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Original question:

LM5121 BOOST CONVERTER IC

LM5121-Q1: LM5121 resets itself LO and HO output suspended

Steve Hankins
Prodigy 30 points
Part Number: LM5121-Q1
Other Parts Discussed in Thread: LM5121

Vin is 27 to 55

Vout is Vin to 65

Lin is 4.1uH

Rosc is 43K or running about 208Khz

Cout is 10,000uF

Inrush limiting works fine, safely charging Cout to Vin, However Boost operation above Vin does not work.

One pulse on LO pin,  and chip resets itself and cycles itself continuously at SS or RES rate. Cout never charges above Vin.

Schematic follows:

  • 0
    TI__Guru* 78070 points
    Hi Steve,

    Thank you for considering the LM5121. It seems the OCP is tripped during soft-start. Your out capacitance of 10 000uF sounds excessive. We need to review your schematic. Somehow your schematic link is broken. Could I bother you to re-attach?

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Steve Hankins
    TI__Guru* 78070 points
    Hi Steve,

    Just let you know that we are reviewing this and we should get back to you in two days.

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    Prodigy 30 points

    hello Youhao,

    Thank you for reviewing the LM5121 design. I wanted to let you know a little more about the application:

    The output capacitor, Cout, is 10,000uF for the purpose of energy storage and not for filtering ripple. Periodically,

    the energy in the capacitor will be partially discharged and the LM5121 will restore the output voltage to the commanded

    level of about 65 volts. Typically the capacitor will be charged to 65 volts and discharged to 48 volts.

    The average output current for the converter will be approximately 5 to 7 amps, so about 400 watts at 65 volts.

    I hope this helps you better understand the application and expectations of this design.

    Thank you

    Steve

  • 0 in reply to Steve Hankins
    TI__Guru* 78070 points
    Hi Steve,

    Thank you for the additional info. Can you consider the circuit of Figure 35 or 36 in the datasheet? This will set the inrush current limit level to be lower than the cycle-by-cycle current limit level, hence avoiding the trip of hiccup mode? It is a simple circuit, can you add to your circuit and do a test?

    Thanks,
    Youhao