A New Ideal Diode for Low- and Medium-Voltage Power Supply Circuitry

Analog Devices has released an IC that provides high-performance diode functionality for automotive applications, portable devices, computing equipment, and photovoltaic systems.

An important part of the journey from student to professional engineer consists of a series of sobering realizations regarding the nature of real electronic components. A resistor is not just a resistor; it also creates noise. A capacitor is not just a capacitor; it also has series resistance. A PCB trace is not just a PCB traces; it also has inductance, and resistance, and capacitance.

The process of awakening to the nonideal reality of electronic devices is particularly discouraging when it is contemporaneous with the gradual—or not so gradual—realization that the world in general is a rather nonideal place. Fortunately, this article offers us a respite from all this nonideality.

The Problem with “Normal” Diodes

Diodes are extremely useful devices and are incorporated into a variety of circuits in which, for one reason or another, current must flow in only one direction. This task is referred to as rectification, and diodes are also known as rectifiers.

This gives you an idea of the current–voltage characteristics of a modern power rectifier. See this article for more information.

The trouble with diodes is that they can’t rectify without dropping a bit of voltage. At some point most of us probably learned that diodes drop 0.6 or 0.7 V when they’re conducting current. This is a major oversimplification, and it’s downright erroneous if you don’t specify that we’re talking about a silicon PN junction diode, because Schottky diodes offer significantly lower forward voltage.

In many applications the diode’s voltage drop is easily ignored, but in high-current situations—such as power supply circuits—this voltage can translate into a significant amount of wasted power. Power, as always, is equal to voltage multiplied by current. If the power supply needs to deliver a certain amount of current for the load circuitry, our only option for reducing the diode’s power dissipation is to reduce forward voltage. Using a Schottky diode instead of a typical silicon rectifier is certainly a step in the right direction, but can we do better?

An “Ideal” Diode

The LTC4376 is described as “an ideal diode with reverse input protection.”

Diagram taken from the LTC4376 datasheet.

As you might have guessed, though, it’s not quite ideal, and furthermore, it’s not really a diode. The LTC4376 is an integrated circuit that provides diode functionality using a MOSFET. It is described as a diode because it rectifies, and it is described as ideal because the forward voltage drop, though not zero, is much lower than what you could get even from a Schottky diode.

The device’s internal circuitry controls the MOSFET such that the voltage drop from input to output is 30 mV—this might be an order of magnitude lower than the forward voltage of a comparable Schottky diode. The following plot gives you an idea of how much you can reduce power dissipation by using the LTC4376 instead of a Schottky.

Diagram taken from the LTC4376 datasheet.

If you’re wondering why anybody uses diodes when we can obtain better performance from a MOSFET, take a look at the LTC4376’s internal block diagram. Apparently it takes an awful lot of circuitry to turn a MOSFET into an improved version of a diode.

Diagram taken from the LTC4376 datasheet.

Ideal Diode Applications

Power supply circuits commonly employ rectifiers for reverse-polarity protection and “ORing.” In both of these situations, potentially high amounts of power supply current are flowing through the diode, resulting in potentially high amounts of wasted power. Also, in low-voltage systems, the voltage drop itself can be problematic.

Rectifiers protect against supply reversal by ensuring that current cannot flow in the direction corresponding to the improper voltage polarity. The LTC4376 can perform this task while reducing the amount of voltage lost across the “diode,” extending battery life, and generating less heat.

The term “ORing” refers to the practice of connecting the outputs of two power supplies. This provides redundancy—if one supply fails, the other automatically takes its place and supplies current to the load. The following diagram gives you an example of an LTC4376 power-supply-redundancy scheme.

Diagram taken from the LTC4376 datasheet.

Do you have any experience with ideal-diode ICs? Does an order-of-magnitude reduction in forward voltage motivate you to start replacing your Schottky rectifiers? Feel free to share your thoughts in the comments section below.