MAX14626 Common Voltage and Current Loop Input Protection

Question

I have an application with generic IO pins configurable as either voltage, 0~10VDC, and current loop, 4~20mA, inputs. The voltage input is always low-current, i.e. <20mA, so I was hoping to use the MAX14626 as a common protection device for the input pin regardless of it's configuration. I have the IC hooked up as indicated on the datasheet, but when I apply my external voltage input to pin 1 of the MAX14626 I see the output, pin 6, jump up to 1.3VDC and just sit there independent of the input voltage. What am I doing wrong? 
 The load resistance connected to IC GND from pin 6 is 500Ohms which should give me a full 0~20mA over the full input voltage range of 0~10V. My understand was that the MAX14626 is fundamentally a pair of back to back FET switches controlled to provide 30mA maximum current limit -- obviously I am missing something. Any ideas? 
 Thanks! 
 ~Jon

YuriyK1 · Accepted Answer

Hi Jon, 
 The MAX14626 requires a minimum input voltage of approximately 2V at the IN pin, with the datasheet specifying a maximum of 2.3V. It will not function for a 0~10VDC range, at least not for the lower 20%. For a 4-20mA current loop configuration, a 4mA current over a 500Ohm load results in a 2V drop across the load, plus an additional 0.1V drop due to a 4mA current across a typical 25Ohm Rdson, totaling a minimum of 2.1V at the IN pin. When initiating a test with 2.3V at pin 1, it is important to observe the response at pin 6. Voltage output devices generally require a minimum load of 1k; therefore, it is advisable to consider switching loads when working with Voltage and Current applications. Regards, 
 Yuriy)

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MAX14626 Common Voltage and Current Loop Input Protection