USB-2537 Logic Keeper circuit details

[pmar]

Please provide application details of your logic keeper circuit on the digital inputs of the USB-2537. A schematic would be good. Or maybe specifications of the current draw in the high and low states. My small-signal BJT with a 2.2K pull up resistor can't switch it high or low from its kept state. Thank you.

[JRys]

Hello,

The logic keeper circuit is inside a FPGA and its inner workings are a bit a mystery. However, it is set to use 3.3 volt logic and will requires very little current to switch when connect similar circuits. Driving it above 3.3 volts requires more current. There was a graph floating around here a few years back that showed to get to 5 volt would require >6mA. Because of this, use 3.3 volt logic or use a level shift circuit to reduce your 5 volt to 3.3 volts. A simple test is to use a voltage divider that will output 3.3 volts when 5 is applied. The input should pull low with no power and high with power as long as it has been programmed to be an input.

best regards,

John