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SN75374: Fan Control using SN75374 causing problems. (See Description)

Daniyal Ali
Prodigy 10 points
Part Number: SN75374

We are  using SN75374 for our fan control circuitry. The connections and schematic is shown in attached file (Image-1). The output of SN75734 which is labelled as Fan_pwr_con-1-OUT etc is used in another IC named FDC3601N (Image-2). The FDC3601N has MOSFETs that are used to drive the fans. The voltage limits are already taken care of as per the instructions on the datasheet.

The problem is once we started the operation. The fans ran for about 10-15 minutes and then the fuse of 0.5 amps attached to the 12V supply got burnt. We further debugged and came to know that the SN75734 is getting heated up. We then removed pins of SN75734 that operate on 12V and tried to test the other functionalities of the board. Everything seemed to be working fine. Then we connected the 12V legs again and this time the SN75734 did not get heat up and everything worked perfectly fine.

We need to know why is this happening. Most of our boards are working fine. But if this issue is time dependent we might face problems in the production side after some span of time.

  • 0
    TI__Genius 12990 points

    Hi Daniyal,

    Welcome to e2e and thats for asking about SN75374. Im an apps engineer with this device and hope to help.

    From what I understand, this device is generating excessive power dissipation, 12V fuse is also blown which tells me there is too much current coming into VCC2/VCC3 pin. Is VCC2 and VCC3 tied together with the same supply? The peak current for the outputs come from the Cvcc caps. Do you have any bypass capacitance on this pin to decouple the supply from the driver?

    Assuming you are operating in 25C, the max power dissipation of SN75374 = (Tj_max - Ta)/Rja = 1.71W ( for D package). With your voltages I calculate that the total package power (from example on page 9 of datasheet) to be about 0.5W for a 500khz fsw. With a load capacitance of 150pF and using the bottom curve in figure 12 you will have to switch in multiple Mhz to exceed package power dissipation. However, There is also switching transition losses to be considered. Can you tell me your mosfet rise and fall times, switching frequency and duty cycle?

    To reduce power dissipation inside the gate driver its common to have a small gate resistor used as a resistor divider to divide the energy dissipation with the internal totem pole resistance and the internal FET resistance. (this is highlighted in equation 10).

    This app note may help for selecting the gate resistor value:

    Thanks,