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This Module on Power Integrated Circuit Switch Products. First Module Introduction PURPOSE: • This module provides information about the Freescale power integrated circuit switch training. OBJECTIVES: • Describe the the basic operation of a mechanical and electrical switch. • Describe switch configurations including, Low-Side Switches, High-Side Switches, Half-Bridge Circuits, and H-Bridge Circuits • List the different protection features and describe what they do. CONTENTS: • 31 pages • 4 questions LEARNING TIME: • 40 minutes Welcome to this module on Power Integrated Circuit Switch products. First, we will teach you some of the basics, then acquaint you with some of the operational features of Power IC switches. In later modules we will further acquaint you with the products using product profiles, and datasheets. The basics will start by first acquainting you with some of the various kinds of switches, their function, and the ideal switch. Important practical switch parameters are pointed out as well as some of the more popular switch configurations used in ICs. The various protection features incorporated in analog ICs will be discussed to acquaint you with their function and operation as well as fault modes and their being reported. Finally, we will discuss the popular input control methods used in analog power IC switches followed by a wrap-up showing some of the more generic applications. 1 Common Switches NPN PNP Mechanical Bipolar Bipolar Collector Emitter ON OFF Base Base Emitter Collector P-Channel MOSFET N-Channel Source Drain MOSFET Gate Parasitic Diode Parasitic Diode (Body Diode) (Body Diode) Gate Drain Source There are many different kinds of switches. In general, they can be thought of as either being mechanical or electronic. It would be fairly safe to say everyone has used a mechanical “wall switch” to turn lights ON in their homes. The most common switches are symbolically shown here. In the electronic switch category semiconductor devices are made up of NPN and PNP bipolar transistors as well as N and P-Channel Field Effect Transistors (or simply FETs ). All of these device structures are used in various combinations in ICs. When a FET structure is used in an IC, it is, more descriptively referred to as a Metal Oxide Silicon Field Effect Transistor, or more simply, a MOSFET. MOSFET switch structures are used in very large numbers in ICs, and bipolar transistors to a lesser extent. The reason being, bipolar transistors use current as a means of control whereas MOSFET transistors use voltage as a means of control and current conservation in ICs is of paramount importance. The need for MOSFET devices has greatly increased in recent years as a result of the market needing lower and lower current devices brought about as a result of smaller and smaller, yet more and more complicated, battery powered portable products. NPN bipolar transistors increase their conduction or turn-ON with small positive voltage increases imposed on the Base relative to the Emitter, causing relatively small Base-to-Emitter currents to flow, allowing large amounts of Collector-to-Emitter currents to flow. PNP bipolar transistors turn-ON in a similar manner but with small negative voltage increases imposed on the Base relative to the Emitter. To turn-OFF the Collector-to-Emitter current, the Base-to-Emitter voltage is reduced to zero. P and N-Channel MOSFET devices use an electric field principle as a means of control and in so doing consume very small amounts of control current. Relatively small decreases in the Gate voltage of a P-Channel MOSFET will cause very large amounts of Source-to-Drain currents to flow. Increases in Gate voltage will turn-OFF the P- Channel Source-to-Drain current. The N-Channel turns-ON with an increase in Gate voltage and turns-OFF with a decrease in Gate voltage. This is just the opposite of that required for the P-Channel and the P and N-Channel devices can be thought of as “mirror opposites” of each other in control. NPN and PNP transistors might also be thought of as “mirror opposites” of each other in regards to control voltage polarity. N and P-Channel MOSFET transistors, and NPN and PNP transistors can be referred to as respective compliments of each other. 2 Basic Switches (Mechanical) +VPWR + VPWR Control L ON O OFF A D L Control O ON A OFF D The Mechanical electrical switch element shown here is the most common and most easily understood. Everyone is familiar with the “inner workings” of the mechanical switch element and for that reason the discussion vehicle around which the basics will be reviewed. The basics apply to all switch varieties. Though it is easily understood that switches connect or disconnect parts of a circuit to control currents, it is another thing to understand in detail what is happening when currents are turned-ON or turned-OFF. Let’s learn a little more about switches. 