I am using the LPC804 in a low power application and am using a 3.00 Volt shunt reference as the external reference for the ADC. My initial testing has revealed that the VREFP pin input current is surprisingly high at approximately 995 uA. Curiously, the input characteristics of this processor pin are not specified at all in the Rev 1.4 12 July 2018 Product data sheet.
I found that if I provide enough shunt reference bias current (1.1 mA or greater) then the shunt voltage reference reaches its expected 3.00 volts and the LPC804 ADC operates properly and converts analog voltages into reasonable digitized values. So while the ADC does indeed work properly with a 3.00 volt external reference, the VREFP input just requires a relatively large amount of input current (995 uA.) This seems odd to me, especially since as far as I can see it's not mentioned in the data sheet or user manual.
This may not sound like much current, but considering that the LM4040 shunt reference itself only needs about 100 uA of bias current, 995 uA additional current into the ADC reference input is a lot of current.
While my current application design actively switches the shunt reference power, it's still disconcerting not to have a VREFP input specification from which to design the hardware circuitry for worst case device characteristics.
Has anyone else encountered this issue? This seems like a very important characteristic to document for a low power microprocessor.
Fredonia Mountain Technologies
Dunlap TN 37327
After contacting NXP technical support about this issue, their own testing has confirmed the VREFP current to be about as I've measured. Subsequently they contacted the designer and the situation is this: the VREFP input current will be about 1 mA per its design (an internal resistor divider is used to derive the internal reference for the ADC.) The formal characterization of the VREFP input current has not been performed as of this date.