AND9129/D Understanding a Digital Transistor Datasheet http://onsemi.com APPLICATION NOTE Introduction Maximum Ratings This application note will describe the common specifications of a Digital Transistor. It will also show how to use these specifications to successfully design with a Digital Transistor. Parameters from the DTC114E/D datasheet will be used to help with explanations. This datasheet describes a Digital Transistor that has an input resistor, R1, equal to 10 kW and a base−emitter resistor, R2, equal to 10 kW. Figure 1 gives a labeled schematic of a Digital Transistor. These labels will be used throughout this application note. Below is the maximum ratings table that can be found in every Digital Transistor datasheet. Table 1. MAXIMUM RATINGS Rating Symbol Max Unit Collector−Base Voltage VCBO 50 Vdc Collector−Emitter Voltage VCEO 50 Vdc IC 100 mAdc Input Forward Voltage VIN(fwd) 40 Vdc Input Reverse Voltage VIN(rev) 10 Vdc Collector Current – Continuous Stresses exceeding those listed in the Maximum Ratings table may damage the device. If any of these limits are exceeded, device functionality should not be assumed, damage may occur and reliability may be affected. The first spec, VCBO, states that the maximum voltage that can be applied from the collector to the base is 50 V. The Collector−Emitter Voltage, VCEO, spec states the maximum voltage that can be applied from the collector to emitter is 50 V. In addition, the table specifies that the maximum DC collector current (IC) that the device can conduct is 100 mA. There are two maximum ratings for the input voltage, forward and reverse. The input voltage is defined as the voltage applied from the base and the emitter. The maximum input forward voltage is determined from the power capabilities of the input resistor, R1. Figure 3 and Equations 1 and 2 describe how the maximum input forward voltage is calculated when the maximum power capability of R1 is 220 mW. It is important to use the minimum value of R1 because it will cause the greatest power to be dissipated on the die. The minimum value of R1 can be found in the Electrical Characteristics Table of every Digital Transistor datasheet. In this case R1(min) = 7 kW. The resulting maximum input forward voltage is 40 V. For this particular device a voltage greater than this would result in the resistor failing. Figure 1. Labeled Schematic For Digital Transistors it is important to realize that the transistor and resistor network is considered as one unit. An example of this is the ICBO parameter. Looking at this parameter one would think that it is not possible. How could the emitter be open when R2 connects the emitter and base of the transistor? Shouldn’t it be ICER? The answer is yes R2 connects the emitter and base of the transistor, but this connection is built into the device. What is considered the emitter and base of the Digital Transistor is left open. In the case of a Digital Transistor ICER would mean that there is an additional resistor placed between the emitter and base of the digital transistor. Figure 2 shows the difference between ICBO and ICER when referring to Digital Transistors. Figure 2. ICBO vs. ICER © Semiconductor Components Industries, LLC, 2013 December, 2013 − Rev. 1 1 Publication Order Number: AND9129/D AND9129/D For ICBO and ICEO, one can expect the leakage current to be below the value that is stated on the datasheet when a reverse voltage is applied between the respective junctions. However, IEBO is dependent on both the input resistor and the base−emitter resistor. The resistor network path is significantly less resistive then the path through the reversed biased base−emitter junction. Ohm’s Law is used to determine the IEBO value. An example for a Digital Transistor with R1(min) = 7 kW, R2(min) = 7 kW, and a VEB = 6 V is shown below. It is important to note that the minimum value of 7 kW was used instead of 10 kW. This is done