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LM73605: Startup Problems

Matthias Fiegel
Prodigy 50 points
Part Number: LM73605

I have a circuit using the LM73605 as Buck converter 10-24V to 5V. It is about the same as the typical Application circuit from the Datasheet.

Cout = 160µ, L = 6.8µ, f = 500k (FORCED PWM), 5ms internal ss

With No load present the converter starts up fine. I can add any load when its runnung.

But with any load it will not start up properly:

4,7µH 47µF LC filter with abput 10mA load = 1.92V output with high ripple (About the Hiccup threshold), same Voltage form on switchnode (not typical Rectangular)

4.7Ohms, 2.35Ohms, 9.4Ohms 0V Output. Just a short spike to 2V then 0V no Hiccup, it doesnt even try. When removing the load long enough it starts up.

I tried: Soft Start time up to 50ms, different switching frequencys (300-900k), Different UVLO Settings. Nothing has helped so far.

Edit:

Output voltages for different loads:

I dont understand why there is a plateau , also switching sems to start at the plateau.
I dont understand why i get only 1.9-2.5V Output

On 10 Ohm load the switching does not even start

500Ohm Load

300Ohm Load


10Ohm Load (Different SCALING)

  • 0
    TI__Mastermind 29950 points

    Thanks for the post, Matthias.

    I want to make sure I am looking at the right circuit that you designed.

    1. Is it Figure 25 in the datasheet , pp25 (see below)?

    2. Can you share an additional waveform capture of the startup with switch node waveform in addition to the output voltage and input voltage

    2.a 500ohm load

    2.b 10ohm load

    3.  Can you share an additional waveform capture of steady state with switch node waveform in addition to the output voltage and input voltage

    3.a 500ohm load

    3.b 10ohm load

    Taking the measurement with a passive probe, DC coupled will be fine.

    Sorry for tasking you with this, but this would be helpful for me to get to the root cause faster and for us to figure out a solution for your design.

    Thanks.

  • 0 in reply to Marshall_Beck
    Prodigy 50 points

    My Schematic: (SS cap added with wire, Swapped R303 for 220n cap Tried different RT Values)

    No Load startup Green = Switchnode, Yellow = Output:

    Switchnode during Ramp after plateau

    Steady State



    500 Ohm Startup (Pulses look the same as no Load)



    Output voltage ans switchnode with 10 Ohms
    This pulse is the same as in other startups before the first ramp (the trigger point)

  • 0 in reply to Matthias Fiegel
    Prodigy 50 points

    Ps i removed one last LC filter dor these.
    Also the steady state of 10Ohms is about 0.1V DC on both nodes nothing else

  • 0 in reply to Matthias Fiegel
    Prodigy 50 points

    Here a 350 Ohm Load , it only outputs about 3 Volt no switching, at 360Ohms switching starts like 500Ohm/NoLoad

  • 0 in reply to Matthias Fiegel
    Prodigy 50 points

    After building a second version i noticed the Problem.

    Bias and VCC caps where shorted. After removing the short it works fine. It seemd the burden overloaded VCC pin