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SN6507: Specifications for SS/ILIM pin

Lukas Rettenbacher
Prodigy 70 points
Part Number: SN6507

Tool/software:

Good day,

I seem to be missing a piece of information over and over while going through the datasheet.
How can the default voltage on the SS/ILIM pin be estimated?
The bias current on this pin is 275 uA typ. according to the datasheet, but the abolute maximum voltage on the pin must not exceed 6V.

Using the bias current (neglecting the leakage current) and an example R_ILIM = 47kΩ I get a voltage of 13V.
Is it internally capped at a certain voltage?

Furthermore, in section 8.3.6.2 "Over Current and Short Circuit Protection (OCP)" below the table 8-3 the following is stated:
"In the event of a transient overload or short circuit, if the resulting voltage dip is lower than 2.5 V (typical) on the SS/ILIM pin, the device considers it as a “soft-short” condition."
It's nowhere mentioned how the voltage behaves in relation to the input voltage/current through the switches.
Why should the voltage on the pin drop and why is the limit of 2.5V not mentioned anywhere else?

I am particularly interested in this in order to be able to select a capacitor with the appropriate DC bias.
If you could clear some things up for me, I would be very grateful!

Thanks in advance and best greetings,
Lukas Rettenbacher.

  • 0
    TI__Guru 55815 points

    Hi Lukas,

    Thank you for reaching and sorry about the confusion in the datasheet.

    SS/ILIM is a dual function pin and the two functions are soft-start and current limit. The internal bias current of 275µA is part of the soft-start circuit is only active at power-up till soft-start is active. Therefore, you can ignore the bias current for current limit function.

    Regarding the voltage on SS/ILIM pin, please ignore this voltage as this is not relevant for device operation. You do not need to monitor this voltage or make any decisions based off of this voltage. The resistor on this pin decides the current limit and if there is a short-circuit at the output causing the input current to increase to >5A then device detects a hard-short otherwise, it detects a soft-short when current exceeds the current limit.

    I hope this helps you with better understanding. Let me know if you have any further questions, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 70 points

    Hi Koteshwar,

    thanks for the swift answer. Generally, i can see why the voltage is not of importance.


    But like I mentioned in my previous post, I would like to get a feeling on what voltage this pin usually is for choosing an appropriate capacitor.
    When using a 16V MLCC the difference between 3V and 6V can be tremendous. As an example, I consider using this one.
    (https://www.murata.com/en-eu/products/productdetail?partno=GCJ21BR71C475KA01%23)
    Or is it negligible, since during startup the voltage on the cap is slowly rising and doesn't affect anything anymore afterwards.

    If i loose 20% of my C_SS cap vor example, I won't fullfil the C_OUT < 10 x C_SS condition anymore.

    Best regards,
    Lukas Rettenbacher

  • +1 in reply to Lukas Rettenbacher
    TI__Guru 55815 points

    Hi Lukas,

    I understand the reason why you would like to know the expected voltage on the capacitor at SS/ILIM pin. This is typically going to be around 1.3V and can go upto 5V under overcurrent condition. Since the voltage is going to be 1.3V under normal operating conditions, you are probably good with 16V rated capacitor.

    I hope this answers your question, thanks.


    Regards,
    Koteshwar Rao