A Very Brainy Diode
Jack Ganssle, publisher of the legendary EmbeddedMuse newsletter, once stated that the embedded space is ideal as things permanently improve. When looking at diodes, the reverse is true – silicon-based diodes have had a drop of 0.7V for ages. Analog Devices now brings a workaround for power supply ORring circuits.
In principle, ORring is simple; two diodes are used to allow one power supply to perform a “soft takeover” against the second one. As long as both power supplies get their energy from an unlimited source, diode losses are insignificant – once a battery is involved, things get less funny.
A FET, obviously, is a better solution; when switched on, its resistance is very low and usually causes less loss than when a diode is involved. Figure one shows how Analog Devices wants to solve the problem.
…Replacing a diode reduces voltage losses
Who Controls the MOSFET
The LTC4412 does its magic via a dedicated sensing pin, which allows the system to find out if external power is present – if this is the case, the MOSFET is enabled. The part makes good use of FETs in that it is strong enough to drive both large and small transistors, thereby ensuring that both large and small loads can be supplied efficiently.
Circuit functionality becomes clearer in the block diagram…
Another interesting aspect of the part is the low quiescent current, which flows in all modes of operation. With a typical value of 11yA, very little energy is lost on management of the backup circuitry.
Analog Devices apparently foresaw the current MLCC capacitor crisis – the LT4412B is one of very few parts which prefer to be decoupled with traditional capacitors or an electrolytic part.
Finally, the zener diode shown in the “basic” version of the circuit can also be substituted with a secondary MOSFET via the STAT output.