A Switching Regulator Circuit that uses IC uA78S40 is often called a buck converter is a special kind of DC-DC switching regulator.
It helps to change a higher input voltage into a lower output voltage in a smart and efficient way.
The uA78S40 is a handy switching regulator chip that can help create effective power supplies.
This chip has everything built in like a voltage reference that adjusts with temperature, an oscillator that controls the duty cycle, a strong switching element, a separate operational amplifier and its own diode.
You can use an input voltage of 25V DC and it will give you an output voltage of 5V at 500mA.
In this article we will explore how the step-down converter circuit works using the uA78S40 IC.
Circuit Working:
Parts List:
| Component Type | Value/Part Number | Quantity |
|---|---|---|
| Resistors | 1.2k 1/4 watt | 1 |
| 3.6k 1/4 watt | 1 | |
| 0.33Ω 1/4 watt | 1 | |
| Capacitors | Ceramic 470pF | 1 |
| Electrolytic 470µF 25V | 1 | |
| Electrolytic 100µF 25V | 1 | |
| Semiconductors | IC uA78S40 | 1 |
| Schottky Diode 1N5822 | 1 | |
| Inductor | 220µH | 1 |
This circuit takes a 25V DC input and changes it into a steady 5V DC output that can provide 500mA.
The main part of the circuit is the uA78S40 IC, which is a special kind of regulator that can step down, step up, or invert voltage.
The 25V input is smoothed out by a capacitor called C3 to keep everything running smoothly and to stop any sudden changes in voltage.
The uA78S40 IC keeps the voltage steady by switching on and off really fast with the timing managed by a capacitor C1.
A resistor R3 helps keep track of the current making sure the output stays just right.
The inductor L1 saves energy when the switch is on and releases it when the switch is off so power keeps flowing.
The Schottky diode D1 changes the current direction and capacitor C2 helps smooth out the output to a nice 5V DC.
A pair of resistors R1 and R2 work together to set the output voltage by sending feedback to the IC.
Formulas with Calculations:
Below are the formulas with calculations for Switching Regulator Circuit using IC uA78S40:
Output Voltage Calculation:
The output voltage (Vout) is determined using the voltage divider resistors R1 and R2:
Vout = Vref × (1 + (R2 / R1))
For IC uA78S40 the reference voltage Vref is 1.25V.
where,
- R1 is 1.2kΩ
- R2 is 3.6kΩ
Vout = 1.25V × (1 + (3.6kΩ / 1.2kΩ))
Vout = 1.25V × (1 + 3)
Vout = 1.25V × 4
Vout = 5V
Inductor Selection Formula:
The required inductor value is calculated using:
L = ((Vin – Vout) × D) / (Iout × f)
where,
- Vin is 25V input voltage
- Vout is 5V output voltage
- D is the duty cycle
- Iout is 500mA = 0.5A load current
- f is the switching frequency assumed 40kHz for this IC
The duty cycle D is given by:
D = Vout / Vin
D = 5V / 25V
D = 0.2
Now calculating the inductor:
L = ((25V – 5V) × 0.2) / (0.5A × 40000Hz)
L = (20V × 0.2) / 20000
L = 4 / 20000
L = 200µH
Thus, the inductor 220µH used in circuit diagram is a standard available value.
Current Sense Resistor R3 Calculation:
The current sense resistor R3 Rsc is calculated using:
Rsc = Vsense / Ilimit
For IC uA78S40 the sense voltage Vsense is 0.165V.
The current limit is 500mA (0.5A):
Rsc = 0.165V / 0.5A
Rsc = 0.33Ω
Thus, the selected current sense resistor R3 is 0.33Ω.
How to Build:
To build a Switching Regulator Circuit using IC uA78S40 follow the below steps for connections:
- Gather all the components as mentioned in the above circuit diagram
- Connect pin 1 of IC1 uA78S40 to cathode of diode D1 and anode of D1 to GND
- Connect pin 2 of IC1 to GND.
- Connect pin 3 of IC1 to one end of inductor coil L1 and other end of coil L1 connect to Vout of 5V.
- Connect pin 8 of IC1 to pin 9 of IC1
- Connect pin 10 of IC1 to the junctions of resistors R1 and R2 and GND.
- Connect pin 11 of IC1 to GND.
- Connect pin 12 of IC to one end of capacitor C1 and other end of capacitor C1 connect to GND.
- Connect pin 13 of IC1 to Vin of 25V
- Connect resistor R3 between pin 13 and pin 14, 15, and pin 16 of IC1
- Connect a positive of capacitor C3 to +25V DC and negative of C3 connect to GND
- Connect a positive of capacitor C2 from Vout and negative of C2 to GND.
Conclusion:
A Switching Regulator Circuit using IC uA78S40 is a smart way to change a big DC voltage into a steady, lower DC voltage.
This design is perfect for small power needs that need a dependable 5V supply.
By using a switching regulator it keeps power loss low and works much better than regular linear regulators.