UC3845BN Detailed explanation of pin function specifications and circuit principle instructions

The UC3845BN is part of the UC384x family of integrated circuits, produced by ON Semiconductor. It is typically used for controlling switching regulators and DC-DC converters, often in Power management applications. The UC3845BN is specifically designed as a current-mode controller, often used in power supply circuits.

Here's the detailed specification and pin function for the UC3845BN:

1. Package and Pin Function Overview:

UC3845BN is available in a DIP-8 (Dual Inline Package) configuration. The device has 8 pins in total. Below is the detailed explanation of each pin.

2. Pin Description:

Pin Number Pin Name Function Description 1 VCC Power Supply Pin (5V to 30V). The UC3845BN requires a supply voltage to operate. VCC connects to the positive supply rail for the chip. 2 Vref Reference Voltage Output (typically 5V). This is used for external regulation or feedback in controlling the output voltage of power supplies. 3 Current Sense (+) Current Sense Input (+). This pin is used to sense the current in the system. It is connected to the positive terminal of the current sense resistor in current-mode feedback circuits. 4 Current Sense (-) Current Sense Input (-). This pin connects to the negative terminal of the current sense resistor. It works in conjunction with pin 3 for current sensing in feedback loops. 5 Ground Ground Pin. This is the reference pin for all signals and is connected to the system's ground (0V). 6 Feedback Feedback Input. The feedback loop controls the output voltage regulation. This pin connects to the feedback network from the output voltage of the power supply. 7 Output Output Pin. This pin drives the power transistor in switching regulator applications. It provides a pulse-width modulated (PWM) output signal. 8 Shutdown Shutdown Pin. This pin is used to disable the UC3845BN. When pulled low, it disables the output and turns off the internal oscillator.

3. Package Type:

DIP-8 (Dual Inline Package with 8 pins).

4. Typical Applications:

DC-DC converters Power supplies Switch-mode power supplies (SMPS) Pulse-width modulation (PWM) controllers

5. Common Usage Circuit:

The UC3845BN is often used in Buck converters and Boost converters. Here is an example of a typical circuit for the UC3845BN:

The VCC pin is connected to the positive supply voltage. The Ground pin connects to the system's ground. Current sense pins (3 & 4) are connected to a current-sensing resistor in the feedback loop. The Feedback pin connects to a voltage divider or other regulation mechanism to set the desired output voltage. The Output pin connects to the base or gate of a switching transistor (e.g., MOSFET) to control the power conversion. The Vref pin might be used for feedback regulation or precision voltage reference applications.

6. FAQ - Frequently Asked Questions:

Q: What is the operating voltage range for the UC3845BN? A: The UC3845BN operates within a voltage range of 5V to 30V on the VCC pin. Q: Can I use the UC3845BN in a boost converter design? A: Yes, the UC3845BN is commonly used in boost converters and other switching regulator topologies. Q: What is the function of the feedback pin (Pin 6)? A: The feedback pin is used to sense the output voltage and provide feedback for regulation purposes in switching power supplies. Q: How does the UC3845BN sense current in a system? A: The UC3845BN uses the current sense pins (Pin 3 & Pin 4) to monitor the current through a sense resistor in the power supply circuit. Q: What happens if the shutdown pin is pulled low? A: If the Shutdown pin (Pin 8) is pulled low, the UC3845BN is disabled, and the output is turned off. Q: Can I use the UC3845BN to drive a MOSFET directly? A: Yes, the Output pin (Pin 7) can drive a MOSFET in a switching regulator circuit, typically through a gate driver. Q: What is the reference voltage output from Pin 2? A: Pin 2 provides a reference voltage, typically 5V, which can be used for feedback regulation. Q: How do I connect the UC3845BN to a power supply circuit? A: Connect the VCC pin to the positive supply voltage, the Ground pin to the system ground, and the Output pin to the gate or base of a switch (e.g., MOSFET) to control the power conversion. Q: Can I use the UC3845BN for high-current applications? A: The UC3845BN can handle moderate current but is typically used in low to mid-power applications such as DC-DC converters and SMPS.

Q: What is the typical output frequency of the UC3845BN?

A: The frequency is internally set by an oscillator and can typically range from 100 kHz to 500 kHz, depending on the external components.

Q: What is the typical application of the UC3845BN in a power supply?

A: It is often used in PWM control for regulating the output of switching power supplies like buck and boost converters.

Q: How is the UC3845BN different from other controllers in the UC384x family?

A: The UC3845BN is optimized for current-mode control applications, while other models in the UC384x family may offer different features like voltage-mode control or integrated gate drivers.

Q: Can the UC3845BN be used in a power factor correction circuit?

A: While the UC3845BN is not specifically designed for power factor correction ( PFC ), it can be used in general switching regulator circuits that can be part of a PFC system.

Q: Does the UC3845BN include over-voltage or over-current protection?

A: The UC3845BN does not have built-in over-voltage or over-current protection. External circuitry is typically required to provide such protections.

Q: How does the UC3845BN regulate output voltage?

A: It uses the feedback signal from the output voltage, processed through an external control loop, to regulate the output voltage by adjusting the duty cycle of the switching transistor.

Q: What is the typical switching speed of the UC3845BN?

A: The switching speed of the UC3845BN is typically high, allowing it to operate at frequencies ranging from 100 kHz to 500 kHz, depending on external components.

Q: Can I use the UC3845BN for both buck and boost converters?

A: Yes, the UC3845BN is versatile and can be used in both buck (step-down) and boost (step-up) converter designs.

Q: Is the UC3845BN suitable for low-power applications?

A: The UC3845BN is more suitable for medium to high power applications but can still be used in low-power designs depending on the required output.

Q: Can the UC3845BN be used in automotive power supplies?

A: Yes, the UC3845BN can be used in automotive power supplies, but ensure the voltage range is within the operational limits and consider the automotive environment.

Q: How can I prevent the UC3845BN from malfunctioning due to voltage spikes?

A: Use decoupling capacitor s on the VCC pin to filter out voltage spikes and ensure stable operation.

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