Hi all,
I want to make my own board that will accept a 3V coin cell and power my Moteino.
I would like to use the LTC3525-3.3 to accomplish this, however, I read the datasheet and do not see anything about reverse polarity protection.
I've found some general schematics online using mosfets or diodes to provide reverse polarity protection, but this usually results in a voltage drop. The LTC3525 has a startup voltage as low as 0.85V which is great so that even low batteries should still work. However, introducing the typical reverse polarity protection circuits incur the voltage drop penalty. This reduces the amount of usable battery life available.
Is there some kind of reverse protection circuit without said voltage drop?
Thanks!
You can do reverse voltage protection with a single P-channel MOSFET. It works down to 0.6V + Vgs(th), very little voltage drop if the MOSFET is hard on. Choose a P-MOSFET with a low Vgs(th) and low Rds(on), and make sure Vgs(max) is larger than applied voltage, see attached schematic.
Quote from: perky on February 14, 2018, 06:13:29 PM
You can do reverse voltage protection with a single P-channel MOSFET. It works down to 0.6V + Vgs(th), very little voltage drop if the MOSFET is hard on. Choose a P-MOSFET with a low Vgs(th) and low Rds(on), and make sure Vgs(max) is larger than applied voltage, see attached schematic.
Good suggestion, Mark, but, um, don't you have the source and drain reversed in your schematic? (Source should be on Vin side).
Tom
No, that's correct Tom. It's opposite to what you expect. When the power is applied the intrinsic diode conducts, and as long as the input is above the forward voltage drop of about 0.6V plus the gate threshold the MOSFET will turn on hard and the output becomes the same as the input. When reverse voltage is applied the diode is reverse biased and the MOSFET is off.
Mark.
Quote from: perky on February 14, 2018, 06:40:44 PM
No, that's correct Tom. It's opposite to what you expect. When the power is applied the intrinsic diode conducts, and as long as the input is above the forward voltage drop of about 0.6V plus the gate threshold the MOSFET will turn on hard and the output becomes the same as the input. When reverse voltage is applied the diode is reverse biased and the MOSFET is off.
Mark.
Ah, very crafty! Thanks for explaining, Mark!
Tom
Just note that you can lose even 0.7v, dependent on mosfet, if it turns on at 3.3v or not... at 3.3v, that is a lot (only diode conducts). It is very important you find mosfet, that will be fully on something over 2.5v give or take or lower, so low Vgs, closer to 0 is better, since fet will be fully on at working voltage
One thing I overlooked that was brought to my attention, that is the most simple solution... just get a battery holder that only allows the connection to be made one way ;D