Questions about 74lvc595A  chip - Page 1
Forum section: electronics / projects

author: Gibson486
author_rank: Frequent Contributor
timestamp: April 04, 2020, 05:18:17 pm
content: I am going over an old design that was inherited to me for analysis. First thing I saw was that it uses a 74lvc595A chip (https://assets.nexperia.com/documents/data-sheet/74LVC595A.pdf). The datasheet says it is a 3.3V part, but on table 4, it says that the max input on the power can be 6.5V. On the design, it is powered by 5V. I suspect that this will be a long term issue?Also, it says the max current out is +/50mA. When I add currents on the outputs, do I add the absolute value of the currents? On some terminals, the device will sink current instead of sourcing it. If the part is 5V tolerant, wouldn't that mean it would take a little more current if more than the intended 3.3V was applied to the outputs as well? Of course, since it is being powered by 5V, I am not sure what this all does to those specs....

author: wraper
author_rank: Unknown
timestamp: April 04, 2020, 05:30:14 pm
content: Inputs/outputs are 5V tolerant but not IC in a whole. 3.6V VDD must not be exceeded. Powering it from 5V likely will cause failure over time. Also 50mA output current is absolute maximum rating. It's not like you should be expecting IC operating properly at such output current. Outputs tolerate 5V only when used in logic low or Hi-Z state (with external pull-up to 5V), logic high shall not be used.

author: Ian.M
author_rank: Super Contributor
timestamp: April 04, 2020, 06:04:36 pm
content: The datasheet gives specs for output voltage when sourcing or sinking 24mA in table 6. That's a good indication that in normal operation you can reasonably expect it to handle about half of the Abs.Max. +/-50mA output current specced in table 4. You could probably push it to about 2/3 the Abs.Max. current but the voltage drop is unspecified and likely to be excessive, so that's a dumb idea for any production design.Also, the total current available from all the outputs is limited by the Abs.Max. Icc (supply) and Ignd (ground) currents, namely 100mA (table 4). That means the total current sourced by all logic '1' outputs must *NOT* exceed 100mA, and similarly but independently, the total current sunk by logic '0' outputs must also not exceed 100mA. If there is any possibility of all Qn outputs being high simultaneously or low simultaneously, its only good for +/-12.5mA per Qn pin!

author: Gibson486
author_rank: Frequent Contributor
timestamp: April 05, 2020, 03:57:40 pm
content: Thanks!....so, does having sinking currents add to the ground current? Or does the direction matter, so 5 mA out cancels out 5 mA in?

author: Ian.M
author_rank: Super Contributor
timestamp: April 05, 2020, 04:08:45 pm
content: The total current sourced from all outputs flows into the chip through the Vcc pin. The total current sunk by all outputs flows out of the chip through the Gnd pin. They do *NOT* in any way cancel.

author: Gibson486
author_rank: Frequent Contributor
timestamp: April 05, 2020, 04:42:01 pm
content: Quote from: Ian.M on April 05, 2020, 04:08:45 pmI see....that makes it easier to understand. Thanks!