Hi,

I have a binary counter hooked to an I/O expander. Both inputs and outputs of the expander are used through the expander. I'm trying to figure out the allowed currents for both ICs in order not to burn them. Therefore I need 4 current values: Counter_IN, Counter_OUT, Expander_IN, Expander_OUT; for them to work I need Counter_IN < Expander_OUT, and Expander_IN < Counter_OUT. Problem is, I can't find some currents. I could really appreciate some help here!

I'm using SN74LV8154 counter and PCF8575 expander (data sheets attached). VCC is 3.3V for both.
Counter_OUT = 6mA (page 4).
Counter_IN = ??? no idea

Expander_OUT = 25mA (7.3, page 4)
Expander_IN = ??? no idea

Where do I find the input currents drawn by each IC?
Thank you in advance!

 

You should look for two different restrictions. The first restriction will involve how much current an output pin can source or sink. For CMOS outputs it is important to know both numbers. For non-CMOS outputs you might find the current sink capability exceeds the current source capability by at least an order of magnitude. The second restriction involves how much current can enter and leave a chip via the Vcc and GND pins.

You should start the search by looking the the section for "Absolute Maximum Ratings" Section 7.1, p.4 of the PCF8575 datasheet

1. Maximum Current through Vcc or GND is ±100 mA. This means current flowing through either pin in either direction.
2. Maximum output low current (\(I_{OL}\)) through any pin is 50 mA
3. Maximum output high current (\(I_{OH}\)) is -4 mA

Since all of these limits apply simultaneously having 3 output pins each of which is sinking 50 mA will violate the maximum for Vcc and GND. the input and output clamp currents will flow only if inputs or outputs are held above Vcc or below GND.

The maximum ratings for the counter are on page 3.

1. Maximum current through Vcc and GND is ±70 mA
2. Maximum Continuous Output Current on any output is ±35 mA. The outputs are CMOS and can sink as much as they source.

It is again obvious that no more than 2 outputs can sink or source the maximum current without damaging the device. Again maximum clamp currents apply to inputs or outputs held above Vcc or below GND.

The operating conditions of the device depend on what it is connected to. There is no law that say an output needs to sink 25 mA. If what you are driving requires less then that you are so much to the good.

The final thing to be concerned about is how high the temperature of the chip will rise per watt of power consumed. This is given in the section on thermal characteristics.

 

Input current considerations:

• These are CMOS devices so typically input currents are very low , <1uA, and the DC loading can be neglected in most cases.
• However, the port pins (Pn) can have pullups internally enabled, which causes significantly more sourcing current.
• Tip: Look for the input signal names as a reference in the electrical characteristics section.
• For the PCF8575 check Ii and Iihl on page 5.
• For the 74LV8154 check Ii and Ioz on page 5.

 

Thank you for your help!

 

The inputs take negligible current. If the inputs are the only loads on the outputs, then you do not have to worry about anything.

Bob