Typical Application Circuit of 7805 Regulator
The typical application circuit of the 7805 voltage regulator shows in the figure. Vi is connected to the rectification and filtering circuit, and V0 outputs +5V voltage. At the input and output terminals of the voltage regulator, C1=0.33uF and C2=0.1uF capacitors should be connected. The purpose of this is to improve the operational stability of the voltage regulator and improve the transient response.
C1 is to reduce ripple, offset the inductance effect when the input terminal wiring is long, prevent self-excitation, and suppress high-frequency interference. When designing, pay attention to keeping the filter capacitor and C1 capacitor as close as possible to the voltage regulator, which can effectively prevent transient overvoltage. C2 is used to improve the quick response of the load and suppress high frequency. The C2 capacitor should use ceramic or tantalum capacitors with good frequency characteristics. In addition, to further reduce the output voltage ripple, a capacitor C3 of 470uF is generally connected in parallel at the output end of the voltage regulator.
The above circuit should pay attention to the following items:
1) Even the lowest value in the input ripple voltage must be more than 2V higher than the output voltage.
2) When the voltage regulator is far from the power filter, the circuit must use C1.
Heat Dissipation Problem of 7805 Regulator and Selection of Radiator
Currently, the CXT105A power board uses 7805 to provide a +5V power supply. A load of +5V voltage includes the main control chip, driver chip, and display circuit, among which the display circuit has a large operating current and is the main load of +5V. The display methods of the display circuit include LCD, LED, multi-color screen, etc., and the load current varies significantly with different display devices. Such as LCD backlight and large-screen colorful screen, the working current is more than 200mA.
Essential characteristics of the 7805 voltage regulator: heat dissipation conditions will affect the size of the output load current. To obtain the larger current output to ensure that the 7805 voltage regulator has good heat dissipation conditions. When the output current is large, it should be equipped with a heat sink to ensure thermal stability and reliability.
Because the 7805 voltage regulator delivers power to the load, it also consumes part of the power. The energy consumed is directly reflected in the device’s junction temperature rise. When the junction temperature rises to a certain level (7805 is about 150°C), it will enter the overheating protection state, stop the output, cause the electronic control to shut down and affect its performance. The allowable power consumption of the device is closely related to the heat dissipation of the voltage regulator. If appropriate measures are taken, it is possible to exert the potential of the voltage regulator and increase the output power. On the contrary, it may cause the junction temperature to rise and be damaged.
We recommend using heat sinks with the following specifications for 7805 regulators: 15*10-20 and 23*16-25.
 |  |
| 15*10-20 | 23*16-25 |
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