Guru
Joined: 21/06/2018
Location: AustraliaPosts: 1292
| Posted: 03:24am 29 Jun 2024 |
If you are using a 74HC86 its output is barely adequate to drive a power board properly. Its absolute maximum current is stated as +/- 25mA but in the fine print it says that this is not a target and if you actually try to drive 25mA the device may no longer function properly (during lower normal use). It then vaguely states that you should use it under recommended typical parameters and of course there is no parameter given for the current value.
Under electrical characteristics for the HC86, with a 4.5V supply and 4mA current drain the output voltage minimum is just under 4V and when sinking 4mA the output drive to the Optos is only 3.7V from a 4.5V supply (why provide parameters @ 4.5V or 6V supply when most logic is either 3.3 or 5V??) so maybe expect max voltage drive of ~4.2V from 5V.
For the FOD3182 the recommended current is 10 - 16mA. So if we aim for the centre of that range 13mA is a reasonable figure. There is no data for 5V or 13mA so I recommended the 74AC86 or 74LVC86 as their absolute maximums are 50mA and they have a similar amount of voltage droop to the HC86 but that is at 24mA not the HC86's 4 mA load and operated at ~13mA I expect the droop value to be halved.
So if you use an HC86 or similar I would not use the resistors instead load U8 & U9.
Then you have ~ 100 times (2A) driving ability with essentially zero droop at 25mA & 5V.
If you use the AC86 or LVC86 the buffers are not really needed so just load the 4 x 33R resistors.