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Nov 15 2019

Small Signal Schottky Diodes in Digital Circuits

Ⅰ. Introduction

 Diodes are widely used in electronics, such as rectification in power supply, as detection and mixing, etc. in communications, and are often used in voltage regulation and protection in various circuits (such as freewheeling diodes, TVS and so on). Due to the wide variety and versatility, the following is an analysis of the simple application of Schottky diodes in digital circuits.

The Schottky diode is structurally different from the PN junction diode. It is made of an anode metal (a barrier layer made of a material such as molybdenum or aluminum), SiO2 (electric field eliminating material), and N- epitaxial layer (arsenic material), the N-type silicon substrate, N+ cathode layer, and the cathode metal, which are as shown in the following figure. A Schottky barrier is formed between the N-type substrate and the anode metal. When a forward bias is applied to both ends of the Schottky barrier (anode metal is connected to the positive electrode of the power supply, and the N-type substrate is connected to the negative electrode), the Schottky barrier layer is narrowed, and the internal resistance becomes small. On the contrary, when a reverse bias is applied across the Schottky barrier, it becomes wider and its internal resistance becomes larger.

Schottky diode structure 

Figure 1. Schottky Diode Structure

Ⅱ. Schottky Diode Applications

The problem with Schottky diodes is that the withstand voltage is relatively low and the reverse leakage current is large. At present, the general condition of the Schottky diode used in the power conversion circuit is that the withstand voltage is below 150V, the average current is below 100A, and the reverse recovery time is between 10 and 40ns. Therefore, Schottky diodes are ideal device for use in high frequency and low voltage circuits.

1. As Dual Power Supply

At present, in the electronic design with the main controller, the real-time clock (RTC) is basically used, and the RTC needs an additional button battery to support, to avoid information lost after the system is powered off. And meanwhile, after the system is started, in order to extend the battery life, the main system is often supplied with power. Therefore, RTC often requires dual power supply, and the diode can be used for power isolation due to its single-conductivity. Taking the small-signal Schottky diode BAT54C as an example, the forward voltage drop is only 0.24v (the forward current is 0.1mA), and the RTC current consumption is uA-level, after adding the Schottky diode to isolate power supply to save info security.

 

2. As AND Gate

As shown in the figure below, n Schottky diodes form the AND gate of the n input. As long as there is a signal output logic 0 in A1~An, the Output is logic 0, only all signals in A1~An output logic 1, Output can output logic 1. That is, the phase sum of the signals A1~An is realized. Since the chip signal input stage is basically high-resistance in the digital circuit, the overall current of the AND gate circuit composed of the Schottky diode is uA-level, and the Schottky diode voltage drop is extremely small. In the case of it, the flat can still meet the design requirements.

 Schottky Diode as AND Gate 

Figure 2. Schottky Diode as AND Gate

3. As OR Gate

As shown in the figure below, n Schottky diodes form an n-input OR gate. As long as there is a signal output logic 1 in A1~An, Output outputs a logic 1. Only all signals in A1~An output logic 0, and Output can output logic 0. That is, the phase sum of the signals A1~An is realized.

 Schottky Diode as OR Gate 

Figutre 3. Schottky Diode as OR Gate

Ⅲ. Necessary Parameters

Note: Because Schottky diodes are used differently in different electronic circuits, we also need to consider the following parameters when using them.

1)Forward voltage drop VF

VF is the forward voltage drop when the diode is forward conducting. The greater the current through the diode, the larger the VF, in addition, the higher the diode temperature, the smaller the VF.

2)Reverse saturation drain current IR

IR refers to the current flowing through the diode when a reverse voltage is applied to the diode. The Schottky diode has a large reverse leakage current, therefore, selecting a Schottky diode with a smaller IR.

3)Rated current IF

It refers to the average current value calculated from the allowable temperature rise when the diode is in a long time operation.

4)Maximum surge current IFSM

Excessive forward current that is allowed to flow. It is not a normal current, but an instantaneous current, which is quite large.

5)Maximum peak inverse voltage VRM

Even if there is no reverse current, as long as the reverse voltage is continuously increased, the diode will be damaged sooner or later. This reverse voltage is not the instantaneous voltage, but the reversed voltage that is added repeatedly. Since the rectifier is supplied with an alternating voltage, its maximum value is a specified important factor. And the maximum reverse peak voltage VRM refers to the maximum reverse voltage that can be applied to avoid breakdown. At present, Schottky's highest VRM value is 150V.

6)Maximum DC reverse voltage VR

VR is the value when the DC voltage is continuously applied. For DC circuits, the maximum DC reverse voltage is important to determine the allowable and upper limits.

7)Maximum operating frequency FM

Due to the junction capacitance of the PN junction, when the operating frequency exceeds a certain value, its unidirectional conductivity will deteriorate. And Schottky diodes have high FM values up to 100 GHz.

8)Reverse recovery time Trr

When the operating voltage changes from a forward voltage to a reverse voltage, the ideal operation of the diode is that the current can be instantaneously turned off. In fact, it usually takes a little delay. The amount that determines the current cut-off delay is the reverse recovery time. Although it directly affects the switching speed of the diode, it does not mean that this value is small. That is, when the diode is suddenly reversed by conduction, the reverse current is greatly attenuated to a time required to approach IR. This indicator is important when the high-power switch is operating in the high-frequency state.

9)Maximum dissipation power P

When a current flows through the diode, it absorbs heat and raises its temperature. In reality, the external heat dissipation condition also has a great influence on P. Specifically, the voltage applied across the diode is multiplied by the current flowing through and the reverse recovery loss.

 Schottky diode

Ⅳ. Example Analysis

In digital circuit design, it is often necessary to make simple phase, or phase inversion of some signals. If the logic chip such as the 74 series is directly used, not only the layout area is greatly increased, but also the wiring is not flexible. The use of small-signal Schottky diodes and OR gates is more flexible and easy to use. The following figure shows a simple two-way reset circuit. JTAG generating a reset signal needs to reset the master, and the external reset button also needs to reset the master when pressed. If the JTAG reset and button reset directly to the reset pin of the master, it may cause damage to the JTAG emulator. For example, when the reset button is pressed, the JTAG output reset pin will be directly lowered. The phase and circuit are formed by the Schottky diode BAT54A, and the signal outputs do not affect each other. The following figure allows the master to reset as long as the JTAG output logic 0 or pressing the button reset output logic 0.

Schottky Diode BAT54A Application 

Figure 4. Schottky Diode BAT54A Application

If it is to be used as a non-gate, a triode can be used. Of course, the triode is widely used in electronics, such as a switching device in a digital circuit, as a current drive, level shifter, and the like.

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1 comment

    • Ail W on 2019-11-29 16:52:06

    Schottky (Schottky) diode, also known as Schottky barrier diode (SBD), is a kind of low power, ultra high speed semiconductor device, which is widely used in switching power supply, frequency converter, driver and other circuits, for high frequency. It is often used as low voltage, high current rectifier diode, current continuation diode, protection diode, or rectifier diode and small signal detection diode in microwave communication and other circuits.

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