Power supply current requirement

[engr_joni_ee]

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 Bus
2- SC18IS606 I2C-bus to SPI bridge

The 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/p...c%2Fgeneral-purpose-ios-gpios%2Fproducts.html

https://www.nxp.com/docs/en/data-sheet/SC18IS606.pdf

 

[FvM]

The 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.

The datasheet says nothing like this. You are confusing absolute maximum ratings with DC operation characteristics.

The required supply current is specified 40 - 100 uA plus current consumed by output load.

 

[engr_joni_ee]

For PCF8574A, I also saw now that Icc in operating mode is 40 uA to 100 uA. This is just 0.1 mA at max so can be ignored.

If the load is connected to each I/O and each I/O is consuming between 20 mA to 25 mA, so all this current will also be provided by the power supply. But again, the datasheet in table 6.1 says that the continuous current through VCC or GND is +/-100 mA. It look like that the sum of all the loads connected to multiple I/O can not be greater then +/-100 mA. Then I guess in power budget I should better use +/-100 mA as current consumption. This will also include the Icc current in operating more.

 

[FvM]

Yes, 100 mA is the maximal allowed power supply current, thus not all IOs can source 25 mA simultaneously.
In most applications, IO current won't be that high, in any case it's predictable. There's no sense in designing the supply for more current than actually required.

 

[KlausST]

Hi,

There's no sense in designing the supply for more current than actually required.

I want to clarify.

In all my applications the power supply surely could provide more current than actually needed.
The drawn current never must be higher than the powersupply can provide .... but the drawn current may be less.

Now time also plays a role. This means you surely may draw shrot pulses that are higher than the power supply can supply permanently.
This current is provided by power supply decoupling capacitors .. but for sure they again need to be charged by the power supply.

So for pulses it´s a matter of pulse current, pulse duration, capacitor size, allowed voltage drop ...
And should not be designed to the limit. Always add some headroom.

Klaus