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LM27951: About selecting capacitor

user4030081
Prodigy 100 points
Part Number: LM27951

Hi dear supporting team,

When selecting a capacitor, I think it is necessary to consider not only
the temperature characteristics of the capacitor but also the DC bias characteristics.
So please tell me the value of the question below.
・voltage of VOUT, VC1(voltage of C1) and CV2(voltage of C1) when VIN = 5V
・minimum capacitance of COUT, C1 and C2

  • 0
    TI__Genius 15776 points

    Hardy, please help in this case.

  • 0
    TI__Genius 12915 points

    Hi,

    For C1 and C2, they are used for internal charge pump circuit. We do not recommended to change the capacitance. The voltage of these pins are quickly switching from 0 to VIN or VIN to VOUT

    For COUT, it is 1x or 1.5x of VIN, in your case you can set VOUT = 5V or 7.5V, which is actively selected to maintain regulation based on LED forward voltage and load requirements. The capacitance is more like a experience value. There is no study for a minimum value. I think you could also keep the recommended 3.3uF in typical application.

    BTW, I am curious why you select this part since it is a quite old device that released in 2005. Could you help to share your system requirements for LED driver, there may have some other parts could be used.

  • 0 in reply to Hardy Wu
    Prodigy 100 points

    Thank you for your reply.
    The reason I selected this device is We need to drive LED Display Backlights
    that consists of 4LED array with cathode common, each LED current is 20mA.
    Do you have anything device you recommend in that case?

  • +1 in reply to user4030081
    TI__Genius 12915 points

    Well.... Common cathode LED is not seen usually. We do not have many choice for it. Most of backlight device integrate current sinks for common anode LED. This device is the most suitable one.