I2CChip.com: Constant Current Pullup Board

Bus Pullups and Drive

Using a constant current pullup will dramatically improve bus reliability.

The I2C Bus normally uses a resistive pullup. The exponential RC pullup characteristic is not ideal. It slows the bus down and reduces the noise margin. Constant Current Pullup is more effective at increasing bus length than reducing pullup resistor value.

The Constant Current Pullup Board plugs straight across the bus, and has connectors for both our ribbon cable, and the 2002 Philips layout.

Obsolete: The I2C2PC adaptor has component footprints on the pcb to fit a constant current pullup to Bus 1

Here are 3 ways to implement constant current pullups:

• Use a current mirror. Our Constant Current Pullup board does this
• Use a resistor to a higher voltage and a clamping diode to VDD (schottky)
• Use a constant current FET or 2 bipolar circuit

How It Works

Q1 acts as a current mirror. I_PULLUP=(VCC-0.6)/R1

To change the pullup current change R1,R2

D1 can be fitted as an idiot diode.

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Contact Information

sales@i2cchip.com
http://www.i2cchip.com
Phone +64 21 623-402

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