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SN74LVC1G125: Question on specifications around Vcc 3V

high speed guy
Expert 3185 points
Part Number: SN74LVC1G125
Other Parts Discussed in Thread: SN74LVC1G175

Hi,

I am looking to use the subject buffer to drive a 3V LED inside a Solid State Relay. Why are there 2 output current specs for a 3V supply?  What are the condition differences that will result in one current output over the other?

For instance can you state why in table 7.3 Vcc =3V there are two current levels specified for Ioh?   Also for table 7.5 with Vcc = 3V why are there two voltage ranges given (obviously this has to do with the currents but please explain)?  

Also does TI offer a 3.3V buffer with a 3.0V min voltage output?

 

Thank you,

HSG 

  • 0
    TI__Mastermind 44655 points

    Hey HSG,

    For logic, output current conditions (aka recommended output currents) are given to show the amount of voltage drop you would see over the Rdson of the pFET and nFET (resulting in Voh/Vol specifications). This is important info since loads vary, and there is typically a desired Vih/Vil trying to be maintained. For your particular application, these values can also be used to determine a resistor value to get the desired If/Vf needed for the LED.

    Your second question is tricky because a lot of our buffers have a current conditions in the uA range (simulating CMOS input leakage)  that then result in a min Voh above 3 V. Though, I can say that you've chosen the best device for LED driving since the LVC family of buffers will have the strongest drivers (resulting in less of a drop => higher Voh). However, I typically see customers using 5 V to drive an LED with a Vf of 3 V since there is little to no head room at 3.3 V. 

  • 0 in reply to Dylan Hubbard
    TI__Expert 3185 points

    Hi,

    We do not have a 5V bus; only a 3.3V bus. The relay we are driving draws a max of 15mA so we need to know if the buffer can maintain a 3.3V output at that load. 

    Also would the buffer require a pull-down resistor at the input; or can the output of the D-Flip Flop (SN74LVC1G175) go straight into the buffer.   Does subject buffer use a push-pull output?

    Thanks,

    HSG 

  • 0 in reply to high speed guy
    TI__Mastermind 44655 points

    Hey HSG,

    The data sheet provides a worst case Voh of 2.4 V with a condition very similar to yours. Pull-downs are not required at the input of the buffer unless there is expectation of potential floating at the input which should be avoided. Yes, this buffer has a push-pull output.