AM26C31: Power Supply Capacity for AM26C31, AM26C32, AM26LS31 and AM26LS32

Hi Shinichi,

The abs max rating for ICC current (ICC to GND) is not what is going to be expected out of the device. This is saying the device cannot handle more than 200mA from VCC to GND without suffering from potential electrical overstress (damage). If digital inputs are kept at or below 0.8V (down to 0V)  for low level inputs and equal to or above 2V  (up to VCC) for high level input then this really shouldn't be much of concern. If the transition times are very slow or the input is kept at an undefined input voltage a low impedance path can from between VCC and GND that will shunt more current. If using the device under the recommended operating conditions this really shouldn't be an issue.

The ICC ratings in the specification tables are the values you should be reading from - but this isn't the expected current draw from the part during operation. 

Total ICC is based on three components - and on these devices generally only 2 can be fully accounted for.

1. The quiescent current - this is the idle current of the device. This current can be thought of as always present on the ICC pin regardless of loading. On these devices it seems to differ slightly how they are measuring this - but they will essentially all act this way. This is what is generally the only part of the ICC that is explicitly mentioned. 

2. The output current - the output current is directly sourced from VCC - so any current coming out of the device is being sourced from VCC and therefore a part of ICC. 

2a) For Drivers this means the differential voltage output divided by the differential load is going to be the differential current. There also is the consideration of the loading from A to Ground and B to Ground. As not only will the driver source the differential current but also the current from A to GND and B to GND - this is dependent on loading - but the differential current is pretty straight forward. For example the AM26C31 has a typical VOD of 3.1V over a 100 Ohm resistor - so 31mA; if all 4 drivers are active that is typically 124mA of current being sourced from VCC to driver outputs. So it will be the standard ICC from part 1 + the additional driver current.

2b) For receivers the logic side output is sourced from the VCC pin - so receivers VOH divided by load will be the current draw. Loading is typically high impedance here however so usually current draw from receivers is lower - but it depends on loading. 

3. Finally there is going to be signaling rate dependent increases in power consumption. Generally speaking the higher the data rate the higher the current consumption is - we don't have data for these parts due their age - but the power usage increases exponentially as you approach the max signaling rate. So when estimating power - there could be a discrepancy due to frequency dependent power usage.  These devices do have a Cpd value however to help estimate power dissipation, not current, based on frequency. 

So essentially you are correct in your line of thinking the ICC value in the datasheet is only quiescent and doesn't include active driving current. 

Please let me know if you have any other questions and  I will see what I can do!

Best,

Parker Dodson