Maria Montero

A new diode ideal for power supply circuits of …

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

An important part of the student’s journey to professional engineering consists of a series of reflections on the nature of real electronic components. A resistance is not just a resistance; it also creates noise. A capacitor is not just a capacitor; It also has series resistance. A PCB trace is not just a PCB trace; It also has inductance, and resistance, and capacitance.

The process of awakening to the non-ideal reality of electronic devices is particularly daunting when it is contemporary with the gradual, or not-so-gradual, realization that the world in general is a rather less ideal place. Fortunately, this article offers us a respite from all this non-ideality.

The problem with “normal” diodes

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

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

The problem with diodes is that they cannot be rectified without dropping a little voltage. At some point, most of us probably learned that diodes drop 0.6 or 0.7 V when they are conducting current. This is a huge simplification, and it’s completely wrong if you don’t specify that we are talking about a silicon PN junction diode, because Schottky diodes offer significantly lower forward voltage.

In many applications the diode voltage drop is easily ignored, but in high current situations such as power circuits this voltage can translate into a significant amount of wasted energy. Power, as always, is equal to voltage multiplied by current. If the power supply needs to deliver a certain amount of current to the load circuits, our only option to reduce diode 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 data sheet LTC4376.

As you may have guessed though, it is not entirely ideal and furthermore, it is 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, while not zero, is much lower than what you would get from a Schottky diode.

The internal circuitry of the device controls the MOSFET so that the input-to-output voltage drop is 30 mV, which could be an order of magnitude less than the forward voltage of a comparable Schottky diode. The graph below gives you an idea of ​​how much you can reduce power dissipation by using the LTC4376 instead of a Schottky.

Diagram taken from data sheet LTC4376.

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

Diagram taken from data sheet LTC4376.

Ideal Diode Applications

Power supply circuits commonly employ rectifiers for reverse polarity protection and “ORing.” In both of these situations, large amounts of current from the power supply flow through the diode, resulting in potentially high amounts of wasted energy. 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 wrong voltage polarity. The LTC4376 can accomplish this task while reducing the amount of voltage lost across the “diode”, prolonging battery life and generating less heat.