EEVblog #831 - Power A Micro With No Power Pin!
Why Electrical Engineering Stack Exchange recommends it
Electronics Stack Exchange answerers and commenters link it whenever unexpected behavior in an unpowered or switched-off IC traces back to ESD protection diodes powering it through signal pins.
Recommended by 4 people across 16 different months, 2016 to 2021.
What it is
A 2015 EEVblog Fundamental Friday episode in which Dave Jones shows how an MSP430 microcontroller can run with its power and ground pins unconnected. It works because the chip's internal ESD protection diodes carry current from the input and output pins into the supply rails. He explains how those diodes work and why the effect applies to almost any CMOS chip.
What people said
Strong rec James Peterson · 2016-06 · Electrical Engineering Stack Exchange · on “RTC Battery Voltage Error; Non-Ideal Op Amp Error?”
An interesting article. Your comment reminded me that I saw an excellent video about just this a while ago from EEVBlog: link [this video] . One of the issues is that it is so difficult to find these diodes mentioned in documentation. And I knew they may be used in certain ICs, but didn't realize they would be in an Op Amp. Thanks!
Recommends rdtsc · 2016-01 · Electrical Engineering Stack Exchange · on “Testing a power chain with a microcontroler”
Note that if he intends on putting the micro into sleep or a zero-powered state, current may be able to flow through the divider and into the micro input, "phantom powering" it. See EEVblog#831 [this video] for an explanation.
Recommends Bimpelrekkie · 2016-04 · Electrical Engineering Stack Exchange · on “Why should I connect Vbat pin of stm32f37x to VDD when there is no battery?”
There is an excellent video by Dave Jones of the EEVBlog [this video] explaining why you can power almost every IC through almost every pin. This applies to your situation as well since you're supplying power through VDD. Vbat will be connected through the ESD protection diodes (which are present on virtually all pins on the IC) to the VDD pin and taking it's supply via this "detour". It is just good practice not to leave the Vbat pin open but instead connecting it to VDD. Although leaving Vbat open works perfectly well as you have found, the circuits feeding from Vbat will operate at a lower supply voltage because they receive their supply voltage through the ESD diodes. In certain situations th
Recommends rdtsc · 2016-10 · Electrical Engineering Stack Exchange · on “When is it okay to exceed the absolute maximum rating on a part?”
Since it wasn't mentioned in other answers, exceeding the maximum ratings on one pin of a microcontroller can also result in the following: If applied before the microcontroller powers up (even by microseconds) it can cause the micro to latch-up and fail catastrophically. If applied while the micro is completely off or is shut down, that current will flow into it's power rails through the protection diodes, powering it or preventing it from powering off completely. Dave Jones of EEBlog has a nice video [this video] demonstrating this behavior.
Recommends J Rodgers · 2017-07 · Electrical Engineering Stack Exchange · on “PIC16F1783 inclinometer project using accelerometer: 12-bit ADC issue”
Have a look at this video from eevBlog by Dave Jones to understand why LED can come on when you have no power to PIC. [this video] Agree with SamGibson, please provide a schematic.
Recommends Bimpelrekkie · 2017-11 · Electrical Engineering Stack Exchange · on “why isp avr MCU doesn't need ground?”
Expanding on pjc50's comment below regarding "phantom supply": even if you do not connect ground and/or supply a chip can still work, it then powers through the **ESD protection diodes** see this EEVBlog video for an explanation: [this video]
Recommends Bimpelrekkie · 2018-01 · Electrical Engineering Stack Exchange · on “How good is to use this zener diode for reverse polarity protection?”
Yes it does / it can be. You can supply power to the opamp through the signal pins via the ESD diodes! See this EEVBlog video for an explanation: [this video] there Dave demonstrates the effect using a uC but it will work the same for an opamp or any IC really.
Recommends Bimpelrekkie · 2018-06 · Electrical Engineering Stack Exchange · on “Battery Tester Circuit with 3 LEDS”
@dmb is right, the dots should not be there! Also, I see another issue (yes even though I said it looked fine) there will be a current flowing into the inputs of the comparators when they have no supply voltage! This current is caused by the ESD diodes between inputs and supply pins. You should be **very careful** when applying a voltage to chips that do not have a supply voltage! See the EEVBlog video about this: [this video] and yes that applies to **all chips** not only micros.
Recommends Bimpelrekkie · 2019-01 · Electrical Engineering Stack Exchange · on “AREF above operating voltage of the microcontroller”
Indeed this information will be in the datasheet, but since it might be difficult to find I'll help a bit: The datasheet will state that the voltages at the uC's inputs/outputs cannot exceed the voltage on the VDD pins . The reason for this is the ESD protection diodes. As soon as the voltage at AREF becomes higher than VDD + 0.5 V the AREF's ESD diode will start conducting. So if you connect 5 V to AREF, the VDD supply of the uC will be pulled up to about 4.4 V ! But you supplied 3.3 V via an LDO. Yes, then the LDO will do nothing. If you want a 5 V input range either power the uC with 5 V or use a voltage divider to lower the input voltage to the ADC. To learn more about what issues the ES
Recommends Bimpelrekkie · 2019-01 · Electrical Engineering Stack Exchange · on “AREF above operating voltage of the microcontroller”
To learn more about what issues the ESD diodes can cause, watch this EEVBlog video: [this video] Making this and comment above an answer because, well it is.