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LM2679: Soft Start used as enabled

Mark Philip Felipe
Genius 9915 points
Part Number: LM2679
Other Parts Discussed in Thread: LM2678, LM2677

Hi Team, seek for some assistance.

LM2679SX-5.0/NOPB and it turns out that we may need to have an enable feature, which this part does not have. However, looking at the block diagram in page 9 of the datasheet, it seems there could be a way to use the soft start pin to enable the chip. 

The block diagram shows that there is a 4.5uA current source hooked on the soft-start pin, to load the capacitor. Meanwhile, section 8.1.8 says the soft-start threshold is 0.63V. If we connect a 10k resistor to the soft-start pin, the 4.5uA would make a 45mV drop over that resistor, which is below the threshold, causing the chip to be disabled. Otherwise, leaving that pin open would enable the chip. A p-mosfet could be use to alternate between pin open and the 10k resistor. 

Would that work without causing adverse effect on the chip ? Furthermore, is the 10k resistor needed, or can we tie the soft-start pin to ground directly ? 

Hoping for your inputs.

Thank you.

-Mark

  • 0
    TI__Guru 65905 points

    Hello

    You can use a simple NMOS on NPN connected to the SS pin to provide a disable function.  

    However, this method has not been tested by TI. 

    It would be best to test this on an EVM before committing to production.

    Also, you may want to look at the LM2677 or LM2678, they provide an EN input.

    Thanks