because it will estimate the largest possible leakage. Figure 3. Input Forward Voltage P R1 + ǒV R1Ǔ R1 2 ³ V R1 + ǸP R1 + Ǹ0.220 R1 7000 + 39.3 V I EBO + (eq. 1) V IN(max) + V R1 ) V BE + 39.3 ) 0.7 + 40 V (eq. 2) IB + Ǔ *1 When R BE ơ R 2, 1 ) 1 + 1 R BE R 2 R2 ǒ Ǔ (eq. 3) *1 R Equivalent + 1 R2 VB + VE + R2 ǒR R) R Ǔ 1 1 IC 5 mA + + 83 mA 60 h FE (eq. 6) The next parameter is the Collector−Emitter Saturation Voltage, VCE(sat). This parameter tells the designer the maximum voltage drop that will occur when the device is ON. In this instance a maximum of 250 mV will be dropped across the transistor when the IC = 10 mA and the base is driven with 0.3 mA (hFE = 33). The hFE spec can be seen as a threshold current ratio of base drive to collector current. If the base current is greater than the collector current divided by the hFE spec then the transistor begins to saturate and the collector−emitter voltage drops to the saturation voltage. In most cases a Digital Transistor will be used as a switch. There are four parameters that characterize this operation: Input Voltage (off), Input Voltage (on), Output Voltage (on), and Output Voltage (off). The first parameter, Input Voltage (off), states that an input voltage less than 0.8 V will turn the device OFF (VCE = 5 V, IC = 100 mA). The Input Voltage (on) parameter defines that at a VCE = 0.3 V and IC = 10 mA a minimum input voltage of 2.5 V needs to be applied to turn the device ON. These two parameters also list typical values. These typical values are the measured voltages when the device is exactly at those ON and OFF conditions. It is not recommended to operate the device at these voltages if one wants to ensure the device is ON/OFF. Figure 5 gives a graphical representation of these specs. Figure 4. Input Reverse Voltage Equivalent Circuit ǒ (eq. 5) Digital Transistors only specify the collector−base, V(BR)CBO, and collector−emitter, V(BR)CEO, breakdown voltages. These ensure that the device will have a breakdown voltage above the specified value. The second section of the Electrical Characteristics Table describes specs for when the digital transistor is ON. Table 2 will be used as an example to help explain the following parameters. First, the DC Current Gain, hFE, is a specification of how much the base current will be amplified in resulting collector current. The hFE spec states that with an IC = 5.0 mA and a VCE = 10 V one can expect the gain to be around 60, but is ensured that it will be above 35. This means that the base current, IB, will need to be 83 mA if the typical gain of 60 is used. The maximum input reverse voltage is determined by the breakdown voltage of the base−emitter junction and the resistor network. The equivalent circuit in Figure 4 can be drawn when analyzing the maximum input reverse voltage and is justified by Equation 3. Looking at Figure 4 one sees that it is just a simple voltage divider. The voltage divider equation is shown in Equation 4. In applications where a large reverse voltage will be applied to the base−emitter junction it is recommended to use a Digital Transistor with a small R2 and large R1. This will cause the majority of the voltage to be dropped across R1, thus a smaller voltage will be dropped across R2 and consequently the base−emitter junction. The worst case for this spec is when R1 is at its minimum value and R2 is at its maximum value. R Equivalent + R BEńńR 2 + 1 ) 1 R BE R 2 6V + 0.43 mA 7 kW ) 7 kW (eq. 4) 2 Electrical Characteristics The first section of the Electrical Characteristics Table has specs pertaining to when the Digital Transistor is OFF. There are three leakage current parameters ICBO, ICEO, and IEBO. http://onsemi.com 2 AND9129/D VOH, spec are: VCC = 5.0 V, VB = 0.5 V, RL = 1.0 kW. Under these conditions the output voltage will be greater than 4.9 V. Refer to Figure 6 for further understanding of the VOL and VOH specs. Vi < 0.8 V, device is OFF