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TCA9802: TCA9802 current source issue

jianfeng john
Prodigy 40 points
Part Number: TCA9802

Please help confirm the following problems about tca9802

1. for the following functions of tca980xb side current source, when vccb is low, current source disable.

(1) Vccb is low. What should vccb do? Floating or resistance to GND? How does "week pull up is connected with back power protection" understand?

(2) When B-side current source is disabled, can sclb/sdab be connected with pull-up resistance to external power supply for level shift?

2.How to understand iilc minimum 1000ua ? If the tca980x is to transmit low current, the B side device must sing at least 1000ua current, and the corresponding RILC is 150 ohm? If the RILC is less than 150 ohm, the B side needs to sing more current to output low?

thank a lot 

john 

  • 0
    TI__Guru 72551 points

    (1) Vccb is low. What should vccb do? Floating or resistance to GND? How does "week pull up is connected with back power protection" understand?
    It's probably best that the VccB line is referenced to GND somehow. 
    'How does "week pull up is connected with back power protection" understand?' This means that even if the B side had an external pull up resistor (ideally you shouldn't add this), the external pull up would not flow from the external rail into the current source into VccB. (back biasing the device). 

    "(2) When B-side current source is disabled, can sclb/sdab be connected with pull-up resistance to external power supply for level shift?"

    Technically you could do this if you don't violate the absolute max ratings on SCLB/SDAB. You would need a way to disable the pull up resistance when B side is enabled though. Also note, the B side current source is only disabled when VccB is in undervoltage mode.

    "2.How to understand iilc minimum 1000ua ? If the tca980x is to transmit low current, the B side device must sing at least 1000ua current, and the corresponding RILC is 150 ohm? If the RILC is less than 150 ohm, the B side needs to sing more current to output low?"

    During a contention situation (when A side is driving low but B side also needs to drive low like an ACK situation or clock stretching), the device on B side must be able to drive the required current inorder for the 980x device to be able to determine the B side is driving low. This allows for the device to drive A side low immediately after the external driver on A side is released. If the I2C device on B side has a RoN/RDSON less than 150ohms, then it will satisfy this condition. Devices which are I2C compliant (VoL of 0.4V at 3mA) will be able to do this with issue.

    Thanks,

    -Bobby