Hello Sir/TI,
Currently, we are using SN74LVC1G79. I would just like to ask if TI has data on VOH typical values. The datasheet only shows minimum values like as shown in screenshot below.
I will used such data to calculate the flip-flop's output resistance at a certain IOH.
Hoping for your feedback on this.
Thanks.
Hi Licenio,
Typically logic gates are used to output into high-impedance loads and the VOH is ~VCC, VOL ~0V. Since this data isn't really new or useful, we don't put it into the datasheets.
If you are looking to calculate impedance, the values given are a good estimation. Since the output FETs are typically operating in the ohmic region, these values don't change much over the device's operating range.
(image from http://www.electronics-tutorials.ws/transistor/tran_6.html)
Impedance of a FET is the inverse of the slope of one of the above blue lines. In the ohmic region, that slope remains fairly consistent until you get too close to saturation (ie VOH ~= 0V and VOL ~= VCC), which are areas that would likely damage the device anyways due to overcurrent.
Hello Emrys,
Thank you very much for your reply. Appreciate it.
Unfortunately for us, we used it to activate or deactivate our photodiode pre-amplifier (not so high impedance) that is composed of 2 PNP transistors. Anyway, block diagram as shown below.
Further experiments have shown us that the output resistance of the flip-flop have an effect on our pre-amplifier pulse amplification (we expressed it in uA/uW).
Actually, in our previous modules, we used the octal type TI SN74HCT574 since we have 8 photodiodes in one module.
However, in our current design we just used the single type TI SN74LVC1G79 since we have at most 2 photodiodes only at each module.
TI SN74HCT574 datasheets have VOH at 25degC as shown below:
We used it to calculate or estimate it's output resistance at 25degC: ro_typ = (4.5V-4.3V)/6mA = 33.33ohms and ro_max = (4.5V-3.98V)/6mA = 86.67ohms
If you could also provide the same data (typical VOH characteristics at 25degC) for SN74LVC1G79, that would be very helpful.
Thank you very much.
