LM5176: hiccup trigger

Hello Fred,

When a converter starts up, it needs to charge the output capacitors, so it could happen that a lot more current is needed than during normal operation. Especially when using a current load. Can you ask the customer to check if he can use a resistive load instead of a current load?

You could as well disable hiccup mode and check how the startup looks then.

Please check if the problem disappears when reducing the sense resistor value.

Best regards,
Brigitte

Hi  Brigitte,

you're right, they turn on the load first before the power on, so large current sink to output and trigger OCP,

so my question is , can they prevent this situation by increase the soft start cap? 

since we don't want to set the OCP higher for this

Regards,

Fred

Hi Fred,

Increasing the SS cap would make the start-up time longer, I am not sure if it would solve your situation.

Did you manage check if the problem is solved after reducing the sense resistor value?

BR,

Haroon

Hi Wardak,

soft start is meant to help this situation, so extend the soft start time supposedly can prevent the inrush power on current, 

so please check again on this.

let me put in this way,  if there's a resistive load , does soft start extension will reduce the current peak during power on?

Hi Fred,

Sorry for my previous message, I misunderstood the question. Increasing soft-start can make sure that the high peak from the inrush current can be suppressed or reduced, as during the start-up when the SS pin voltage is below the internal Vref, the SS pin takes over the FB voltage regulation, once the SS votlage goes above the Vref, the soft-start is complete and during this soft-start the Vout is slowly ramping up to it's value and in this way the peak of the inrush current can be reduced or suppressed by extending the time of the soft-start. However, you still might face the same issue as before, so it is important to check your over current limit margin in this case so that the OCP is not initiated during the start-up.

Hope this helps.

BR,

Haroon

Hi Wardak,

they change soft start cap to 2uF,

12Vin stop hiccup and 9V vin still has this issue,

should we keep increasing the SS cap?

issue has been solved after they change to 4.7uF

thanks