Vi < 1.2 V, device is typically OFF Vi > 1.8 V, device is typically ON Vi > 2.5 V, device is ON Figure 5. Output Voltage vs. Input Voltage The output voltage of a switch is also very valuable to a designer. The Output Voltage (on), VOL, spec states that the output voltage will be less than 0.2 V under the following conditions: VCC = 5.0 V, VB = 2.5 V, RL = 1.0 kW. The VOL is similar to the VCE(sat) of the device in that they both describe the maximum voltage that will be dropped across the collector−emitter when the Digital Transistor is in saturation(ON). The conditions for the Output Voltage (off), Figure 6. Output Voltage Test Schematic Table 2. ELECTRICAL CHARACTERISTICS Characteristics Symbol Min Typ Max Unit Collector−Base Cutoff Current (VCB = 50 V, IE = 0) ICBO − − 100 nAdc Collector−Emitter Cutoff Current (VCE = 50 V, IB = 0) ICEO − − 500 nAdc Emitter−Base Cutoff Current (VEB = 6 V, IC = 0) IEBO − − 0.5 mAdc Collector−Base Breakdown Voltage (IC = 10 mA, IE = 0) V(BR)CBO 50 − − Vdc Collector−Emitter Breakdown Voltage (IC = 2.0 mA, IB = 0) V(BR)CEO 50 − − Vdc OFF CHARACTERISTICS ON CHARACTERISTICS DC Current Gain (IC = 5.0 mA, VCE = 10 V) hFE 35 60 − VCE(sat) − − 0.25 Vdc Input Voltage (off) (VCE = 5.0 V, IC = 100 mA) Vi(off) − 1.2 0.8 Vdc Input Voltage (on) (VCE = 0.3 V, IC = 10 mA) Vi(on) 2.5 1.8 − Vdc Output Voltage (on) (VCC = 5.0 V, VB = 2.5 V, RL = 1.0 kW) VOL − − 0.2 Vdc Output Voltage (off) (VCC = 5.0 V, VB = 0.5 V, RL = 1.0 kW) VOH 4.9 − − Vdc Input Resistor R1 7.0 10 13 kW Resistor Ratio R1/R2 0.8 1.0 1.2 Collector−Emitter Saturation Voltage (IC = 10 mA, IB = 0.3 mA) Product parametric performance is indicated in the Electrical Characteristics for the listed test conditions, unless otherwise noted. Product performance may not be indicated by the Electrical Characteristics if operated under different conditions. http://onsemi.com 3 AND9129/D because the ratio of R1/R2 is more pertinent when designing with Digital Transistors. The R1/R2 ratio is what controls the critical parameters of the Digital Transistor. However, if desired it is possible to calculate the minimum and maximum values of R2. Equations 7 and 8 help explain how to do this using the values in Table 2. There are two curves present on all Digital Transistor datasheet that provide more data on the Vi(on) and Vi(off) parameters. The first is the Output Current vs. Input Voltage, Figure 7. This curve characterizes the Vi(off) parameter, and the data was taken when the VCE or the output voltage is 5 V. One can use this curve to determine at what voltage the current will drop rapidly, or in other words turn OFF. For this device the voltage at which the current rapidly drops off is around 1.4 V at 25°C. R 2(min) + R 2(max) + R 1(min) (R 1ńR 2) max R 1(max) (R 1ńR 2) min + + 7 kW 1.2 13 kW 0.8 + 5.8 kW (eq. 7) + 16.3 kW (eq. 8) Example of Designing with Digital Transistor Datasheet Design requirements: Polarity: NPN IC = 10−20 mAdc VCE(max) = VCB(max) = 40 Vdc VIN(fwd) = 20 Vdc VIN(rev) = 10 Vdc Micro controller output voltage: Off: 0.4−0.6 V On: 1.5−4.2 V Micro controller max output current: 250 mA Figure 7. Output Current vs. Input Voltage The other datasheet curve is the Input Voltage vs. Output Current, Figure 8. This curve characterizes the Vi(on) parameter and the data was taken with the VCE = 0.2 V. This chart provides the designer the voltage needed for a desired collector current while maintaining an output voltage of 0.2 V. For example, to operate the device characterized in Figure 8 at an IC of 30 mA and a VCE of 0.2 V, an input voltage of 5.3 V would need to be applied. 