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LM3492HC: Query on the CDHC function diable

Sunil Kumar Nagendra
Prodigy 90 points
Part Number: LM3492HC

As per the datasheet, 10Mega ohm pullup on the CDHC pin will disable the CDHC function.

In my design, I am planning to disable completely the CDHC function by providing a strong pullup. And configure the LM3492HC to a constant voltage (9.6V typical) by setting the VFB to 2.25V using the feed back resistor.I am planning to control the current though the PWM (Dimming). Please let me know your thoughts?

Regards

Sunil

  • 0
    TI__Mastermind 25500 points

    Hello Sunil,

    I will forward this to a couple people that probably know this device better than I do to see if they have any comment. But from what I can tell this device isn't set up for a constant voltage output. It may be possible (which is why I will forward this on), but from what I gather in the datasheet if you pull up on DHC it will cause VFB to rise to the OVP threshold, not 2.25V. It also looks as though it only overrides DHC at low PWM duty cycles. Plus depending on the output voltage and the variation in the LED string Vf you may get into an IOUTx OV situation.

    Is there some reason for a constant voltage output? The DHC function greatly improves efficiency and performance.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 90 points

    Hi Clint,

    My appication is  :The input voltage is 5V and required output typical is 9.6V and maximum output voltage is 10.5V. The current through LED is 200mA (fix).

    I am not able to design the Feedback voltage as 2.88 during maximum output voltage (10.5V) and  less than 2 V when typical (9.6V). So i was thinking to overide the DHC function and use as a normal LED driver which can drive the LED current based on the PWM input.

    Please suggest on this

  • 0 in reply to Sunil Kumar Nagendra
    TI__Mastermind 25500 points

    Hello Sunil,

    Perhaps I am still not understanding exactly what you are trying to do. But it sounds like you may not want to disable DHC actually. A variable output voltage (LEDs change Vf over temp) is what this device is designed for. It does not regulate a FB voltage, it only monitors it. It regulates the current through the IOUTx pins and the DHC adjusts the output voltage to maximize efficiency.

    So in this case you would set your LED current and then adjust the feedback resistors to get 1.5V at the FB pin at your nominal output voltage of 9.6V as described in the datasheet. Then when the output is 10.5V the FB voltage will be about 1.64V and the output will increase (via DHC) to account for the extra voltage and keep the LED current in regulation. Then you can control the average current by PWM dimming.

    Let me know if I'm missing something, but it sounds like this device is designed for what you are trying to do as is. Thanks.

    Regards,

    Clint