Hi
We have a inrush current problem.
And we have some question about current limit operation at startup. On the datasheet, it is mentioned as the following.
Q1. As it is mentioned, the current limit ramp up from an initial 400mA. However at Figure 17 the start up current is more than 400mA at Vout=0V.
Why is it more than 400mA?
Q2: How is the startup time and inductor current determined?
Regards,
Koji Hamamoto
The current limit in this paragraph means the input current. In buck mode the inductor current is the output current which is much higher that the input current.
Moreover this value of 400mA is not accurate (not in the electrical specifications), it is just giving a rough value. From a control point of view the average inductor current is reduced during start up and short circuit to avoid the current to run away. As a consequence, this is reflected at the input with a limited input current.
Regarding the 1.2V, it is an internal Voltage threshold for the current limit to ramp up. As you see in the scope plot it can be lower than 1.2V, but it is hard to give an exact value.
What is the main concern of the customer? What is the application? Do they want to extend the start up time?
Hi Sabrina-san,
Thank you for your reply.
I understand. However our concern is how much the inrush current(peak) flows at worst case during startup.
I think the current will flow until max current limit at worst case.
In addition, the above specification is at VIN=VINA=3.6V. Do you have the data at other condition ?(VIN=5V)
>What is the main concern of the customer? What is the application? Do they want to extend the start up time?
*I can not post the more information because of confidential. However I put the information as possible.
The upstream power supply will be hit to current limit if the inrush current is larger than expected.
That is why we asked the inrush current at worst case.
Regareds,
Koji Hamamoto
I could not find data for the other condition, we need to measure them, but I don't know if this would solve the customer problem.
I think good solution would be to build a soft-start circuit as described in this app note to make sure the inrush current is reduced : 
This circuit is applicable to all our buck-boost TPS63xxx series and I believe this would address the customer's concern,
You can send more information of the application in private message if it is confidential
