TCA9617A: buffer internal block diagram

Hi Fred,

The resistance of the open-drain FET when in the ON position is not spec'ed directly in the datasheet. I believe we can estimate this value using the IOL parameter. 

If we look at section 6.5 electrical characteristics in the datasheet, we can estimate the resistance of the section you circled by using IOL = 30mA & VOL = 0.58V. 

Based off these numbers, R = VOL / IOL = 0.58V / 30mA = ~19.33ohm. I think this makes a lot of sense since we expect the open-drain driver resistance to be much stronger than that of the pull-up resistor to VCCB in order for I2C to work. Otherwise if the driver of the buffer is not stronger than the pull-up resistance, then we can never fully drive the I2C bus LOW since the driver doesn't have enough strength.

Please let me know if this helps!

Regards,

Tyler

Hi Fred,

I was suggesting that the buffer should have a stronger pull-LOW condition than the pull-up resistors on the bus, otherwise the buffer will have a difficult time driving bus to a LOW level condition. 

Fred Cheng said:

so the pull-low resistance  of the buffer  should be lower than the pull-low resistance of the master , right?

I would say this statement depends on the pull-low strength of the master. Consider that a master device could be a microcontroller with weak I2C open drain drivers. It may only have a certain amount of pull-down strength and could potentially struggle to pull-down the entire SDA bus without help from a buffer/redriver. 

We add buffers/redrivers between master(controller) and slave(target) devices either to help increase drive strength or to separate parasitic bus capacitances to stay underneath the 400pF limit for I2C. Of course, the more buffers/redrivers that we add to an application, we will demand more current from our supply and more space on our board. 

Regards,

Tyler