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Sinking current higher than Sourcing current, why?

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ZaeemAhmad

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Why is the sinking current higher than Sourcing current for IC's like 74 series etc. and microcontrollers? e.g. for 74LS series Sink current is 0.4 mA whereas 20 uA only
 

srizbf

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sourcing current is from 'pullup' device.
sinking current is through the 'pulldown' device.

the ic in question has to supply the sourcing current from internal circuits.

but , it just passes the sinking current through 'pulldown' device.

the external device is only responsible for supplying this sinking current.
 

asimov

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Hi,
This is a good question, reason I could think of was, generally the output stage is made up of a PNP/P-channel mosfet and an NPN/N-channel mosfet in a totem pole fashion. For a given Si area PNP/Pchannel mosfet have lower current capacity in comparison to N channel devive. This is because holes have a lower mobility than electrons.

Asimov
 

srizbf

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you are right in terms of area of p-pullup and n-pulldown.

but from my understanding ,they size the output stages ,
with the result that p-area is larger than n-area.
 

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