LM5068: How to Disable FDC for Short Period

Shinichi Yokota

Part Number: LM5068

Hello,

Could you give me an idea on how to disable LM5068's FDC (Fast Discharge Current) function for a short period of time? A circuit must continue its normal operation against a lightening surge of about 20 to 40 µs, but I'm concerned if it could trigger the FDC and could turn off the MOSFET switch.

I guess adding an RC filter of short time constant to the SENSE pin could work, but any feedback is appreciated.

Best regards,
Shinichi Yokota

over 2 years ago

0 Avishek Pal over 2 years ago

TI__Mastermind 30915 points

Hi Shinichi,

Thanks for reaching out. Fast Discharge Current Limit can't be disabled. Regarding work around I will get back to you on Friday.

+1 Avishek Pal over 2 years ago in reply to Avishek Pal

TI__Mastermind 30915 points

Hi Shinichi,

Adding an RC filter of short time constant to the SENSE pin will disable actual short circuit protection for that duration. Now it's the decision of the customer, which is more important.

Surge protection has to be taken care by placing appropriate TVS diode at the input in order to make sure that the absolute maximum rating of the controller is not violated. Series or parallel combinations of 5.0SMDJ or 8.0SMDJ Series TVS diodes need to be used based on the surge requirements.

https://www.littelfuse.com/media?resourcetype=datasheets&itemid=22eaf6f1-66e8-40a7-8916-2057b1d7cb37&filename=littelfuse-tvs-diode-5-0smdj-datasheet

https://www.littelfuse.com/media?resourcetype=datasheets&itemid=a0cea0c4-2291-4cbd-a55d-658acdb7512f&filename=littelfuse-tvs-diode-8-0smdj-datasheet

0 Shinichi Yokota over 2 years ago in reply to Avishek Pal

TI__Genius 9715 points

Avishek,

Thanks for your feedback.

By the way, what's the response time of the LM5068's SENSE pin? It's helpful if even a rough number is available.

Best regards,
Shinichi Yokota

+1 Avishek Pal over 2 years ago in reply to Shinichi Yokota

TI__Mastermind 30915 points

Hi Shinichi,

Circuit Breaker (CB) & Analog Current Limit (ACL) have response times of around 25 μs and Fast Discharge Current (FDC) has a response time of around 1 μs.

0 Shinichi Yokota over 2 years ago in reply to Avishek Pal

TI__Genius 9715 points

Avishek,

Thanks for your quick feedback.

Best regards,
Shinichi Yokota

Power management

Power management forum

LM5068: How to Disable FDC for Short Period