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PCA9306: PCA9306 EN and VREF2

JENITT MARIA FRANCIS
Intellectual 380 points
Part Number: PCA9306

Hi,

In one of my design ,I wrongly connected the EN and VREF2 pin of PCA9306 i.e ,I connect EN pin to VCC_B supply through 200K resistor and VREF2 directly to the VCC_B. I observed the voltage lifting at the VREF1(1.8 v becomes 2.2).When I connect the EN and VREF2 pin according to the recommended path(EN and VREF2 shorted and connected that to VCC_B through 200K) there is no voltage lift at VREF1.

Can anyone give clarification on this scenario? I came to know it is due to leakage path after reading some of the E2E posts, but can anyone describe the leakage path on IC with some more data(like internal block diagram of IC) if possible? I am curious to know about this.

Regards,

Jenitt

  • 0
    TI__Mastermind 49010 points

    Hello Jenitt,

    The initial set up you described can damage the device. In the case where Vcc2 is first applied and then Vcc1 is then applied, the gate is biased at 3.3V and fully on. The path from Vref2 to Vref1 (below) results in an effective short where only the internal resistance of the FET will limit the current, with the initial connection causing a large current spike. The purpose of the 200kohm resistor is to prevent this from occurring.

    EDIT:

    The 2.2V shift: When in this set up, the 3.3V rail will try to push current to the Vref1 point and try to push the Vref1 point up to 3.3V. As Vref1 grows larger, the Vgs will grow smaller and change the resistance of the FET from a short to a large value until Vgs reaches it's threshold voltage which is about 0.7V~when Vref1 reaches 2.6V. What Vref1 is connected to may influence what happens next. If a device is connected to this rail and has ESD protection utilizing a zener diode set to turn on at 2.0V, Vref1 will clamp to that voltage. In the case that it may be connected to a FET which may start to break down, current will begin to flow through the FET breaking down and generate a voltage proportionate to V=I*R.This will lower the voltage at Vref1 and the 3.3V rail would fight with the Vref1 point and try to push Vgs back to 0.7V or Vref1=2.6V. The system would try to balance itself out (stabilize) at 2.2V.

    We would need to look at what is connected to your 1.8V rail to really try to narrow it down.

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    Intellectual 380 points
    Hi Bobby,

    Thanks for your input.

    Regards,
    Jenitt