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LM3445: Role of Q3

Hirotsugu Kobayashi
Mastermind 8705 points
Part Number: LM3445

Hi, team.

What is the role of Q3 by a reference circuit?

When connecting between E-C of Q3, is there a problem?

Best Regards,

  • 0
    TI__Mastermind 25500 points

    Hello Hirotsugu,

    The LM3445 operates as a floating buck regulator. That means that regardless of your LED stack voltage VBUCK will be the same. So Q5 and R14 set a COFF charging current based on the LED stack voltage. The higher the stack voltage the shorter the OFF time, the lower the stack voltage the longer the OFF time. Of course the higher the stack voltage the longer the ON time and so on...

    So what Q5 does is compensate for the stack voltage so that your switching frequency remains relatively constant. This is good for having multiple designs with different load voltages, but it is also good for changes in the forward voltage as the LED stack heats/cools.

    You can disconnect the base and short across the emitter/collector and it will work fine, but you will have much wider variations is switching frequency.

    Regards,

    Clint