LM61480: Short Circuit protection

hi,Julien,

sorry to hear you have issue when using LM61480. Could you please provide some of you waveform or more information ,SCH and layout is better.

Elena

Hi Elena,

I send you a private message.

Julien

hi, Julien,

For you SCH it can work well under stable input and output, only when you do short test, I am thinking this is related with system issue not a device OCP issue. So first check if remove L2L3, is workable, if not next try add input bulk cap ,like 47u or 100uf see if any improve.

BTY, how you did short on Vout ,suggest do not use a cable or long wire, just hard short.

Elena

Hi Elena,

Ok, I'am going to replace L2 and L3 by a wire and add a 100µF between VIN1-VIn2 and PGND1.

For the short on Vout, I used two 30cm wires. I don't understand the link with the lenght cable?

Anyway, do you  confirm me that the chip is tolerant with short-circuit?

Julien

hi, Julien,

LM61480 have hiccup mode when OCP. you can refer to detail in DS section.

Elena

Hi Helena,

Yes, I have seen this section about hiccup but this is not very clear for me.

Is the protection is available only for a short time short-circuit or for a permanent short-circuit too?

Also, I am not sure to understand the 3 conditions:

 "A time greater than tss2 has passed since soft stat has started" => So, the protection is not available if the is short-circuit at the startup?

"Output voltage is below approximately 0.4 times output setpoint" => Ok, in my case (+12V), the protection is available if Vout <4.8V

"The part is not operating in dropout defined as having minimum off time controlled by duty factor" => Not clear for me.

Julien

Let me loop our expert here ,he will reply to you soon. 

Elena

Hello Julien,

In the case of a hard short circuit, the device will be in hiccup mode until the physical short between VOUT/GND is removed.

In this instance, the inductor current will periodically pulse up to the short circuit current limit specified in the datasheet.

This means that in a short circuit fault condition the LM61480 will have inductor current up to 13.8A momentarily, turn off, wait 40ms and retry periodically. 

Is the damage happening on the LM61480 or on the efuse. If the efuse is destroyed, then I would suggest looking into the efuse current rating specs and see how much current it can handle. It could just be that the current from the inductor current that is delivered into the efuse is too much for current and damaging the fuse, of which a higher current rated fuse needs to be swapped for.

Regards,

Jimmy

Hi Jimmy,

"Is the damage happening on the LM61480 or on the efuse" => This is the LM61480 which is destroyed, not the fuse.

I don't undestand wihy the hiccup mode doesn't work.

Do you have an idea?

Julien

Hello Julien,

Please attach both the full schematic and PCB layout for review here.

For the damaged LM61480 do you have any pin continuity and ohm meter readings that can help me understand what happened here? Is VIN shorted to GND, etc?

Regards,

Jimmy 

Hello Jimmy,

How can I send you schematic and layout by private message ? 

When the chip is damaged, the VIN is shorted to GND (0.2ohm).

If I remove and replace the chip, the board works fine.

Julien