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AMC6821: Power supply capability, requirement and spin up register settings

bhargav goud
Prodigy 160 points
Part Number: AMC6821
We are using a 12V,0.6A regulator which supplies brushless fan. The fan needs 1.2A for 500ms startup current. If AMC6821SD is used, will my regulator current capability be sufficient to meet the start up current by programing the spin up registers? if so what are the recommended values for register settings.
Thank you!
  • 0
    TI__Mastermind 41325 points

    Hi Bhargav,

    I would say that a 0.6A regulator is not sufficient for your 1.2A fan, but it's not really my place to speculate on these matters.The AMC6821 datasheet diagrams show that a discrete FET is needed to allow the AMC6821's PWM output to control the on/off state of the fan. This discrete FET would have some contribution to the voltage/current requirements of the fan circuit, but the AMC6821 itself would not contribute to this limit. If the source and FET are sufficient to switch the fan on/off, then the AMC6821 will be able to regulate fan speed. 

    thanks,

    Ren

  • 0 in reply to Ren Schackmann
    Prodigy 160 points

    Thank you Ren for your quick reply!

    We would like to elaborate more about our application. Fan start up current is attached below(this is captured without PWM control) and max current is 1.2A for 500ms duration and reaches 0.6A in normal spinning condition

    Please refer to our application circuit diagram. Basically the Fan has 4 inputs pins for wiring. Two pins for power (12V ad GND) and two pins for PWM input and Tach output.

    Our question is, Can we control the inrush current of 1.2A by programming the AMC6821SDBQR device?

     

    12V Fan startup current without PWM control (12 V supply applied) :


     

    Here is the proposed circuit :


  • 0 in reply to bhargav goud
    TI__Mastermind 41325 points

    No, you cannot use AMC6821 to stop the in-rush current. Toggling the fan's PWM input during start-up will only make the spin-up process take longer. If the fan needs 1.2A to start spinning, it won't prevent the peak current of 1.2A from occurring.

    The spin-up feature of AMC6821 is intended to reduce fan noise.  

    Your schematic is missing the recommended FET between the PWM Out pin and the fan's PWM input.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 160 points
    Thank you for the explanation on spin up functionality of AMC6821.
    The PWM pin of the fan is a discrete input pin (Fan has a internal pull up 10K connected to 12V supply). AS per the datasheet of AMC6821 PWM output is an open drain , therefore we have connected the AMC6821 PWM output to the PWM input of the Fan. Could you please explain what is the need of an external FET?
    Also I couldn't find the absolute max pull up voltage of PWM open drain output for AMC6821,can you please provide max Vdd of this pin?
  • 0 in reply to bhargav goud
    TI__Mastermind 41325 points

    Typically, the fan input is not a buffered, high impedance input and will take significant current to activate. AMC6821's open drain outputs are specified to sink up to 6mA (Vol spec in Electrical Characteristics table.) All of the functional diagrams in the datasheet depict this external, discrete FET for this reason. In the case of 2 and 3 wire fans, it is absolutely necessary to have the FET, because the only way to control speed is by controlling the fan's power supply directly. 

    As a digital pin, PWM out cannot exceed 6.5V per Absolute Maximum Ratings table on page 2 of the datasheet. This is further evidence that PWM out cannot be directly connected to a 12V pull-up resistor.

    thanks,

    ren