Simple LM2678 Based 5V DC-DC Buck Converter Circuit

A simple circuit using the IC LM2678 makes a 5V DC-DC buck converter, which is a type of DC-DC converter that lowers a higher voltage to a lower one with a switching regulator.

The IC LM2678 is known for being very efficient as a step-down (buck) switching regulator and can deliver an output current of up to 5A.

This chip has an efficiency of over 90% and performs well in both load and line regulation.

The LM2678 IC comes in three fixed output voltage choices 3.3V, 5V and 12V and also has an adjustable output version.

It includes useful features like thermal shutdown, current limiting and ON/OFF control.

You can power this circuit with a DC voltage supply between 8V and 40V.

This article explores how the circuit works, the design calculations and how to put together a 5V buck regulator circuit using the LM2678.

Circuit Working:

Parts List:

The circuit uses an IC LM2678 step-down switching regulator chip to change an input voltage between 8V and 40V into a steady output of 5V.

This regulator works in a switching mode making it much more efficient than regular linear regulators.

The circuit takes a direct current DC input voltage that ranges from 8V to 40V.

To reduce noise and voltage spikes three capacitors, C1, C2 and C3 are used to filter the input voltage.

The LM2678 IC manages the voltage and switches the current through the inductor L1 at a high frequency.

For internal frequency stabilization a capacitor C4 is used from pin 2 of the IC1.

The inductor L1 stores energy while the switch is ON and releases it when the switch is OFF keeping the current flowing continuously.

Pin 7 acts as the ON/OFF switch and the regulator will turn off if this pin is linked to the ground.

The Schottky diode D1 provides a path for the current when the switch is OFF helping to reduce energy losses.

Capacitors C6 and C7 are used to smooth the output voltage ensuring a stable 5V DC is produced.

Formulas with Calculations:

The formulas and calculations for Simple LM2678 Based 5V DC-DC Buck Converter Circuit is mentioned below:

1. Inductor selection l is calculated using the formula:

l = (Vout * (Vin – Vout)) / (f * Iout * Vin * k)

where,

• Vout is the 5V output voltage
• Vin is the 12V assuming input voltage
• Iout is the 5a output current
• f is a 260khz switching frequency of LM2678
• k is a 0.3 constant factor for current ripple

substituting values:

l = (5 * (12 – 5)) / (260000 * 5 * 12 * 0.3)

l = 22uh

2. Output capacitor selection is calculated using the formula:

Cout = (Iout * k) / (f * Vripple)

where,

• Iout is 5a
• k is 0.3
• f is 260khz
• Vripple is 50mv

substituting values

Cout = (5 * 0.3) / (260000 * 0.05)

Cout = 115uf

to ensure better performance two 180uf capacitors are used in parallel

How to build:

To build a Simple LM2678 Based 5V DC-DC Buck Converter Circuit follow the below steps:

• Assemble all the components as mentioned in the above circuit diagram
• Connect pin 1 of IC1 LM2678 to output voltage of 5V DC
• Connect pin 2 of IC1 to input voltage of 8 to 40V DC
• Connect pin 3 of IC1 to one terminal of capacitor C5 and other end terminal of C5 connect to pin 1 of IC1
• Connect pin 4 of IC1 to GND
• Connect pin 6 of IC1 to the junction of L1 coil and positive of capacitor C6
• Pin 7 acts as the ON/OFF switch.
• Connect positive of capacitors C1 to C3 in parallel from pin 2 of IC1 and negative of capacitors connect to GND.
• Connect capacitor C4 from pin 2 of IC1 and GND
• Connect diode D1 cathode from junction of pin 1 and pin 3 of IC1 and anode of D1 connect to GND
• Connect positive of capacitor C6 and C7 in parallel from junction of L1 coil and output voltage of 5V DC and negative of capacitors connect to GND
• Connect inductor coil L1 between cathode of diode D1 and positive of capacitor C6

Conclusion:

Simple LM2678 Based 5V DC-DC Buck Converter Circuit is a powerful step-down regulator that delivers 5A of output current while maintaining high efficiency.

Its switching design helps reduce heat loss which makes it ideal for uses like powering microcontrollers devices that run on batteries and industrial electronics.

By choosing the right parts like the inductor, capacitors and diode the circuit can be fine-tuned for improved performance and stability.

References:

LM2678 SIMPLE SWITCHER® High Efficiency 5-A Step-Down Voltage Regulator