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PCA9306: EN pin to VREF1 instead of VREF2

Tran Lin
Prodigy 20 points
Part Number: PCA9306
Other Parts Discussed in Thread: LSF0101,

If VREF1 is 3.3V and VREF2 is 5V and both VREF1 and VREF2 comes up at the same time.  Is there any concern for tying EN pin to VREF1 @ 3.3V thru a pullup?

  • 0
    Guru 318700 points

    The Vref pins are not power supplies.

    The A and B sides are symmetrical, but the EN pin must always be tied directly to the higher-voltage Vref pin, and the 200 kΩ resistor must be at that pin. (See the application note Voltage-Level Translation With the LSF Family for how this device works.)

    An easier way to use this device is to connect EN directly to the lower supply (3.3 V), and to leave Vref_A and Vref_B open. (Actually, Vref_A and Vref_B can then be used as a third I/O channel. So you could use the smaller LSF0101 for I²C.)

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Section 8.1.8 of the PCA9306 datasheet says that "The 200k resistor can be changed to a lower value; however, the bias

    current proportionally increases as the resistor decreases.

    Ibias = (VCC2 - Ven)/RCC : Ven = VREF1 + Vth

    where

    • Vth is approximately 0.6V (5)

    Keep in mind RCC should not be sized low enough that ICC exceeds the absolute maximum continuous channel

    current specified in section 6.1 which is described in Equation 6."


    Currently I have Vref2 as my pull up voltage, I'm planning to change the 200K ohm to 150 ohm which should meet the "continuous current specified in section 6.1 of 128mA.  What is driving the 200K ohm value for Rcc?

  • 0 in reply to Tran Lin
    TI__Guru 53575 points

    Hi Tran,

    From my understanding, there is a leakage path that exists between Vref1 and Vref2. The 200k-ohm resistor is sized to limit this leakage current and allow generation of a valid bias voltage for EN and Vref2. If this resistor value is sized to small (or not included) the leakage path between the two supplies could cause the one or both sides to lose regulation and a potentially large amount of current through PCA9306. 

    Regards,
    Eric Schott