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LM5021: How to disable hiccup-mode

Josef O.
Prodigy 10 points
Part Number: LM5021

Tool/software:

Hello,

I am looking for a way to deactivate the hiccup mode of the LM5021.

To do this, I removed the soft start capacitor and connected a 200k resistor between VCC pin and SS pin. This disables both the soft start function and the hiccup function.

I have tested this in various operating states and it works.

Is this method suitable or do you see the possibility of a malfunction?

The reason why the LM5021 detects overload is understood and can be tolerated (voltage drop at NTC thermistor during startup). In my opinion, increasing the size of the soft start capacitor is not expedient.

 

Provided the method it is suitable:

With a VCC voltage of 8.5V and a voltage of the SS pin of 5.2V, a current of 16.5µA flows through the 200k resistor, which compensates for the overload detection timer current.

However, the SS pin can theoretically be raised to 8.5V, which is outside the absolute maximum ratings. Do I have to limit the pull-up voltage to a value below 7V?

In my tests, the voltage of the SS pin always remained below 6V.

  • 0
    TI__Guru* 85926 points

    Hello,

     

    We will never recommend disabling an over power protection feature.  I have not used this device in this fashion.  So, I can’t reliably tell you what issues you will or could face.  I do recommend that you evaluate this concept with reliability, validation and production testing to make sure you don’t run into any hidden or unknow issues.

     

    This will disable soft start and you will be charging the output capacitor of the design the energy storage in the flyback transformer.  The current will by cycle by cycle limited by the 500 mV comparator inside the controller.  So, I would check for over shoot during startup just to make sure you don’t need soft start.  I would test it over line and load.

     

    Another issue I can think of is short circuit protection.  You will have no protection against it.  If you disable the over current hiccup mode, you might want to have a discreate over current protection that shuts down the design in case of short circuit on the output.

     

    Rather than completely disabling this feature you could adjust the SS capacitor so it takes longer to trigger over power protection hiccup mode. 

     

    Regards,