What is LM7805 voltage regulator?
LM7805 is a commonly used linear voltage regulator which can regulate the output voltage to a fixed voltage of 5V. Though it is not the most efficient voltage regulator, it is an easily available IC. Suppose we have a 12V supply and we require a microcontroller to be powered at 5V, then a very quick and low-cost solution is LM7805.
LM7805 IC features :
- Output Current up to 1.5A.
- Thermal-Overload Protection.
- Short-Circuit Current Limiting.
- A thermal dissipative pad.
Power and Heat dissipation in LM7805
$$P_{heat}=(V_{in}-V_{out})I_{load}$$
If the input voltage is 12V and the output voltage is 5V, and the current drawn is 1A, then the power dissipation would be:
$$P_{heat}=(12-5)1=7\,\text{W}$$
This voltage regulator can accept an input voltage of 36 volts; it is recommended to limit the voltage to 2-3 volts higher than the output regulated voltage. If the dropout voltage is too high, as mentioned in the above calculation, the regulator will dissipate more power, heating it up and reducing the efficiency.
LM7805 pin configuration
LM7805 has a three-pins:
- Pin – 1: Input pin for connecting to supply. (> 7V)
- Pin – 2: Ground pin.
- Pin – 3: Output pin which has regulated 5V output.
LM7805 voltage regulator circuit
The connections are simple in case of fixed output LM7805 as shown in above figure. The input capacitors are recommended when there is long supply routing from the main power source. Output capacitors improve the transient behavior (minimize overshoot and undershoot during load/line transient). The reverse biased diode protects when input is shorted as described in following section.
Reverse biased protective diode
Sometimes large capacitors at the output of the LDO are charged up to a designated output voltage and the input of the LDO is shorted to the ground. This puts the internal circuit of the LDO at risk because all the junctions inside the LDO are reversed biased and may enter avalanche breakdown.
Adjustable output voltage circuit
Internal circuit