Power supply current requirement  - Page 1
Forum section: electronics / projects

author: joniengr081
author_rank: Regular Contributor
timestamp: September 20, 2024, 12:53:45 pm
content: I am wondering how do we calculate the power supply current requirement for the following two devices.1- PCF8574A Remote 8-Bit I/O Expander for I2C Bus2- SC18IS606 I2C-bus to SPI bridgeThe datasheet of PCF8574A says that the I/O current is around 25 mA. Is this the power supply current ? I don't think so. The I/O current need to be provided by the FPGA/MCU and is not the power supply current, is that right ? The datasheet also says in table 6.1 that the continuous current through VCC or GND is +/-100 mA. Kindly let me know which number should I use for power supply current.The datasheet of SC18IS606 says that high and low I/O currents is 20 mA and 8 mA respectively.The datasheet also says that the operating supply current is 4 mA. Which one should I use as power supply current ?The links to the datasheets:https://www.ti.com/lit/ds/symlink/pcf8574a.pdf?ts=1726766000684&ref_url=https%253A%252F%252Fwww.ti.com%252Finterface%252Fi2c%252Fgeneral-purpose-ios-gpios%252Fproducts.htmlhttps://www.nxp.com/docs/en/data-sheet/SC18IS606.pdf

author: ajb
author_rank: Super Contributor
timestamp: September 20, 2024, 03:03:17 pm
content: You have to consider the current required to operate the device itself, plus the current drawn by any loads connected to the device's output pins.The first is given by the "operating supply current" spec for the NXP device, and the "Icc" spec for the TI. The actual output current for any given IO pin depends on what it's connected to. If you are driving a bunch of LEDs configured to operate at 2mA, then the operating current is 2mA per LED. The IO currents specified in the datasheet are limiting values, so it's up to you to make sure that you don't connect the IO pins to anything that will cause currents in excess of those values to flow into or out of the pin. You also have to observe the total IO current limit (continuous vcc/ground current as TI calls it), which as the name suggests is a limit for the total power consumption of the part, including all output currents. The exception to that is the input current per pin, which is basically how much current an input pin will consume from the device it's connected to -- in terms of sizing a power supply, the input currents are often negligible unless you have a LOT of them.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 20, 2024, 04:30:58 pm
content: Quote from: joniengr081 on September 20, 2024, 12:53:45 pmThe spec sheet says the AbsMax for Vcc or GND is 100mA.I/O pin specs are sink and source currents. 20mA source, 8ma sink. (verify spec sheet for actual max for source and sink)It's good to back off AbsMax 10-20% to be a design max (a max that will not kill the IC), and I would design psu to be rated ~80mA with ability to go 100mA for short period. These days, very small IC's are 1A rated.It's interesting that Vcc AbsMax is 100mA in an 8 I/O IC where I/Ox8 could potentially be more than 100mA.Note: in reading spec sheet the source & sink max for I/O is confusing.Some times doing a calculation just from spec sheet can be misleading, doing actual test to measure supply current would be better.Side note: you could buffer the I/O's with a silicon switch (fet or the ike) to gain higher current control ability, this would then reduce the power the IC "uses" (Vcc current, etc), but there's no psu savings, even fet's waste energy too.