SN6507: SS/ILIM voltage level after soft start

Gilles Lopez

Prodigy 20 points

Part Number: SN6507

I need to select the voltage rating of the Css capacitor.

What is the maximum voltage at the SS/ILIM pin?

I suppose it depend of Rlim.

In my application, Rlim is 18.2kOhm.

During soft start, Iss is 275uA. What is this bias current afer soft start?

5 months ago

0 Andrew Jackiw 4 months ago

TI__Genius 17555 points

Hello Gilles,

Thank you for reaching out. The Absolute maximum for the SS/ILIM pin is 6V. After soft start the I_bias is off and I_Leakage is 0.7uA as shown below. The Css capacitor should be rated to handle at least the abs max value of 6V

Best,
Andrew

0 Gilles Lopez 4 months ago in reply to Andrew Jackiw

Prodigy 20 points

Thank you for you return.

So with this current leakage, I don't understand why I measure 1.2Vdc between SS/ILIM pin and GND with Rlim=18.2k and Css=4.7uF.

Have you an explanation?

0 Michael J Schultis 4 months ago in reply to Gilles Lopez

TI__Mastermind 24665 points

Hello Gilles,

That is a fair question and unexpected. Can you share a schematic of the setup and a scope shot of the operation upon startup?

Best,

Michael

0 Gilles Lopez 4 months ago in reply to Michael J Schultis

Prodigy 20 points

Here are the schematic and an oscillope acquisition of the SS/ILIM pin upon startup

Rlim=18.2k

Css=4.7uF

Rclk=40.2k

NC= Not Connected

Output snubber 22.1R +680pF

On pins 2 and 9: Resitors are 0R

BR,

0 Andrew Jackiw 4 months ago in reply to Gilles Lopez

TI__Genius 17555 points

Hello Gilles,

680pF is too small of a value for Css. Css should be between the ranges listed in the recommended operating conditions.

Also, "To ensure a smooth transition between soft-start and the steady state, it's recommended to have a CSS value between 50 nF and 5 µF with an output capacitor, COUT, of less than 10 times the value of CSS." 8.4.1.1 Soft-Start.

I believe what you are seeing in the scope shot is a result of having too small of a capacitor as Css.

Please let me know if making this change helps.

Best,
Andrew

0 Gilles Lopez 4 months ago in reply to Andrew Jackiw

Prodigy 20 points

Css is not 680pf but 4.7uF as i wrote in the previous message.

680pF is the value of the capacitor used on the snubber on the SW pin.

Gilles

0 Aaditya Vittal 4 months ago in reply to Gilles Lopez

TI__Expert 7975 points

Hi Gilles,

Thank you for the information. Please give us an additional 24 hours to review and respond.

Regards,
Aaditya Vittal

0 Andrew Jackiw 4 months ago in reply to Aaditya Vittal

TI__Genius 17555 points

Hello Gilles,

I have reviewed the post with the team and a 1.2Vdc between SS/ILIM pin and GND is okay, and this is an expected behavior.

I apologize for the confusion.

Best,
Andrew

0 Gilles Lopez 4 months ago in reply to Andrew Jackiw

Prodigy 20 points

Hello,

So 1.2Vdc is the typical value?

What could cause the value to change?

Does it depend on the value of Rlim?

BR,

Gilles

+1 Koteshwar Rao 4 months ago in reply to Gilles Lopez

TI__Guru 52660 points

Hi Gilles,

Thank you for following up.

Yes, 1.2V observed at SS/ILIM pin is an expected typical value for RILIM of 18.2kΩ.
Yes, the voltage on SS/ILIM is dependent on the RILIM resistor value.

Is there any specific concern you have with the voltage on SS/ILIM pin?
I will not be able to disclose exactly how the SS/ILIM circuit is implemented but I will be able to address any concerns you have on this. Let me know, thanks.

Regards,
Koteshwar Rao

Isolation

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SN6507: SS/ILIM voltage level after soft start