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LM5118: Switcher get stuck while ramping to setpoint

Chris_CBC
Intellectual 470 points
Part Number: LM5118

I have a strange problem that I am trying to overcome. The buck/boost switcher below converts a 12 volt rail to a 5 to 55 volt rail (output voltage is controlled by a micro controller via PWM to an optocoupler). The output rail powers two low frequency coils (125KHz) that are antennas for an RFID exciter. The load on the switcher is pulsy as the coils are OOK to produce a signal the receiving tags understand.

The design works and is currently in production, but what we are finding is that 5% to 10% of units exhibit a problem where the switcher gets stuck while ramping to set point. Usually the switcher gets stuck at an output voltage of about 3.3 to 3.5 volts. At the time the switcher gets stuck there is no load on the output. At the FB pin we can see that the control loop is trying to push the output higher, but the switcher is unresponsive.

Any ideas?

  • 0
    TI__Guru* 76785 points
    Hi Chris,

    Thank you for using the device in your design. Does this only happen with no load? Could you measure the COMP and FB pin voltages when the circuit get stuck in ~3.3V output?

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    Intellectual 470 points
    I assume you want DC voltmeter measurements. I can do scope captures if you would like.

    FB = 0.04 V
    Comp = 4.70 V
  • 0 in reply to Chris_CBC
    TI__Guru* 76785 points
    Yes your are right, I want to see if the error amplifier behaves correctly. Your results show that the error amplifier is good. Now we need to find out what causes the low Vout.

    You have not answered my question: does the problem (getting stuck in 3.3Vout) only occur under no load? If you add some load, what does it look like?
  • 0 in reply to Youhao Xi
    Intellectual 470 points

    You are right. I neglected to answer that. Loading helps.

    Any load below about 250 ohms helps. The switcher no longer gets stuck.

    Thanks for the help.

  • 0 in reply to Chris_CBC
    TI__Guru* 76785 points

    Beside your case, we never heard anything like this with the LM5118 in the past.  We will look into this and get back to you.

    Thanks,

    Youhao