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TPS7B7701-Q1: adjust current or voltage

Part Number: TPS7B7701-Q1
Other Parts Discussed in Thread: LM317,

Hello team,

My customer would like to use tps7b7701 to power a two-wire fan, the problem is speed control. We could either change the output voltage or limit the output current, need check with you: Can we change the feedback resistor or the current limit resistor during operation?

Do you have any other recommendation for this kind of application? As far as I know, most of the next-generation infotainment processors power dissipation is very high, fans are needed to help cool the system.

Thanks.

Dongbao 

  • Hi Dongbao,

    the advantage of powering a fan by a voltage regulator is that you can achieve a low noise performance. In comparison to the voltage regulator a PWM control can add nasty click noise. Even a switcher regulator (sometimes with a post filter) can be used for low noise fan operation.

    On the other hand, with a voltage regulator the fan speed control range is a bit limited, because you need a minimum voltage to be able to keep the fan running. A PWM control is better when it comes to very low fan speeds.

    A good trick when driving the fan with a voltage regulator is to start the fan always with the nominal voltage and reduce it to the desired speed afterwards. This works pretty well, provided the speed is not decreased too much.

    Another issue is the inrush current when starting the fan. The inrush current can be two or three times higher than the nominal current. This can be troublesome when using a linear regulator with too tight current limit. A switcher regulator is clearly better in this regard.

    Speed control with a linear voltage regulator can work very well. I have done this many times in the past with a LM317, where I used NPN transistors to switch between the programming resistors.

    Kai
  • Hi Dongbao,

    Can you provide more technique details on the two-wire fan that your customer plan to drive with TPS7b7701?

    Regards,
    Jason Song
  • Hi Jason,

    Two wire fan:

    input voltage: 7-10V, output current =200mA. 

    Dongbao

  • Hi Dongbao,

    Like Kai suggested, there are different approaches for you to control the speed of the fan speed. For your application, do you plan to have PWM to control the fan? If that's the case, what's the frequency you plan to control the output? Can you provide more details on the fan?

    Regards,
    Jason Song
  • Hi Jason, Kai

    Thanks for your guys comments.
    Customer prefer to use a voltage regulator to control the speed of the fan, because this is quiet and robust. The speed of the fan basically propotional to the voltage, and the voltage is controlled by pwm. We have discrete solution to achieve this function, but the size is too big. So we would like to try TPS7B7701-Q1, the diffculty of knowing how to adjust the ouput voltage propotional to the duty cycle.

    Regards,
    Dongbao
  • Hi Donbao,

    you could use the method shown here:

    www.edn.com/.../Control-an-LM317T-with-a-PWM-signal

    Kai
  • Hi Dongbao,

    The one Kai posted is definitely one approach to use regulator with PWM. I sent you an friend request, if you would like to share some of your current design, we could communicate via private message system. I would like to understand your current approach and see if there is anything we can improve.

    Regards,
    Jason Song