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LM2776: LM2776: flying capacitor value

PRAHLAD Supeda
Expert 5545 points
Part Number: LM2776

LM2776 data sheet suggest: "In typical high-current applications, TI recommends 0.47-μF or 1-μF 10 V low-ESR ceramic capacitors for the

flying capacitors".

What is "typical high-current" value referred in the data sheet for TI recommended flying capacitor value?

My customer is using 0.1-uF flying capacitor & want to know, if 0.1-uF can be used for < 20mA load?

With 1-uF capacitor LM2776 output voltage is doesn't increase beyond -1V for 3.3V input. Output increases to -2.7V with 0.1-uF with 3.3V input. 

  • 0
    TI__Intellectual 1120 points

    Hi Prahlad,

    the LM2776 can operate just fine with 1uF flying capacitor when driving small (<20mA) load. In fact, evaluation board has 1uF flying capacitor.  As you mentioned, minimum recommended flying capacitor value is 0.47uF, but flying capacitor must be selected so that capacitance does not fall below 0.2uF.

    If circuit is not operating correctly with 1uF flying capacitor, there might be issue in layout. Proper layout is important for stable operation of the LM2776. 

    Can you provide schematics, PCB layout figures and BOM list? Additionally oscilloscope waveforms showing VIN,VOUT,EN and C1+ signals at start-up would be helpful.

    Regards,

    Juha