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    Dimming the MP2489 right down to zero (Buck LED driver)

    Hi,
    We wish to analog dim this chip down to 25% output, and then we obviously have to PWM dim to further get the leds dimmer from there. I trust that it is possible to pwm dim the chip with a 0.3v control voltage into the EN/DIM pin?.
    The datasheet seems to show pwm dimming only being done when the EN/DIM pin is pulled up to 1.27v.

    MP2489 datasheet
    https://www.monolithicpower.com/pub/...2489_r1.23.pdf

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    Re: Dimming the MP2489 right down to zero (Buck LED driver)

    If I understand the data sheet, the input is always analog but the voltage range is limited to the internal Vref. It gives you the option of controlling output current by dropping the enable voltage linearly -or- by chopping it with PWM -or- both. In other words to use it in PWM mode they are saying use the maximum voltage so you can get maximum brightness with 100% PWM drive but if your PWM voltage only went up to a lower voltage, that would be the ceiling at 100% PWM.

    Brian.
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    Re: Dimming the MP2489 right down to zero (Buck LED driver)

    Thanks Betwixt, i believe what you describe is the most logical thing, but i am not sure if the datasheet is purporting that...

    .....We are wanting to use the MP2489 LED driver……we realise that to get smooth dimming from max light down to almost zero light, we will need to first analog dim it, then pwm dim it to get down to really low light levels from the minimum analog dimming point….
    However, page 7 of the datasheet seems unclear as to whether or not this can be done. Page 7 seems to suggest that PWM dimming can only be done if the voltage applied to the EN/DIM pin is 1.25V. Would you agree with this?
    The circuit attached is how we intend to use it…..so in other words…….when we are applying 0.3V to the EN/DIM pin. We would like to then simply PWM Q4 in order to PWM dim it down from there…but is this possible?
    MP2489 datasheet
    https://www.monolithicpower.com/pub/...2489_r1.23.pdf



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    Re: Dimming the MP2489 right down to zero (Buck LED driver)

    I don't read it that way. That device has analog dimming with the level set by the voltage on the EN/DIM pin. Read 'EN' as digital enable and 'DIM' as the analog voltage but you would normally use one or the other, not both.
    With PWM drive, the logic low voltage will always turn the LED off, the logic high will turn it on at whatever the high voltage is. The data sheet suggests a 45K pull-up is already inside the IC to lift the pin up to internal Vref so all you have to do is pull it lower with external circuits. An open collector/ open drain PWM driver would do the job without additional components.

    Brian.
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    Re: Dimming the MP2489 right down to zero (Buck LED driver)

    Page 7 seems to suggest that PWM dimming can only be done if the voltage applied to the EN/DIM pin is 1.25V. Would you agree with this?
    The circuit attached is how we intend to use it…..so in other words…….when we are applying 0.3V to the EN/DIM pin. We would like to then simply PWM Q4 in order to PWM dim it down from there…but is this possible?
    The question is, why does the datasheet suggest minimal 1.25V control voltage for PWM operation? I see two possible explanations:
    1. They simply don't consider mixed analog and PWM dimming as useful option.
    2. Control voltage below 1.25V doesn't achieve stable PWM.

    The datasheet doesn't answer the question. Why not try in real hardware?


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    Re: Dimming the MP2489 right down to zero (Buck LED driver)

    Thanks, we will try in real hardware but must pay for the PCB to be manufactured first.

    Do you believe the cct in #3 will be OK for MP2489........we are using the opamp to over-ride the internall pullup and force the EN/DIM voltage to be lower than the 1.25v that it woudl otherwise default to.

    We simply must use combo analog and pwm dimming, since its the only way to get down to really low levels of light.



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    Re: Dimming the MP2489 right down to zero (Buck LED driver)

    We simply must use combo analog and pwm dimming, since its the only way to get down to really low levels of light.
    I don't understand - if you use PWM you can take it down to zero or any step up to full brightness.

    I think the references to 1.25V are because that is the internal reference voltage. I would guess that the PWM 'high' logic level is decided to ensure control over full range otherwise it would not achieve brightness higher than the high logic level, treating it as an analog voltage. Note that the internal pull-up resistor on the pin goes to the reference voltage source.

    Brain.
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