Summary of BUCK CONVERTER PCB DESIGN REPLACES TO-220 REGULATORS
Linear regulators maintain a steady, lower output voltage by dissipating the excess input voltage as heat. Fixed-output 78xx devices need no external setting resistors, while adjustable LDOs like LM1117 use external resistors and have lower dropout (~1.25 V). Internally, regulators use a high-gain op-amp, feedback resistor divider, and a pass transistor to force the sensed output to match the reference. Because input current equals output current and Vin>Vout, excess power becomes heat, often requiring heatsinks; buck converters are more efficient alternatives.
Parts used in the Linear Voltage Regulator Project:
- 78xx series linear voltage regulator IC
- LM1117 low-dropout (LDO) regulator IC
- High-gain operational amplifier (internal to regulator IC)
- Pass transistor (internal to regulator IC)
- Feedback resistor divider network (internal for 78xx, external resistors for LM1117)
- Reference voltage source (internal to regulator IC)
- Input voltage supply (Vin)
- Heatsink (when required for heat dissipation)
- Buck converter (as an alternative)
Linear voltage regulators are used to maintaining a steady output voltage. They are step-down converters where the input voltage is always higher than the output voltage. The output voltage can be adjusted by external resistors. 78xx series is a very popular linear voltage regulator series. 78xx series, however, do not require external components for setting the output voltage.
The regulators have some dropout voltage which can be defined as the input voltage minus the desired output voltage. It should be noted that if the desired output voltage is higher than Vin-Vdropout, then the linear regulator can’t regulate the output at the desired output voltage. In LM1117, the dropout voltage is around 1.25V which is very small compared to 78xx and so it is called a Low-Dropout (LDO) regulator.
The voltage regulator ICs consists of a high-gain operational amplifier, a feedback network, and a pass transistor. The gate of the pass transistor is controlled by the output of the operational amplifier. The output voltage is sensed via a resistor divider network and is fed to the inverting pin of the OP-AMP. The sensed voltage is compared with the reference voltage and the high gain of the OP-AMP forces the reference voltage and the sensed voltage to be equal. In the 78xx series, the voltage divider ratio is fixed internally. On the other hand, in LM1117, the voltage divider ratio can be set via an external resistor. This gives the LM1117 IC flexibility in terms of output voltage.
Linear voltage regulators have a major issue. The input current required to the regulator is always the same as the output current. Additionally, the input voltage is always higher than the output voltage. Therefore, the input power is always higher than the output power. The difference is dissipated in the form of heat by the regulator. This means that the heatsink is necessary for many applications and sometimes, substantial power is wasted during regulation. To overcome these issues, buck converters can be preferred over linear regulators as they are more efficient and do not require heatsink.
Read more: BUCK CONVERTER PCB DESIGN REPLACES TO-220 REGULATORS
- What is the main function of a linear voltage regulator?
To maintain a steady output voltage by stepping down input voltage and dissipating excess power as heat. - Do 78xx series regulators require external resistors to set the output voltage?
No, the 78xx series has its voltage divider ratio fixed internally and does not require external resistors to set the output voltage. - Can the LM1117 output voltage be adjusted externally?
Yes, the LM1117 allows the output voltage to be set via external resistors. - What is dropout voltage and how does LM1117 compare to 78xx?
Dropout voltage is Vin minus the desired Vout; LM1117 has a low dropout around 1.25 V, smaller than typical 78xx devices. - Why do linear regulators often require heatsinks?
Because input current equals output current and the voltage difference is dissipated as heat, causing significant power loss that may need a heatsink. - How does the regulator internal circuitry maintain the desired output voltage?
An internal op-amp compares the sensed output via a resistor divider to a reference and drives the pass transistor to force equality between sensed voltage and reference. - Is input power always higher than output power in linear regulators?
Yes, because Vin is always higher than Vout and the same current flows in, so the excess power is dissipated as heat. - What is a preferred alternative to linear regulators for efficiency?
Buck converters are preferred for higher efficiency and reduced heat dissipation compared to linear regulators.
