Hello all,

I have encountered an AHC244 chip used to drive LEDs (Something like in the attached image) so I wondered why not just drive the LEDs with the uC pins?



And also I have seen this IC used with some PWM signals as inputs to it! in this case, I am guessing the output would be the same PWM! so what's the trick here? what's special about the output from this chip?

I have read the below paragraph in the datasheet but it isn't clear enough for me yet.!!
" AHC244 is a low drive CMOS device that can be used for a multitude of bus interface type applications where output ringing is a concern. The low drive and slow edge rates will minimize overshoot and undershoot on the outputs. The inputs can accept voltages to 5.5 V at any valid VCC making it ideal for down translation. "

Can anyone clarify this for me, please..?

Thanks,

 

hi YT,
Look at this way of driving a single LED from two sources, TRI-State is effectively an Open Circuit.
E

 

A line driver can supply more current than a microcontroller pin.

 

To drive an LED you typically need 5-20mA depending how bright you want it. The '244 datasheet I have seen can source or sink up to 8mA using 5V logic levels or 4mA at 3.3V logic levels. I tend to use the PIC microcontrollers, I have typically seen source and sink currents for them of 25mA. So to drive an LED with that microcontroller, you can just drive it directly.
I agree with BobTPH: The (244 can 'normally') source or sink more current than a microcontroller. Though a lot of microcontrollers can drive LED's directly depending on the part number.

 

hi YT,
Look at this way of driving a single LED from two sources, TRI-State is effectively an Open Circuit.
E

Does this apply to the PWM signal as well?, meaning I can source 2 PWM signals to the same pin (eg. gate driver)?

 

There is no compelling reason to use a 74xx244 bus driver.
You can use a single transistor to boost one output line.

 

There is no compelling reason to use a 74xx244 bus driver.
You can use a single transistor to boost one output line.

This is not actually a microcontroller who's driving those LEDs but high-side and low-side signals that are going to gate drivers who are inturn gonna drive some IGBTs.
And these LEDs are just status LEDs which tell if there is a signal or not.!
Maybe because of this type of application they are using a AHC244 so that they don't interfere with the signals going to the gate drivers.

 

Again, there is no compelling reason to use 74xx244 bus drivers.
You can use whatever buffer or driver that does the job required.

 

Hello all,

I have encountered an AHC244 chip used to drive LEDs (Something like in the attached image) so I wondered why not just drive the LEDs with the uC pins?

View attachment 285870

And also I have seen this IC used with some PWM signals as inputs to it! in this case, I am guessing the output would be the same PWM! so what's the trick here? what's special about the output from this chip?

I have read the below paragraph in the datasheet but it isn't clear enough for me yet.!!
" AHC244 is a low drive CMOS device that can be used for a multitude of bus interface type applications where output ringing is a concern. The low drive and slow edge rates will minimize overshoot and undershoot on the outputs. The inputs can accept voltages to 5.5 V at any valid VCC making it ideal for down translation. "

Can anyone clarify this for me, please..?

Thanks,

There are three reasons that come to mind on why you might want to use a line driver (does not have to be THAT particular line driver).
1) As a voltage buffer -- you need to protect the signal from the load. If the load being driven has the potential to change the signal that is driving it, that has the potential to cause a number of problems. If that is a concern, then buffer the signal so that the buffer output stage bears the brunt of it, leaving your signal clean.
2) As a current driver -- you need more drive capability than the device providing the signal can deliver.
3) As a line driver -- you are driving a load that needs controlled edges or other considerations. When driving a long line, the output of many logic devices have edges that are too fast and this results in ringing and overshoot, which can cause signal integrity issues as well as EMI issues. A line driver is often designed with edge-rate limit control to mitigate this.

 

Thank you, your answer was really helpful.

I don't think it's because of reason 1 because these signals are going to a gate driver which has output pins isolated from input pins.
And reason 2 also isn't applicable.
But reason 3 is the most reasonable one as i know that the output of these gate drivers is going to drive some IGBTs using cables.

There are three reasons that come to mind on why you might want to use a line driver (does not have to be THAT particular line driver).
1) As a voltage buffer -- you need to protect the signal from the load. If the load being driven has the potential to change the signal that is driving it, that has the potential to cause a number of problems. If that is a concern, then buffer the signal so that the buffer output stage bears the brunt of it, leaving your signal clean.
2) As a current driver -- you need more drive capability than the device providing the signal can deliver.
3) As a line driver -- you are driving a load that needs controlled edges or other considerations. When driving a long line, the output of many logic devices have edges that are too fast and this results in ringing and overshoot, which can cause signal integrity issues as well as EMI issues. A line driver is often designed with edge-rate limit control to mitigate this.