TCA9555: TCA9555 PWR Consumption Question

The supply current (I_CC) flows into V_CC and out of GND.

The input leakage current (I_I) flows into or out of an input pin.

The load current flows into or out of an output pin. The datasheet specifies the current-vs-voltage characteristics, but the actual load current depends on your load.

For practical purpose, the first two currents are low enough that you can ignore them in most cases; what matters are the load currents and the currents through the I²C pull-up resistors.

Hi Elvis,

Elvis Han said:

1.The third row IOH = -10mA (Negative means current source from IC to the outside); The 4th row IoL=24mA  (Positive means current source from outside to IC)

These are specs based off different types of loading. 

For example, when VCC = 3V, a VOH is measured of 2.5V, you can expect 10mA of current to be sourcing out of the pin. In general, the VOH measured is probably much higher close to 3V due to the loading that is most likely present on the output. For example, a load of 500 ohms is much stronger and thus will draw more current than a load that measures 10kohm. 

This is the same idea for IOL. When VCC is 1.65V to 5.5V, and VOL = 0.7V, the typical current draw is ~24mA. However, the VOL in practice will probably measure much lower when using a smaller load on the output. 

Elvis Han said:

2. Test Condition =>Output H under current 8mA/10mA but  Output L under Voltage 0.5V/0.7V =>May I know the the reason why the conditions are different

This is similar to my previous response. Imagine on P00 you have a 10kohm pull-up resistor when VCC = 5V. Then you drive an output LOW on P00.

The estimated current is then 5V / 10000ohm = 0.5mA.

Now imagine your load is a 1k pull up resistor. 

The estimated current is then 5V / 1000 ohm = 5 mA. 

Depending on the load that you choose on the output will determine the amount of current that the IO expander will sink/source. 

Elvis Han said:

3. The last 2 rows=>Input sink current H=>1uA L=>-100uA  (Also negative means the current flow from Vdd to outside)

These are leakage specs when the input is held at a voltage either VCC or GND. It gives the worst case leakage which is in the uA range.

Elvis Han said:

4. The more detial one; There is 75uA needed for the IC working 

75uA is not necessarily the needed current supply for the IC to work. 

This spec is the maximum current that the IO expander would draw under operating mode conditions (when I2C clock signal is present). The uA can fluctuate but is always less than <75uA under those conditions. 

Regards,

Tyler

Hi Elvis,

Elvis Han said:

So when VCC=3V VOH is measured of 2.5V =>10mA (Sourcing Out ) would be like the figure below;

R251=(3V-2.5V)/10mA=50 Ohms

Based on R251 resistance, Souring Current can be varied.

Also the IOL across R237 will be (3V-0.7V)/4.7k ohms=>0.489mA

This equation is incorrect (R251 = (3V - 2.5V) / 10mA = 50 ohms). R251 is referenced to GND. The math would imply that there is a 0.5V voltage drop across the resistor during an output HIGH (VOH). 

TCA9555 is powered with VCC = 3.3V in this example. 

VOH varies due to the current that is sourced out of the device. The current that is sourced out of the device is controlled by R251. 

So if R251 = 50 ohms then

3.3V / 50 ohms = 66mA. This is a rough estimate to the amount of current that would be sourced out of the pin for a 50 ohm load. THis is way too much current and is above the abs. max rating of the pin. My suggestion would be to choose a pull-down resistor that is much larger. 

As for IOL we can estimate again by taking 3V / 4.7kohm = ~0.638 mA. I am not taking into account the resistance of the push-pull driver internal to the TCA9555 IO expander for IOH or IOL cases. 

Elvis Han said:

Also there are notes on Page 14 of Spec

I think that for all Ports Current 200mA would be the max one ;The remaining will be I2C load currents.

Output H:

(2) Each I/O must be externally limited to a maximum of 25 mA, and each octal (P07–P00 and P17–P10) must be limited to a maximum
current of 100 mA, for a device total of 200 mA.

Output L:

(3) The total current sourced by all I/Os must be limited to 160 mA (80 mA for P07–P00 and 80 mA for P17–P10).

Pxx < 25mA max

Port (8 Pxx pins) < 100mA max

Port 0 + Port 1 (16 Pxx pins) < 200mA 

Each IO adds to the overall current that ICC will sink. It is recommended to follow each of the mA levels for your system application. 

Regards,

Tyler