please be aware that the input current measurement can also include the current to charge the input 100uF capacitance. This depends on the input slew rate and EN pin connection. 

    In order to see the actual current flowing into the output of LM74800-Q1, Enable the controller after the input  is stable at 12V.

  • 0 in reply to Praveen GD
    Prodigy 185 points

    Hi Praveen,

    Thanks for the inputs. we have found and root caused the issue in our application circuitry with the following changes

    1. We have modified the HGATE control to 100ohm and 10nf

    2. We also on output side LM5060 high side protection controller where we have modified the gate slew rate control with 10nf like below.

    After changing above without modification of any input and output capacitors the inrush current looks below at the battery input

    We are seeing around 4A inrush current about 11ms and input voltage drop of 234mV.

    Regards,

    Chaitanya

  • 0 in reply to Chaitanya Satuluri
    TI__Guru 57737 points

    Hi Chaitanya,

    Good to know that your issue is resolved.

    As your issue is resolved, I am closing this thread. Incase you have any other questions please raise a new e2e query/thread. 

  • +1 in reply to Praveen GD
    Prodigy 185 points

    Hi Praveen,

    Yes we can close this thread.

    Regards,

    Chaitanya