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LM5069: dv/dt start-up circuit question

Jiaqi Yuan
Prodigy 230 points
Part Number: LM5069

Tool/software:

Hi Experts

I am using the LM5069-1 for input protection with a 25.6V battery. I wanna use the soft-start function of the dv/dt startup. I ran a PSPICE simulation with LM5069 model numerous times. However, I found that the OUT of LM5069 is always zero and the External MOSFET is OFF, even though I used the parameters from TI's LM5069 design calculator. The schematic is as follows. 

If I remove this dv/dt startup circuit, the simulation works, and the OUT is expected. The rise time of the GATE is ~3.5ms in simulation

If I removed the dv/dt startup circuit, I added a simple RC directly; the schematic is as follows. Both RC connections work.  The simulation is also working, the OUT is expected. If R=100; C=50nF, the rise time of the GATE is ~100ms

.

So my question is that can I use an RC circuit instead? The purpose of that circuit is to slow down the speed of the MOSFET turning ON to reduce the inrush current.

And why it's not working of the dv/dt startup circuit in PSPICE

I am looking forward to your reply.

Thank you

Jiaqi

  • +1
    TI__Mastermind 39270 points

    Thanks for reaching out. If the value of Cdv/dt capacitor is less than 50nF in your design, please use only RC circuit. 

  • 0 in reply to Avishek Pal
    Prodigy 230 points

    Hi Pal,

    Thanks for your answer. I noticed that two different RC connection methods work in PSPICE simulation. I attached the figures above. Do you think I can connect R and C in series on the board? because it's easier to achieve based on my existing board. 

    I am looking forward to your reply.

    Thank you

    Jiaqi

  • 0 in reply to Avishek Pal
    Prodigy 230 points

    Hi Pal,

    Just curious, anything happened in the PSPICE simulation today? I ran an RC circuit with LM5069 yesterday; it was working, but today, both RC circuits are not working. I am sure I am running the same file. Looks strange, does it often happen?

    Thank you

    Jiaqi

  • +1 in reply to Jiaqi Yuan
    TI__Mastermind 39270 points

    This circuit should be used. 

    Jiaqi Yuan said:
    Just curious, anything happened in the PSPICE simulation today? I ran an RC circuit with LM5069 yesterday; it was working, but today, both RC circuits are not working. I am sure I am running the same file. Looks strange, does it often happen?

    I never seen like this.

  • 0 in reply to Avishek Pal
    Prodigy 230 points

    Hi Pal,

    Got it, thank you

    Jiaqi

  • 0 in reply to Avishek Pal
    Prodigy 230 points

    Hi Pal,

    I ran the LM5069 PSPICE model with the dv/dt startup circuit, and it works. The base resistor is 1k, and Cdv/dt=10nF. I understand I can use  RC circuit, but I think a dv/dt startup circuit is better to handle the overcurrent situation. I tested it on the hardware with the same circuit at 24V input, same parameters from the TI design calculator. It is not working once the dv/dt startup is added. The N-MOS is partially ON, OUT=6.5V, and Gate =9.1V. Once I removed the dv/dt startup circuit, it worked. The OUT=24V. Do you know what the reason for that? The simulation works, but the hardware does not. Do you have any suggestions for that? 

    I am using Q2: MMBT2907A and D2: BAS16 on hardware.

    I am looking forward to your reply.

    Thank you

    Jiaqi

  • 0 in reply to Jiaqi Yuan
    TI__Mastermind 39270 points

    Please send ne the following waveforms for both the successful and unsuccessful start-up cases.

    VOUT, GATE, Input current, TIMER

    All these four signals should be captured in a single scope-shot. 

  • 0 in reply to Avishek Pal
    Prodigy 230 points

    Hi Pal,

    I solved it. It works. Thanks for your help.

    Best regards

    Jiaqi