LEVEL SHIFTERS AND NOT USED INPUTS - PULLING HIGH CURRENT

Thread Starter

longpole001

Joined Sep 16, 2014
62
Hi guys
have found a pcb on equipment that uses a 74lvc8t245 8 bit dual supply level shifter, the chip was running warm and signals that go through it seem to be a bit off on occasions when diagnosing fault with signals connected to it.

The output , the direction and OE pin were tied to fixed levels so that the level shift was from bside 3v3 to aside 5v, thought it was just a faulty chip but the new one did the same, measuring the current of the supplies to the chip showed about 55ma on the 3v3 side which seems off for what its doing.

Out of the 8 inputs only 3 were active , where the other 5 inputs were connected to chips that for this option were not fitted , as a result the input pin were floating.

Output side of these 5 inputs were connected , so needed to tie the inputs to logic levels that would not effect the existing functions.

After this, the current dropped to 12ma on 3v3 side for chip , no over heating and signals were nice.

Design bottom line - always check that options on the inputs when not fitted on a level shifter have a logic state, else you will run into issues.

cheers
 

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Alec_t

Joined Sep 17, 2013
13,147
Design bottom line - always check that options on the inputs when not fitted on a level shifter have a logic state, else you will run into issues.
Thanks for sharing.
Unless its datasheet says otherwise, it is good practice always to connect unused inputs to an appropriate point, for any integrated circuit.
 

Thread Starter

longpole001

Joined Sep 16, 2014
62
looking at that report indicated that lvc devices have internal buss hold but i can tell you that the inputs floating effected the chip badly
 

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