3 Switch Basics (Function, Operation, and Design Traits) +VPWR L O A D Leakage Current Control ON Path OFF OFF-Resistance (Very Large) ON-Resistance (Very Small) An electrical switch is used to connect or disconnect two points of a system so as to accomplish a goal or make something happen or not happen. Being more specific, the purpose of a switch is to control electrical currents. Switch manufactures diligently strive to make switches electrically more efficient. Every effort is made to minimize the switch’s ON-resistance to current flow, when the switch is CLOSED. Another important trait of an efficient switch is that it exhibit an extremely high resistance to current flow when OPEN (or OFF) and be able to accommodate a large voltage across the switch without breaking down. When a switch is turned-ON and large currents are made to flow through the switch, the switch will heat up. The heat is generated as a result of the current flowing through the inherent ON-resistance of the switch. The heat can be calculated in theory by applying Ohm’s Law and multiplying the voltage drop across the switch ‘squared’ times the resistance through the switch (Remember the equation P = I2R from basic electrical classes?). In reality, the inherent ON-resistance is hard to determine by direct measurement as a result of it’s temperature dependency. In practice, it is better to measure the voltage drop across the switch and the current flowing through the switch directly and then apply Ohm’s Law by multiplying the voltage across the switch times the current through the switch (Again, do you remember the equation P = IE ?). Both of these power calculations produce the same end result. Heating represents lost system energy and is sought to be minimize by making the switch ON-resistance as low as economically possible. When a switch is turned-OFF, no current should flow. In practice, a very minute amount of current will flow “around” the switch as a result of leakage current paths having very high resistances. The leakage current again gives rise to heating and represents lost system energy which can again be calculated by applying Ohm’s Law by multiplying the voltage applied across the OPEN switch times the leakage current. Good switches will exhibit many meg-Ohms of OFF-resistance. 4 The Ideal Switch +VPWR L O A D ON Control OFF O FF-R esistance (infinity) ON-Resistance (None) An ideal switch is capable of switching an infinite amount of current, exhibit zero resistance to current flow when CLOSED (or ON) and present infinite resistance when OPEN (or OFF) and be able to accommodate, or stand-off, an infinite voltage. Ideal switches do not occur in real life and for that reason are engineered to meet various parametric requirements. 5 Important Practical Switch Parameters • Some primary parameters of importance for practical switches are: 1. Switch ON Current Capability • Maximum operating current 2. Switch OFF Voltage Capability • Maximum operating voltage 3. Switch OFF Leakage Current • Maximum switch OFF leakage current with maximum operating voltage applied across the switch. 4. Switch ON Resistance • Maximum resistance across the switch with maximum operating current flowing through the switch. 5. Switching Speed • Switch transition time. 6. Switch Propagation Delay Time • Switch input stimulus to output response (results) time. • All switch parameters are influenced by temperature and the parametric temperature is always specified on datasheets. DC switch parameters of prime importance are maximum operating current, maximum operating voltage, switch-OFF leakage current specified with maximum operating voltage applied, and the switch-ON resistance with maximum operating current flowing. Dynamic switch parameters of prime importance are Switching Speed and Switch Propagation Delay Time. Switching Speed is a measure of how fast the switch can transition from ON to OFF or from OFF to ON and is commonly referred to as output rise and fall times. The internal circuitry of IC switches, acting in conjunction with external circuitry greatly influences rise and fall times. Datasheets always specify the values of any external components and conditions having an effect on rise and fall time parametrics. IC switches are routinely characterized for this parametric. Propagation Delay Time is a time duration measurement from switch input stimulus time to output response (or results) time. Here again, external circuit components have a great influence on Propagation Delay Time measurements and external circuit conditions are always specified on datasheets. 6 Question What are the correct resistance levels for the ON-Resistance and the OFF- Resistance? Click on your choice. a) Very small ON-Resistance, very small OFF-Resistance. b) Very small ON-Resistance, very large OFF-Resistance. c) Very large ON-Resistance, very small OFF-Resistance. d) Very large ON-Resistance, very large OFF-Resistance. Here’s a question on switching basics.
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