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74LV244 buffer output current

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cosmicboy

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current sink buffer

Hello folks,

I am having some trouble trying to understand how much current I can get out of a 74LV244 Octal buffer chip and I am a bit confused!

Here is the extract from the datasheet regarding the current sink/source ability:
27293180.png


I need around 30mA per channel, in total 240mA for all the 8 buffers.

If the quoted "DC source or sink current" figure, 40mA, is per channel then I am fine but if it is the total amount of power the chip can source (all 8 buffers) then I am in trouble...

Also, I don't quite understand what they mean with "DC Vcc or GND current for types with - bus driver outputs". Could this possibly be the total maximum power that the device can source?

These are probably really basic questions but your comments will be much appreciated :)


Thanks!
 

betwixt

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74lv244

IO is per channel.
IGnd/IVcc is for the whole device.

Basically the total of IOs must not exceed IGnd/IVcc.

You could run all 8 IO pins at say 5mA simultaneously but not at 10mA as that would take you over the limit.

If you are trying to get 30mA out per channel, I would suggest your design should be reconsidered. the LV244 is a bus driver, not a power driver.

What are you using it for?

Brian.
 

cosmicboy

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logic buffer 50ma

Hello Brian, thanks for your reply,

betwixt said:
IO is per channel.
IGnd/IVcc is for the whole device.

Basically the total of IOs must not exceed IGnd/IVcc.

You could run all 8 IO pins at say 5mA simultaneously but not at 10mA as that would take you over the limit.

Ah, I think I understand it now, cheers :)


betwixt said:
If you are trying to get 30mA out per channel, I would suggest your design should be reconsidered. the LV244 is a bus driver, not a power driver.

What are you using it for?

I have these high brightness LEDs that I am trying to drive from a microcontroller (that can only source 4ma per pin) so I thought I could get away with a simple logic buffer to act as a 'driver'.

The other alternative I was looking at was a darlington transistor array, like the ULN2803, but the voltage drop across the transistors will be too much for the system (Vdd is only 3.3V)


Do they make 'high current' version of the 244? :)


Regards.
 

FvM

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ti uln2803a max current

The limitation is most likely in the single bond wire connecting GND/VCC with standard 74 chips. The 16 bit LVC16244 versions have 100 mA for each of the 8 GND or 4 VCC pins. Individual small MOSFETs should be considered as alternative to darlongton drivers.
 

    cosmicboy

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betwixt

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high current sourcing buffers

I agree with FvM, with little voltage overhead you should be looking at MOSFETs. They will draw almost no input current so you can connect them directly to whatever was driving your LV244 (or the LV244 itself if it has to be there) and their low 'on' resistance will let you sink lots of mA without any problems.

As an alternative, the ULN2803 can just about be driven with 3.3V so if you have a higher voltage available elsewhere, you could use that at the top of your LEDs and use the 2803 as a controlled switch to ground.

Brian.
 

    cosmicboy

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cosmicboy

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darlington buffered output

Many thanks for your replies FvM and betwixt!

Having mosfets to drive the LEDs seems like the 'proper' solution but that adds 8 MOSFETs and 8 resistors (pulldowns on Gates to GND, right?) so I think I will go for the LVC16244 route. However, I am slightly confused (again!)

The dataasheet I am now looking at (https://focus.ti.com/lit/ds/symlink/sn74lvc16244a.pdf) specifies Io (Continuous output current) as 50mA, which is enough per buffer but it also specifies "Continuous current through each VCC or GND" as 100mA and it does not partition the buffers per power pair, so am I right in thinking that if I connect all the VCC pins upto a source I can source a total of 400mA for all of the 16 buffers? (Or if I use all the GND pins can I sink a total of 800mA?)


Sorry if I am being a bit thick, just want to get the design right the first time :)


Many thanks!
 

betwixt

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driving uln2803 with 3.3v

Sadly no.

The current for all the outputs has to pass through the VCC pin when driving high or through the GND pin if sinking current when low. As pointed out earlier, the bottleneck in the system is the wire-bond between the physical pin on the package and the actual silicon inside. The connection is through a very fine wire, welded at each end and it will either burn out or overheat the silicon to the point of destruction.

You are also looking at "Absolute Maximum" figures for the chip which are the ones the manufacturer specifies as the "point of no return" in terms of chip reliability. In other words, if you exceed them you almost certainly cause permanent damage. You will note they are generally higher ratings than the normal operating ones.
Try a 2803A, it might just work, if not, you will have to look at power drivers of some sort.

Brian.
 

    cosmicboy

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cosmicboy

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buffer 50 ma output current

Hmm, thanks Brian.

I can't use a ULN2803 as it's a darlington array. I only have 3.3V to work with (and some unregulated voltage which I can not trust) so the voltage drop across the transistors will render it useless. I looked at Farnell for a simple (not darlington) transistor array but couldn't find anything sensible, do they make such things? :)

Coming back to your previous suggestion, I think I will end up using small logic level mosfets if I can't find an 'all-in'one' packaged component.


Cheers!
 

betwixt

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uln2803 alternative

I searched but couldn't find MOSFET driver arrays except ones with serial inputs and latches. Its strange that such a device doesn't seem to exist.

You could go with plain old NPN transistors, ones with a low VCEsat. At least they are cheap and being current driven, should be easy to saturate from your low supply.

Brian.
 

    cosmicboy

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helios31

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source or sink current and buffer

You can use simple SOT23 transistors with integrated resistors, like PDTC143TT. Models exist with different resistor values, and with 1 or 2 resistors (B or B and BC). They are cheap and easy to find. You nead only to add the led resistor.
 

    cosmicboy

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cosmicboy

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244 buffer for 3.3v designs

betwixt said:
I searched but couldn't find MOSFET driver arrays except ones with serial inputs and latches. Its strange that such a device doesn't seem to exist.

Odd isn't it? Well, if any manufacturer out there comes across this post - GUYS, WE NEED STUFF LIKE THIS :)


helios31 said:
You can use simple SOT23 transistors with integrated resistors, like PDTC143TT. Models exist with different resistor values, and with 1 or 2 resistors (B or B and BC). They are cheap and easy to find. You nead only to add the led resistor.

That is great, that'll save me some resistors!


Thanks for the replies everyone.
 

turnup224

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hi guy i need h bridge schematic diagram of ka3524 cos i want to use it for 3kva transformer
 

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