MC1489 Current Consumption Question - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: April 16, 2025, 03:36:40 am
content: I have a few questions regarding the datasheet for the MC1489 line receiver:https://www.ti.com/lit/ds/symlink/mc1489.pdf?ts=1744773729853&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FMC1489Page 3 shows 'Output Voltage Io' is 20mA. Since it's specifying 'voltage' and the specification is 'current', I'm assuming this is an error, but wanted to confirm I'm not overlooking something.My other question is current consumption. Long story short, I'm powering only this chip with a regulator that has a 1mA minimum load spec (https://www.onsemi.com/download/data-sheet/pdf/ncp1117lp-d.pdf) - spec is on page 3.If I understand the MC1489 spec correctly, per the graph on page 9, at a 5V input (I assume this is the data input pin) on a single pin, the current consumption is 1mA. I'm guessing two inputs would be 2mA, etc...The chip must use some current when the inputs are zero, correct? Would this be on the datasheet or it's so small the manufacture doesn't list it?When the signal(s) swing to 0v, does this mean the regulator is generating noise due to the minimum load being <1mA?

author: moffy
author_rank: Super Contributor
timestamp: April 16, 2025, 04:24:17 am
content: Quote from: bostonman on April 16, 2025, 03:36:40 amI think they use the term 'Output Voltage', ambiguously, to refer to the 'Output Voltage' pin, and the 20mA is the maximum sinking current it should ever draw.2. Current consumptionRefer to the schematic on page 2, for 0V input the middle 5k resistor is low, plus the 9k is pulling current, so at 5V it should pull at least 1mA per channel.

author: bostonman
author_rank: Super Contributor
timestamp: April 16, 2025, 04:44:22 am
content: AhhhhI was diving too much into the specs and graphs without looking at the schematic. Good to know I'm pulling enough current through the regulator to meet the minimum. If it's approximately 1mA per channel, at four channels, that's 4mA, and the minimum on the regulator is 1mA.Also makes sense about using the 'output voltage' ambiguously. Well, doesn't exactly make sense, but more of a logical explanation.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 16, 2025, 07:25:03 am
content: Quote from: bostonman on April 16, 2025, 03:36:40 amThe current consumption of the chip alone (disregarding other components and signals to its pins) is in the datasheet tables under the name of "ICC Supply current", and it is between 20 and 26mA. Then, the currents through each pin is another story, but this can only increase the total consumption. You'll never get less than Icc (20mA).

author: bostonman
author_rank: Super Contributor
timestamp: April 16, 2025, 09:58:09 pm
content: QuoteI thought that was the total current the chip can draw.So Io on the top (one of the two questions I initially had) of 20mA, means if all four outputs are used, then the total maximum current consumption would be (4 * 20mA) + (Icc) of 20mA = 100mA?

author: jwet
author_rank: Frequent Contributor
timestamp: April 17, 2025, 12:06:37 am
content: You're mixing up the meaning of these data sheet sections.The first spec is in "absolute max" ratings and says that you shouldn't let the part sink more than 20 mA from any output- this is a fail limit. These are all never exceed values but have no other value.The real specs are in the table below- called "electrical characteristics"- supply current of 20 mA is here. These are the guaranteed specs and limits and give you design guidance.Its a bit confusing but all data sheets are written this way.

author: Benta
author_rank: Super Contributor
timestamp: April 17, 2025, 12:12:38 am
content: Dunno how you read data sheets, but:"Supply current 20 mA typ. 26 mA max {outputs open}".On top of that, you can add the maximum output short circuit current, which is +/- 3 mA per output.Four outputs: 12 mA.Absolute worst-case current consumption: 26 + 12 mA = 38 mA.

author: bostonman
author_rank: Super Contributor
timestamp: April 17, 2025, 01:11:35 am
content: QuoteI know the differences on datasheets, but it's always good to reiterate in case I didn't know. How do you know it's "sink" and not "source" a max of 20mA?QuoteIf the outputs are not "shorted" (I assume this means the outputs are tied directly to ground), then why add the 3mA?Also, if the max "sink" current is 20mA, then why is the short circuit 3mA?