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LM3447 Constant Voltage Output

MunChung Wong
Prodigy 30 points
Other Parts Discussed in Thread: LM3447

Hello, can i use the pin of INV to accept the opto-isolator feedback from the secondary output to achieve constant voltage output?

Thanks.

Regards,

MC

  • 0
    TI__Mastermind 25500 points

    I am not one of the experts on this particular device so I will forward this to somebody that knows it better. However it does say you can use an opto to regulate secondary side current so I see no reason why you could not use an opto and a resistor divider to set an output voltage.

  • 0 in reply to Clinton Jensen
    TI__Mastermind 28655 points

    Hello,

    I think it would work that way.  You could also try regulating the primary bias voltage to get a somewhat regulated output voltage without the use of an opto.  It would depend on power level and transformer leakage.  It would probably need a minimum output load to work this way.

    Is there a reason you are looking at the LM3447 for a voltage regulator?

    Thanks,

  • 0 in reply to Irwin Nederbragt
    Prodigy 30 points

    Hello,

    It is to drive the constant voltage LED lamp, the power of the lamp is around 10W.

    Thanks for the great idea of eliminate the use of opto as a feedback. Is it possible that I directly use back the auxiliary winding that is use to power the IC as feedback to regulate the output voltage?

    In order to regulate the output voltage, the feedback need to connect to the INV pin. Is this INV pin being pulled up to a fixed voltage or at what voltage range does it function?

    Thanks.

    Regards,

    MC

  • 0 in reply to MunChung Wong
    TI__Mastermind 28655 points

    Hello,

    The INV pin is the negative input to the error amplifier.  The reference is internal at 1.0 volt so with feedback it will try and regulate the pin to 1.0 volts.  This is power factor corrected so bandwidth needs to be low.  With a flyback transformer the output voltage will be the ratio of the number of turns of the windings.  So if the primary bias is 10 turns and the secondary (or output winding) is 20 the output will be about twice the voltage of the primary bias.  So if you set the bias winding to regulate at 15 volts the output will be about 30 volts.

  • 0 in reply to Irwin Nederbragt
    Prodigy 30 points

    Hello,

    Since the INV is the input to the error amplifier, is it possible to make connection below as a feedback to the INV pin?

    Thanks.

    Regards,

    MC

  • 0 in reply to MunChung Wong
    TI__Mastermind 28655 points

    Hello,

    That's how I would start, I'm not sure how it will behave.  If the bias supply capacitor is large there may be stability issues with the loop (which would need to be slowed down).

    Thanks,