How to Use CN3791: Examples, Pinouts, and Specs

The CN3791 is a high-efficiency step-down (buck) voltage regulator designed for low-power applications. It is capable of converting a higher input voltage to a stable, lower output voltage with high efficiency. The CN3791 features a wide input voltage range, adjustable output voltage, and built-in protection mechanisms such as overcurrent protection and thermal shutdown. These features make it ideal for use in battery-powered devices, portable electronics, and other compact electronic systems requiring efficient power regulation.

• Battery-powered devices (e.g., lithium-ion battery chargers)
• Portable electronics
• LED drivers
• Low-power embedded systems
• Industrial control systems

The CN3791 is typically available in an 8-pin SOP (Small Outline Package). Below is the pin configuration:

1. Input Voltage Supply: Connect a DC voltage source (4.5V to 28V) to the VIN pin. Ensure the input voltage is within the specified range.
2. Output Voltage Adjustment: Use a resistor divider network connected to the FB pin to set the desired output voltage. The output voltage can be calculated using the formula: [ V_{OUT} = V_{REF} \times \left(1 + \frac{R1}{R2}\right) ] where ( V_{REF} ) is typically 1.25V.
3. Inductor Selection: Choose an inductor with appropriate current rating and inductance value to ensure stable operation. A typical value is 10µH to 22µH.
4. Capacitor Selection: Use low ESR capacitors for input and output filtering. A 10µF ceramic capacitor is commonly used for both input and output.
5. Enable Pin: Connect the EN pin to a logic high voltage to enable the regulator. Pull it low to disable the regulator.
6. Thermal Considerations: Ensure proper heat dissipation by using a PCB with adequate copper area around the CN3791.

Below is a basic example of a CN3791 circuit for a 12V input and 5V output:

VIN (12V) ----+----[10µF]----+---- VIN (Pin 1)
              |              |
             [10µH]         [GND]
              |              |
             SW (Pin 3)      GND (Pin 2)
              |
             FB (Pin 4) ----[R1]----+----[R2]---- GND
                                    |
                                   VOUT (Pin 8)

The CN3791 can be used to power an Arduino UNO. Below is an example code to monitor the output voltage using the Arduino's ADC:

// CN3791 Output Voltage Monitoring with Arduino UNO
// Connect the CN3791 output to the Arduino's A0 pin for voltage monitoring.

const int voltagePin = A0; // Analog pin connected to CN3791 output
const float referenceVoltage = 5.0; // Arduino reference voltage (5V)
const float resistorRatio = 2.0; // Adjust based on your resistor divider

void setup() {
  Serial.begin(9600); // Initialize serial communication
  pinMode(voltagePin, INPUT); // Set A0 as input
}

void loop() {
  int adcValue = analogRead(voltagePin); // Read ADC value
  float voltage = (adcValue / 1023.0) * referenceVoltage * resistorRatio;
  
  // Print the measured voltage to the Serial Monitor
  Serial.print("Output Voltage: ");
  Serial.print(voltage);
  Serial.println(" V");
  
  delay(1000); // Wait for 1 second before the next reading
}

• Ensure the input voltage is always higher than the desired output voltage.
• Use proper decoupling capacitors to minimize noise and ensure stable operation.
• Avoid exceeding the maximum current rating (3A) to prevent damage to the component.
• Place the inductor and capacitors as close as possible to the CN3791 to reduce EMI.

1. No Output Voltage:

   • Check if the EN pin is connected to a logic high voltage.
   • Verify the input voltage is within the specified range (4.5V to 28V).
   • Inspect the feedback resistor network for proper connections.

2. Output Voltage is Unstable:

   • Ensure the inductor and capacitors have appropriate values.
   • Check for loose or poor solder connections.
   • Verify the compensation network (COMP pin) is correctly configured.

3. Overheating:

   • Ensure proper heat dissipation by using a PCB with sufficient copper area.
   • Verify the load current does not exceed the maximum rating (3A).

4. Low Efficiency:

   • Use low ESR capacitors for input and output filtering.
   • Ensure the inductor has a low DC resistance (DCR).

Q: Can the CN3791 be used for charging batteries?
A: Yes, the CN3791 is suitable for charging batteries, especially lithium-ion batteries, due to its adjustable output voltage and current capabilities.

Q: What is the maximum output voltage of the CN3791?
A: The maximum output voltage is adjustable up to 25V, depending on the input voltage and feedback resistor configuration.

Q: Can the CN3791 operate without a load?
A: Yes, the CN3791 can operate without a load, but it is recommended to have a small minimum load for stable operation.

Q: How do I calculate the feedback resistors for a specific output voltage?
A: Use the formula ( V_{OUT} = V_{REF} \times \left(1 + \frac{R1}{R2}\right) ), where ( V_{REF} ) is typically 1.25V. Select ( R1 ) and ( R2 ) accordingly.