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Interpreting Datasheet

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wuchy143

Member
Hi All,

I'm trying to figure out how much current a CD74HC154 can drive on the single output pin for example YO when being driven low. How much can the output pins sink when driven low? From the datasheet it's not easily seen and would like to know because I have a feeling it cannot sink enough current which is why I'm having issues in my design.


Datasheet:
https://www.ti.com/lit/ds/symlink/cd74hc154.pdf

Thanks All.
 

ronsimpson

Well-Known Member
Most Helpful Member
How much current do you want?
What are you driving?
I can't find it now but there is a graph for 74HCxxx family that shows voltage verses current. At 1mA there is 0.1V and 10mA there is 2 volts across the part.....(I made up numbers)
 

crutschow

Well-Known Member
Most Helpful Member
Look in the DC Electrical Specifications table under High Level and Low Level output voltage with TTL loads. The Test Condition currents given are the maximum source or sink current the outputs should be required to provide (for example 4mA with a 4.5V supply). If you need more, then you will need to add an output buffer transistor or buffer circuit.
 

audioguru

Well-Known Member
Most Helpful Member
The absolute maximum ratings are 25mA for one output or a maximum of 50mA from the entire IC.
 

wuchy143

Member
Thanks guys. I'm confident the 4.5V input will yield a max of 4mA as the datasheet states. My system is +5V. Below is my circuit. What I'm trying to do is when the 74HC154 drives it's port pin "low" I want the NFET to turn off. The 500 ohm resistor is simulating a restive switch for a keypad. Essentially my issue was it appeared that the 74HC154 couldn't provide enough current to pull down the gate to zero volts. Clearly I had an issue.

When the 74HC154 drives a port pin "low" with my set-up it will yield 419uA. Given my Vdrop on the diode is .6V and using KVL. I hate to admit this but I suspected my breadboard I was using(was too lazy to do a real prototype) had a short and was causing me issues. So, I used a real wired prototype and sure enough the 74HC154 easily can drive the gate of the FET close enough to ground to turn it off.


Sometimes I really have a way of humbling myself. Thanks for the help!!

Untitled.jpg
 
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misterT

Well-Known Member
Most Helpful Member
The PIC input is not going to draw (practically) any current. It is high impedance. You do not need the FET.
 
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wuchy143

Member
Hi misterT,

Correct. I don't really need the FET but part of the design consideration was to have exact firmware for our resistive keypad that we have used on our capacitive keypads. This required the inversion of the signal. I could have used a single inverter as well.
 

misterT

Well-Known Member
Most Helpful Member
Essentially my issue was it appeared that the 74HC154 couldn't provide enough current to pull down the gate to zero volts.
Maybe my brain does not work well now that I'm tired, but you have a diode in the circuit. How do you expect to drive the gate to zero when there is a diode there?
 

crutschow

Well-Known Member
Most Helpful Member
What's the purpose of the diode? I see no reason for it to be there.
 

wuchy143

Member
Vgs in my circuit is 1.34v. Luckily the IRFH5104 transistor turn on voltage can be between 2-4volts. So it's off when it's gate is 1.34v. That makes sense? Or am I mistaken?
 

wuchy143

Member
What's the purpose of the diode? I see no reason for it to be there.
It's a keypad. You will have ghosting and general issues scanning if you do not have a diode in series with EACH key. My circuit is a very very simple version of the actual keypad to show the electronics side so I can come on here and not sound like a do-do....hopefully :)
 

crutschow

Well-Known Member
Most Helpful Member
Okay. Then what's the purpose of the 500 ohm resistor in series with the diode? It also seems unnecessary.
 

wuchy143

Member
That is in there to simulate a resistive key being down. I just short the gate to +5v so make sure the transistor can turn on.(which is actually where it should be unless someone hits a key) The rubber key(resistor) that I'm using which makes contact with the traces I'm going to short when a key is pressed is about ~250 ohms. It should be better than that but I wanted to be conservative so I jacked it up to 500 ohms to make sure I can switch the transistor without an issue or tolerances of parts. Bootstraps and suspenders I guess.

I know the circuit is hokey. It looks weird but that was because I had to use the exact same firmware that I use in my capactively sensed keypads using almost the same hardware. Not my choice. Was my bosses. I personally would have designed the hardware much much differently to make the design more solid for a resistive keypad.
 
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