What limits how much open collector outputs can be grounded simultaneously in an IC

[matrixofdynamism]

Using open collector output means that when the output is driven low, some current will enter the IC as the transistor at the output is going to grounded. Remaining current shall enter the device on the other end. Now how much current shall enter the input device on the other end and how mucn into the driving open collector output buffer depends on the impedance of the two terminals. I assume that usually the input buffers of ICs have much lower input impedance than the output buffers but I am not sure how this can be confirmed.

Since some tiny current shall enter the driving buffer, there shall be some power dissipation in this driving IC. How does one know e.g for a 100s of pin FPGA or CPLD about how many outputs that are configured as open collector outputs, can be grounded at the same time? All devices have a limit on how much power they can dissipate before they burn out.

 

[Audioguru]

An IC has a maximum power limit stated on its datasheet at a certain ambient temperature. An output has a maximum saturation voltage at a certain current stated. Heating is the power caused by the saturation voltage times the output current. Then use simple arithmetic to see if your IC will overheat

Many ICs show the maximum allowed power supply current which tells you how many outputs can be used at the same time.

 

[KlausST]

Hi,

a simple drawing of your circuit with the currents you refer to would be of big help.


Klaus

 

[D.A.(Tony)Stewart]

You can treat it as a power resistor with a heat sink.

Each open collector or open drain when saturated as a switch has a finite ESR, measured by the Vol @Iol = Rce or =RdsOn and the power dissipation Pd= Vol*Iol

Then take the number of devices switched with load currents and add up the switch losses and compute from the package,Rjc + layout thermal resistance Rca , the junction temperature rise due to those loads. THe resulting temperature rise should not exceed 85'C for Tj in good design above worst case ambient. Which is usually >40'C. ambient plus internal self heating of other parts for typical outdoor... unless you live in india...