Sinking current higher than Sourcing current, why?

[ZaeemAhmad]

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]

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]

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]

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.