Using A Step Down Converter Instead of A Linear Voltage Regulator

Linear voltage regulators are widely used in electrical applications to regulate the voltage and maintain a fixed output. The regulating device is made to act like a variable resistor and it continuously adjusts a voltage divider network.

The load current passing through the resistor network causes heat dissipation on the device. For instance, let we have a +12V dc power supply and we want to obtain +5V dc to feed our load that tends to draw 0.5A under 5V. From the series 78, the LM7805 is suitable for this purpose. When we set up the circuit in Figure-1, we can see that the circuit draws a little more than 0.5A from the power supply that results in approximately 12V x 0.5A = 6W power delivery. But our load works with 5V and it draws 0.5A that results in 5V x 0.5A = 2.5W significant power usage. The delivered power and the used power difference is about 6W – 2.5W = 3.5W which is dissipated as heat on the regulator device.

The heat on the regulator will cause malfunction when the heat generation exceeds the maximum value that the package of the device can dissipate. So we have two options to prevent this. One of them is reducing the input, and the second is connecting a heatsink to the device. Sometimes doing those may not be possible. Then we can use “Step Down Voltage Regulators” which are much more efficient (75% - 98%) than the linear voltage regulators.

A step down converter (buck converter) is a device that chops the input voltage by making switching action and then it stores the output waveform on a capacitive or inductive device. By this way, a lower voltage level is obtained from the output. Since the efficiency is high and the heat dissipation on the device is low there is no need to use a heatsink in most cases.

Step down voltage regulators generally require external components such as feedback resistors, inductors and capacitors. In our application we will use LM2575-ADJ (Figure-2) which can deliver up to 1A with excellent line and load regulation, thermal shutdown and current limit protection.

LM2575 requires a minimum number of external components. There are fixed output versions such as  LM2575 - 3.3V, 5V, 12V, and 15V. The external components are an input capacitor, an output capacitor, an inductor, and a catch diode. The adjustable version additionally requires two feedback resistors. The basic circuit structure of the adjustable version can be seen from Figure-3.

The output voltage can be calculated by using the equation below;

Vout = Vref ( 1 + R2 / R1) , Vref = 1.25V , R1 = between 1k and 5k

R2 can be practically found by the equation,

R2 = R1 ( Vout / Vref  - 1)

In the fixed output configuration, feedback resistors are placed inside the device. Figure-4 shows the schematic for the fixed output configuration.

For a more accurate inductor rating, an input versus load current plot is provided in the datasheet. You can check out the regions on the graph for the most suitable inductance.

If we return to the example that we give at the beginning, by using a LM2575 – 5.0 (or adjustable version with proper R1 and R2), we can much more efficiently convert +12V to +5V with no heatsink requirement. There will be very low heat generation and this won’t bother us.

Do you use step down converters? Please share your experience with us.