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power down input protection for LPC213x ADC

Started by Kevin Lynch August 7, 2008
Hi Folks I have a question about power down input protection for the
ADC circuit for the LPC213x processors - does the LPC213x have a JFET or
similar protection for the input ports of the ADC circuit to isolate the
circuit from any external voltage when the power is down?

Im working on an existing LPC2138 based product that uses a 3V lithium
battery to back up the RTC circuit (Vbat) when the device is powered down.

I need to mod the board to monitor Vbat in order to notify users when
battery must be replaced. The easiest way to do this is to simply use a
jumper wire from Vbat to one of the unused ADC inputs. If I was starting
from scratch, Id gate it with a JFET, but its a low volume product and
we dont want to spin another board rev right now. I have tested using
just a simple jumper from Vbat to an ADC port and it works fine. The ADC
circuit does not draw any power from Vbat when the main power supply is
off even if I program the ADC mux so it is set to monitor Vbat when
the power goes down (so the ADC mux is set to that port).

However the 213x data sheet (static characteristics) says that Via
should not exceed Vdda, which of course will happen when the board is
powered down because Via stays at 3V battery voltage. While this doesnt
seem to be a problem in practice, I dont want to do something that
violates the spec, although I suspect that this spec assumes that the
processor is powered up. It appears to me that when the unit is powered
down, that the ADC inputs are completely isolated by the pin connect
block because I can not detect any current which might otherwise bleed
through the ADC power supply from the jumper to Vbat

Ive searched through this forum and while Ive seen that there are a
few other Philips parts where this is a problem, I didnt see anything
on the LPC213x family. Is there any anyone who knows for sure does the
LPC213x already have a JFET or similar protection for the ADC circuit?

Thanks - Kevin

An Engineer's Guide to the LPC2100 Series