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SN74LVC14A: SN74LVC14A drive transistor, if no limitation resistor, does it have problem?

Part Number: SN74LVC14A

We use an SN74LVC14APWR as FAN PWM control circuit, we do not add the current limitation resistor between 74lvc14 output to transistor (V9). And the output of SN74LVC14APWR will be fixed to about 1V,could you help let your R&D  evaluate the risk of this application? And could you share the internal block circuit of SN74LVC14APWR for review?

  • Driving small MOSFETs without a resistor is fine. Driving BJTs is likely to exceed the absolute maximum ratings and burn out the output transistors.

    Please show the schematic.

  • BJTs spec as below:

    schematic:

    PCO_FAN0_PWM is always 0V is worst case, then the BJTs is always driving. Ic current of BJTs is about 0.12mA and plus an current from FAN module. we are requesting from FAN vendor. and Ib current is not measured, we are plan to capture the thermal image by camera, to see the temperature rise if over spec.

    BR

    Ted

  • The images did not attach. Please try again.

  • Hi Clemens,

    please kindly check the schematic and BJT datasheet below. In my opinion, the Ibias current is very small, it should make no difference to use a small  resistor(below100ohm) or not. The current is only 0.1mA, It only work when the BJT is damaged, but you know the source side also has the current limit function to several mA. Could you please double check the risks? Thanks.

     00024441.pdf

  • This is the VBE(sat) curve of the transistor (when IB is forced to be 1/10th of IC):

    These are the output voltages of an LVC device:

    A saturated BJT does not limit its base current. An LVC CMOS output cannot protect itself and must not be short-circuited.

    The transistor has no specification for such high base currents, but I'd estimate that these curves intersect at roughly 80 mA, which exceeds the SN74LVC14A's absolute maximum ratings.

  • Hi,

    we have measure the case temperature of the SN74LVC14A, when Ta=27C, the Tc is about 61C, and Tc of BJTs is about 40C.

    how about the Tj of SN74LVC14A, does it must know the power loss ?

  • The bond wires (or other small structures) will melt before the case gets hot. Logic devices are not designed for high currents and cannot dissipate much power.

  • Hi,

    Tj Max for this device is 150 C. I wouldn't recommend using a temp reading to verify if you are exceeding the output current spec of the device. It's best just to find a way to measure with a wire and current probes or a sense resistor.

  • Hi,

    I have measure the current flow the device. at 25C, each pin output 80mA, totally 160mA. and at 90C board temperature, each pin flow 70mA, totally 140mA.

    base on the thermal resistance, 113C/W , the Tj=90+2.3*0.14**113=126.4C. and in our system, the worst case board temperature is about 77C. So to Tj=125, we have about 12C margin at worst case.  we have done this for 16h without failure.

    So could you give some comments, which level risk for the application if only current over the spec ? we will replace the BJTs by MOSFET , but we need to evaluate the previous board risk.

    thanks

    BR

    Ted

  • TI does not guarantee functionality if you are driving over the absolute maximum ratings. Maximum junction temperature is an additional restriction and does not allow for ignoring any other limiting specifications in the absolute maximum ratings table.

  • Hi ,

    I'm sorry that you don't like my answer, but it is the correct one.  Don't exceed maximum restrictions in the datasheet.