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LM3405A: Can remove D2 in LM3405A

Part Number: LM3405A
Other Parts Discussed in Thread: LM3407, LM3409

Hi,

I see in the LM3405A datasheet, need D2 and boost capacitor to generate boost voltage? why LM3405A need D2 and boost capacitor? in other buck LED driver, only need boost capacitor for boost voltage, thanks.

  • Hello Betty,

    The LM3405A uses a high side N-channel MOSFET.  To drive the gate to turn the MOSFET on requires a voltage higher than Vin.  Many parts use this configuration for generating a switch node referenced voltage to drive the N-channel MOSFET.  The TPS92515A is one example.  There are other parts that do not need the boost voltage such as the LM3409 since it uses a P-channel MOSFET or a part that has a low side MOSFET such as the LM3407.  There are a few parts that include the boost diode internal however this part has multiple methods for generating the boost voltage since it does not have a VCC to generate boost from.  So depending on input voltage, output voltage and other voltages available on the board different methods can be used to generate the boost voltage.

    Best Regards,

  • thanks Irwin.

    On the datasheet, the function block diagram of LM3407 has a mistake abnout the FET. It should be the NMOPS, but the diagram show a PMOS.

  • Hello Betty,

    So diode D2 is needed or see section 7.3 on page 9 of the datasheet.

    Best Regards,