MrVictorM

ON pin of multiple ADA4870

Discussion created by MrVictorM on Jun 7, 2018
Latest reply on Jun 12, 2018 by MrVictorM

Hello,

 

We have built a board with several ADA4870 working in parallel. These opamps share the ON pin for activation/protection purposes. I recently found two questions in the AD forums dealing with a shared signal for several ON pins:

 

Link1: https://ez.analog.com/thread/92651-ada4870-in-parallel-protection-features

Link2: https://ez.analog.com/thread/92633-ada4870-short-circuit-protection-using-issue

 

During regular operation the ON pins are floating. When one of the amplifiers detects a shortcircuit, it seems that its ON pin sets a voltage that drives the ON pin of the remaining active amplifiers.

 

According to the answer of Link2, the ON pin floats between Vee+1.8 and Vee+2.4V. However, I do not understand why this range falls within the values setting a power-down in the amplifier (ON pin between Vee+1.8V and Vee+5V, according to the datasheet).

 

Link2 also states that the ON pin voltage is set to Vee+1.71V when a shortcircuit is detected. This value is too high to set a low state in the ON pin of the other amplifiers, and too low to disable them. It is difficult to predict the behavior of this voltage driving the ON pins of the amplifiers.

 

According to the datasheet, the ON pin is simply modelled as a 20k resistor. However, the behavior explained above seems more complex. Do you have any model or explanation that helps to understand how the ON pin behaves?

 

Thank you in advance,

 

Victor M.

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