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WEBENCH® Tools/LM5160-Q1: The operation of LM5160-Q1 when the short circuit protection.

Part Number: LM5160-Q1
Other Parts Discussed in Thread: LM5160DNTFBKEVM

Tool/software: WEBENCH® Design Tools

Dear Ti Team:

I got a problem with LM5160-Q1.

First, the current limit is 2.5A typ. When Vin is more then 6v. The operation is normal. The Isw reach 2.5A , IC operation at short circuit mode.

But , when Vin equal 5v, the current limit  become about 5.2A.

The input current become very high. Can you explain it? Thanks.

 Spec. and The waveform below

Fly-buck topology

CH2: Vsw  CH3: Isw

  • Hi Ti Team:

    I found that when Vcc < 5.3V,OCP won't active

    Vcc > 5.3V,OCP will active.

    But, the datasheet recommend input voltage is 4.5V - 60V

    So, when input voltage is below 5.5V,Vcc charged by internal regulator,and the Vcc voltage is 5.3V or smaller.

    The OCP function should be normal.

    I test those function on LM5160EVM.

    So,the schematic is right, can you solve this problem?

    Thank you so much.

  • Hi,

    Could you please tell the input/output conditions? Do you use the board LM5160DNTFBKEVM and any modifications on it? Thanks.

  • Hi  Iven:

    I use the board LM5160DNTFBKEVM and the spec below.

    Input Voltage : 5V-12V
    Non-iso output Voltage : 3V     output current : 0.1A

    Iso output Voltage : 5V             output current : 0.1A

    Switch Frequency = 350kHz

    The transformer made by myself, Np : Ns = 5:12, but I think in this case, it doesn't matter.

    So, when input voltage equal 5V. The waveform below

    CH1 : Vin   CH2 : Vsw    CH3 : IL     CH4 : Vo

    When Input voltage equal 9V

    CH1 : Vin   CH2 : Vsw    CH3 : IL     CH4 : Vo

    We can see that, when input voltage = 5v, IL reach 2.5A , the duty doesn't turn off until the current reach 5.72A

    And I did one more experiment, take another power source set 6V and connect to Vcc pin.

    Vin  = 5v, the OCP function work normally. The IL reach 2.5A ,the duty turn off immediately.

    Thanks.

  • Hi,

    At which point you probe the input voltage? As the current is high on the wire or trace, the input pin voltage might be lower. Thanks.

  • Hi Ivem,

    I measured the input voltage on input cap C7.

    And, I added one more 47uF cap to input. To avoid the input voltage drop under 4.1V

    Thanks.

  • Hi,

    The Vsw waveform looks not normal because when on-time is done, it drops to a very low voltage. Here Vsw is almost equal to Vin which looks lower than UVLO threshold.

  • Hi Iven :

    Because of the large current, the Vsw is abnormal.

    In normal satiation , when IL peak reach 2.5A(typ), the high side mos turn off immediately.

    But, in1st pic , IL peak until it reaches 5.72A , high side mos turn off.

    It's that OCP didn't active when Vin = 5V in my circuit.

    And I did one more experiment, take another power source set 6V and connect to Vcc pin.

    Vin  = 5v, the OCP function work normally. The IL reach 2.5A ,the duty turn off immediately.

    Thanks.

  • Hi,

    I understand this, but it still makes no sense that the Vsw is abnormal when current is high because high-side Rds(on) is only 0.29Ohm typical. Please double check this and from your first waveform Vin is ~4V which is below the minimum input of this device.

  • Hi Iven:

    At the beginning, I doubted the Vin drop under the uvlo of IC(3.98V typ).

    So I added two more 47uF CAP at input.

    Here is the waveform.

    CH1 : Vin  CH2 : Vsw  CH3 : IL   CH4 : Vcc

    Obviously, the Vin min is 4.6V. It's far away from 3.98V.

    Mentioned before,I provided a 6.5V to Vcc pin

    CH1 : Vin  CH2 : Vsw  CH3 : IL   CH4 : Vcc

    We can see that when IL peak current reach 2.73A, the high side mos turn off immediately.

    The different of those experiment is the value of Vcc.

    First one, Vcc is regulated by internal regulator. when input voltage is 5V. Vcc is 5V - VCC(LDO). Maybe 4.8V.

    The second  experiment, Connected a  7V power source to Vcc. It passed by  the diodes , must deduct the Vf.  So , the value is 6.6V.

    So , I wonder why Vcc voltage effect the OCP function active or not.

    And , How did the Mos trun off at first experiment. What condition to turn off the mos at 5.72A.

    Thank you so much.

  • Hi,

    OK, I understand. I notice in the waveforms when you apply a 4.6V min Vin, the abnormal switching-off current happens and here CH4 : Vcc is about 5V and it's not connected to external source, is that right? If so, my concern is here Vcc should be lower than Vin because it's step-down from Vin. Could you please check this? Also, it looks you can have an external source to power Vcc, could you please lower the voltage from 7V to<5V to see if the issue happens? Thanks.

  • Hi  Iven

    At normal operation mode.

    CH1 : Vin  CH2 : Vsw  CH3 : IL   CH4 : Vcc

    Input voltage and Vcc voltage measured are 5.47v and 5.45V.

    So, the Vcc voltage is normal.

    And, the waveform below.

    CH1 : Vin  CH2 : Vsw  CH3 : IL   CH4 : Vcc

    Vin is 5.516V and Vcc is 5.463V without external voltage sourced.

    CH1 : Vin  CH2 : Vsw  CH3 : IL   CH4 : Vcc

    Vin is 5.545V and Vcc is 5.539V with external voltage sourced.

    I think 5.5V is the boundary of whether OCP acting normally or not.

    Thanks.

  • Hi,

    I see, could you please leave your email and I will forward it to dedicated engineer and copy you at the same time. Thanks.

  • Hi Iven

    Could you leave your email here and I will send you my email.

    Thanks.

  • Hi,

    My email address is iven-su@ti.com. Thanks.