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MC7808ABD2T Datasheet(PDF) 21 Page - ON Semiconductor |
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MC7808ABD2T Datasheet(HTML) 21 Page - ON Semiconductor |
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21 / 34 page  MC7800, MC7800A, MC7800AE, NCV7800 http://onsemi.com 21 APPLICATIONS INFORMATION Design Considerations The MC7800 Series of fixed voltage regulators are designed with Thermal Overload Protection that shuts down the circuit when subjected to an excessive power overload condition, Internal Short Circuit Protection that limits the maximum current the circuit will pass, and Output Transistor Safe−Area Compensation that reduces the output short circuit current as the voltage across the pass transistor is increased. In many low current applications, compensation capacitors are not required. However, it is recommended that the regulator input be bypassed with a capacitor if the regulator is connected to the power supply filter with long wire lengths, or if the output load capacitance is large. An input bypass capacitor should be selected to provide good high−frequency characteristics to insure stable operation under all load conditions. A 0.33 mF or larger tantalum, mylar, or other capacitor having low internal impedance at high frequencies should be chosen. The bypass capacitor should be mounted with the shortest possible leads directly across the regulators input terminals. Normally good construction techniques should be used to minimize ground loops and lead resistance drops since the regulator has no external sense lead. I O + 5.0 V R ) I B Figure 9. Current Regulator Figure 10. Adjustable Output Regulator Figure 11. Current Boost Regulator Figure 12. Short Circuit Protection The MC7800 regulators can also be used as a current source when connected as above. In order to minimize dissipation the MC7805C is chosen in this application. Resistor R determines the current as follows: For example, a 1.0 A current source would require R to be a 5.0 W, 10 W resistor and the output voltage compliance would be the input voltage less 7.0 V. IB ^ 3.2 mA over line and load changes. Input 0.33 mF R IO MC7805 Constant Current to Grounded Load The addition of an operational amplifier allows adjustment to higher or intermediate values while retaining regulation characteristics. The minimum voltage obtainable with this arrangement is 2.0 V greater than the regulator voltage. Input MC7805 Output 0.33 mF 10 k MC1741G 7 6 4 1.0 k VO = 7.0 V to 20 V VIN = VO ≥ 2.0 V 0.1 mF 3 2 The MC7800 series can be current boosted with a PNP transistor. The MJ2955 provides current to 5.0 A. Resistor R in conjunction with the VBE of the PNP determines when the pass transistor begins conducting; this circuit is not short circuit proof. Input/output differential voltage minimum is increased by VBE of the pass transistor. XX = 2 digits of type number indicating voltage. MC78XX Input Output R 1.0 mF MJ2955 or Equiv. 1.0 mF The circuit of Figure 11 can be modified to provide supply protection against short circuits by adding a short circuit sense resistor, RSC, and an additional PNP transistor. The current sensing PNP must be able to handle the short circuit current of the three-terminal regulator. Therefore, a four-ampere plastic power transistor is specified. XX = 2 digits of type number indicating voltage. 1.0 mF MC78XX MJ2955 or Equiv. Output RSC R 2N6049 or Equiv. ≥ 10 mF RSource 0.33 mF Input RSource 0.33 mF ≥ 10 mF |
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