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LM5069-2 Power Limit out of scale

JUAN VALENCIA
Prodigy 50 points
Other Parts Discussed in Thread: LM5069, LM5066

Hi,

I am having a strange issue with the expected values at PWR limit feature.

Application: Inrush control at 50Vdc bus

Capacitor: 1000uF

Current limit aprox 22A (Rshunt 2mOhm)

Power limit programmed aprox 5W (R pwr limit 1K)

Ctimer: 1u. Should not work with the previous values, but as the circuit is charging higher than expected it works ok and IC does not enter the failure cycle.

With the values described, if I am not wrong, the maximum initial inrush current should be around 0.1A but I am measuring 1A (10 times more)

I have checked the sense voltage at IC pins to verify no problems with Kelvin measurement and voltage is ok (aprox 2.2mv at 1A)

Attached a waveform:

Light Blue: Output Voltage

Dark Blue: Input Voltage

Green: Input current

(ignore the rest as they are references..)

Any idea of what is going wrong??

 

TEK00000.TIF
  • 0
    TI__Intellectual 1005 points

    Juan,

    The power limit engine has it's limitations. Specifically this occurs at high current limit and low power limit, due to the sense voltage becoming very small.

    Unfortunately this was not captured in the LM5069 datasheet, but is explained in the LM5066 datasheet on pg 22 equation 3 and the following paragraph. (DS attached) The hotswap portion of the LM5066 uses the same IP as the LM5069.

    The minimum recommended sense votlage is 3mV. Idealy, I would recommend keeping it above 5mV.

    In addition the power limit becomes a funtion of Vds at low sense voltages <10mV.  This effect is captured in equation 2 on page 3 of the LM5066 DS.

    I apologize for the inconvenience.

    7801.LM5066_DS.pdf

  • 0 in reply to Artem Rogachev
    Prodigy 50 points

    Hello,

     

    Ok, I see...I will see how can I mnage a compromise with Rs, current limit and power limit (SOA) in order to work a little bit more close to your recomendations..

    Great to know there was a reasonable issue behind that behaviour.

     

    Regards