TPS562209 - About current limit value (min/typ/max)

Hi Team,

 

Would you tell me about the progress on this matter?

If you could reply by today, it would be very helpful.

I’d greatly appreciate your verification.

 

Regards,

Kanemaru

Hi Kanemaru-san,

I am working on this case and tried to contact IC designer to know the voltage threshold on OCL. I did not get any feedback yet. I am thinking about other ways to calculate or get the current limit value. I will get back to you soon.

Best regards,

Anthony

Hi Anthony-san,

Thank you always for your kind support.

I’m looking forward to hearing from you.

Regards,

Kanemaru

Hi Kanemaru-san,

I did not get the actual data from our designer team. But I figure out a way to get a rough estimated current limit level. The OCL function detects the valley inductor current. The current measurement method for inductor current is using low side MOSFET Rdson(72m ohm). The inductor current flowing thru low side MOSFET will generate a negative voltage on SW pin( IL*Rdson), this voltage will compare with the OCL threshold to determine when the current limit will start to work. In this case, no matter what kind of inductor, if the OCL threshold is a fixed reference, the valley current limit level will be the same under all working condition. We could use current limit Iocl in the datasheet( output current) with the input, output and inductance to calculate the valley current level( Iocl-1/2*inductor ripple). With this valley current level, we could add a new half inductor ripple under a new working condition. I think in this case, we could get an estimated current limit value under other working condition.

Best regards,
Anthony

Hi Anthony-san,

 

I’m sorry for the delay in my reply.

Thank you for the information.

 

I understand that the calculation value of current limit(Iocl) is around 2.051A(min) , 2.93A(typ) , 3.809A(max) at the customer’s conditions.

Is my understanding correct?

 

[Calculation process]

1.To calculate the 1/2 inductor ripple at the customer’s conditions and the datasheet conditions using the equation(3).

  Customer conditions : 0.606A

  Datasheet conditions: 0.335A

  So, the differential value is 0.271A

 

2.To calculate typ of current limit(Iocl) at the customer’s conditions.

 This current is 2.93A(typ) (3.2A-0.271A).

 

3.To calculate min and max of current limit(Iocl) at the customer’s conditions.

 I refer to min and max fo current limit in the datasheet.

 Therefore, I included an error of 30%.

 Min: 2.93A * 0.7=2.051A

 Max: 2.93A * 1.3=3.809A

 

Regards,

Kanemaru

Hi Kanemaru-san,

Since ripple for customer working condition is higher than datasheet condition. The current limit for customer should be 3.2A+0.271A=3.471A(typ).

Best regards,
Anthony

Hi Anthony-san,

 

Thank you for your response.

I understand that current limit for customer is lower than spec of datasheet, if the OCL threshold is a fixed and ripple for customer working condition is higher than datasheet condition.

Because, I guess that the inductor current reaches the threshold early if the inductor ripple current is high.

So, the current limit for customer should be 3.2A-0.271A=2.93A(typ).

Is my understanding correct?

 

I am terribly sorry that I confuse you…

I would appreciate it if you would give me some advice.

 

Regards,

Kanemaru

Hi Kannemaru-san,

Let us just see an example. In the datasheet, for the current limit level, we have two sets of data. One is for 2.2UH and the other is 1.5UH. Since the other working condition is the same, smaller inductance means higher ripple. The current limit for 1.5UH is higher than that for 2.2UH. This tells us that higher ripple leads to higher current limit level.

Best regards,
Anthony

Hi Anthony-san,

 

Thank you for your prompt reply.

I understand that "A" and "B" are the different OCL threshold.

A: OCL threshold = IL/Rdson(72mohm) for TPS562209

B: OCL threshold = IL/Rdson(39mohm) for TPS563209

 

Therefore, I believe that I can’t compare "B" with "A" only with the inductor value.

Is my understanding correct?

 

Regards,

Kanemaru

Hi Kanemaru-san,

Yes, maybe you are correct. Since I am not the editor of this datasheet and it is not well explained, I did not figure out it is not for the same IC. Your understanding is more reasonable. But I still hold my idea on the current limit calculation.

In datasheet, there is a sentense:"The load current is higher than the over-current threshold by one half of the peak-to-peak inductor ripple current." Since our current limit method is based on detecting the valley inductor current. Only if the valley current is higher than the OCL threshold, then our current limit will work. In this case, the valley current level that will trigger the current limit function will be an constant no matter what kind of working condition it is. But if the valley current reach the OCL threshold, the inductor current or the current limit level will depends on the working condition. This current limit level will be higher if the current ripple is higher, which is what I meant in the previous posts.

Best regards,
Anthony

Hi Anthony-san,

 

Thank you for the detail information.

I understand. Thank you very much.

 

Regards,

Kanemaru