5.3 V Figure 9. Schematic for Example Now that the design requirements have been established it is time to indentify the Digital Transistor that works best for the application. Table 3 will help with this selection. The first step should be to find a Digital Transistor that meets the max current and voltage requirements. The majority of Digital Transistors within ON Semiconductors portfolio are rated at 100 mA. The remaining Digital Transistors are rated above 100 mA. Also, the majority of them have a VCEO and VCBO of 50 V, which will meet the above requirement. Next, is to find a Digital Transistor that meets the forward and reverse input voltage requirements. Using Table 3 it is seen that the VIN(fwd) requirement of 20 V is reached by all digital transistors except for ones that have a R1 of 1 kW or 2.2 kW. Digital Transistors with a R1/R2 ≥ 1 will meet the 10 V requirement for VIN(rev). Figure 8. Input Voltage vs. Output Current The last two parameters describe the built in resistors of the Digital Transistor. First, is the typical R1 value along with the minimum and maximum values that are ±30% of the typical value. The typical resistor ratio of R1/R2 is also defined along with a minimum and maximum spec of ±20% of the typical value. The actual value of R2 is not defined http://onsemi.com 4 AND9129/D The next requirement to look at is Vi(off). In the design requirements it states that the micro controller will give a voltage of 0.4−0.6 V when it wants the Digital Transistor to be OFF. This eliminates the Digital Transistors that have a maximum Vi(off) spec of 0.5 V because these should be supplied with a voltage of less than 0.5 V when the device is desired to be OFF. The design requirements also state that the maximum output voltage of the micro controller is 4.2 V. A Digital Transistor needs to be selected that can provide a maximum IC of 20 mA when the input voltage is 4.2 V. It also has to provide an IC of 10 mA with an input voltage of greater than 1.5 V. Looking at the Input Voltage vs. Output Current charts of the four remaining Digital Transistors only two meet the input voltage requirements. They are the Digital Transistors with R1/R2 = 10 kW/10 kW and 22 kW/22 kW. It is also important to consider the max output current of the controller. In this case the micro controller can source a maximum of 250 mA. For the 10 kW/10 kW digital transistor the typical voltage needed to drive an IC of 20 mA is 3.3 V. In this case the micro controller would have to source approximately 260 mA which is above the max output current of the micro controller. 3.3 V * 0.7 V + 260 mA 10 kW (eq. 9) However, for the 22kW/22kW the typical voltage needed for an IC of 20 mA is 3.8 V resulting in a current of 140 mA. 3.8 V * 0.7 V + 140 mA 22 kW (eq. 10) After considering all the design requirements it is found that the Digital Transistor that will work best for this specific application is the Digital Transistor with R1/R2 = 22 kW/22 kW. The final step would be to pick the package. ON Semiconductor offers single Digital Transistors in six packages ranging from SC−59 to SOT−1123. Table 3. INPUT VOLTAGE SPECIFICATIONS VIN(fwd) (V) R1 (kΩ) VIN(rev) (V) Vi(on) (V) Vi(off) @ 0.1 mA (V) R2 (kΩ) PNP NPN PNP NPN PNP NPN PNP NPN 1 1 10 10 10 10 2.0 @ 20 mA 2.0 @ 20 mA 0.5 0.5 2.2 2.2 12 12 10 10 2.0 @ 20 mA 2.0 @ 20 mA 0.5 0.5 4.7 4.7 30 30 10 10 3.0 @ 20 mA 2.5 @ 20 mA 0.5 0.5 10 10 40 40 10 10 2.5 @ 10 mA 2.5 @ 10 mA 0.8 0.8 22 22 40 40 10 10 2.5 @ 5 mA 2.5 @ 5 mA 0.8 0.8 47 47 40 40 10 10 3.0 @ 2 mA 3.0 @ 2 mA 0.8 0.8 100 100 40 40 10 10 3.0 @ 1 mA 3.0 @ 1 mA 0.5 0.5 2.2 47 12 12 5 6 1.1 @ 5 mA 1.1 @ 5 mA 0.5 0.5 4.7 47 30 30 5 6 1.3 @ 5 mA 1.3 @ 5 mA 0.5 0.5 10 47 40 40 6 7 1.4 @ 1 mA 1.4 @ 1 mA 0.5 0.5 22 47 40 40 7 8 2.0 @ 2 mA 2.0 @ 2 mA 0.5 0.5 47 22 40 40 10 10 4.0 @ 2 mA 4.0 @ 2 mA 1.2 1.2 2.2 Inf. 12 12 5 6 1.3 @ 10 mA 1.1 @ 10 mA 0.5 0.5 4.7 Inf. 30 30 5 6 1.3 @ 10 mA 1.3 @ 10 mA 0.5 0.5 10 Inf. 40 40 5 6 1.7 @ 10 mA 1.7 @ 10 mA 0.5 0.5 47 Inf. 40 40 5 6 4.0 @ 10 mA 4.0 @ 10 mA 0.5 0.5 100 Inf. 40 40 5 6 1.5 @ 1 mA 1.5 @ 1 mA 0.5 0.5 Conclusion a variety of resistor combinations is pivotal in helping designers fulfill their design requirements. 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