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LM2904: frequency range on Vin-

Shu-Cheng LIN
Guru 11785 points
Part Number: LM2904

Hi Sirs,

Sorry to bother you.

We would like to know what is the acceptable frequency range on Vin-?

Thanks!!

  • 0
    TI__Guru**** 150296 points

    Hello Shu-Cheng,

    That depends how the signal is applied (through what) and whether the LM2904 should amplify the input signal, ignore it, or simply survive it.

    Please be more specific about the amplifier application.

  • 0 in reply to Ron Michallick
    Guru 11785 points

    Hi Sirs,

    Application as below
    MCU PWM - LM2904DGKR Vin-, integral line - MOS switching Power- DC Voltage to DC FAN
    If the Vin- of the LM2904DGKR receives the following signals:
    MCU PWM (3.3V level), 33Hz~40KHz
    Is this frequency range ok?

  • 0 in reply to Shu-Cheng LIN
    TI__Guru**** 150296 points

    Shu-Cheng,

    Thank you; I understand the input signal very well.

    Would you like the LM2904 to change the PWM duty cycle to a voltage and drive a FAN?  If yes, what is the LM2904 supply voltage and fan maximum voltage and current draw?

     

  • 0 in reply to Ron Michallick
    Guru 11785 points

    Hi Sirs,

    Our schematic as below

    Actually we have use LM2904 on x86 platform of Desktop/AIO MB.
    Before FAN PWM control is through SIO, code/FW is from our side.

    But this time is use MCU base, code/FW is from end customer side

    That why end customer would like to know recommended frequency of the PWM

    MCU PWM -> LM2904DGKR Vin-, integral line -> MOS switching Power-> DC Voltage to DC FAN

    Not OP directly push FAN

    About LM2904 datasheet, looks frequency max is 0.7Mhz

    From the picture below, it seems that 400Khz will begin to decay.
    So it is recommended that the frequency of the input signal fall between 0 and 400 KHz?

     

  • 0 in reply to Shu-Cheng LIN
    TI__Guru**** 150296 points

    Shu-cheng,

    The LM2904 is an error amplifier setup whose output ideally a DC voltage. The error amplifier's job is to get the fan voltage (after the voltage divider) to equal the average DC voltage of the PWM input signal. Therefore the amplifier doesn't need to follow input frequency. It needs to follow the DC value of it. 

    R0402 and C0401 have a time constant of 1ms and a cut off frequency of 160 Hz.  Because the fan driver is a DC circuit, the input PWM should be well above that, 1 kHz and above should be fine. A lower frequency would make more ripple in the FAN driver voltage, but that might not be noticable. There is no maximum input PWM frequency concern.

    Channel 1 seems to be unused, hover it is terminated incorrectly; see section 7 "Unused Amp Connections and Inputs Connected Directly to Ground" in the application note http://www.ti.com/lit/pdf/sloa277

    Q402B gate seems to have no shutoff path. The gate is either driven high or becomes high impedance. 

  • 0 in reply to Ron Michallick
    Guru 11785 points

    Hi Sirs,

    Thanks for your reply.

    Our update schematic as below.

    Could you help double confirm OPA schematic at U0401/U0501/U1201?

    Any suggest are welcome.

    Schematic:

    LM2903 and LM2904.pdf

    Thanks!!

  • 0 in reply to Shu-Cheng LIN
    TI__Guru**** 150296 points

    Shu-Cheng

    For U0401, it looks good if label "FAN_OP_FB" is not connected to anything else.

    For U0501, R0513 and R0512 values are swapped.

    For U1201, it looks OK