Uc3843s: Pin Configuration, Features, and Applications

Introduction to UC3843S

The UC3843S, belonging to the UC384X family of controller ICs, is designed for use in SMPS applications. It integrates a high-gain error amplifier, fixed-frequency oscillator, current-sense comparator, and high-current output driver into an 8-pin package. The IC operates over a wide frequency range and can be used in isolated or non-isolated topologies.

UC3843S Pin Configuration

To effectively utilize the UC3843S, it is essential to understand its pin configuration. The following table provides a detailed overview of each pin:

Pin No.	Pin Name	Description
1	COMP	Output of the error amplifier and input to the PWM comparator
2	FB	Inverting input of the error amplifier
3	CS	Current sense input to the PWM comparator
4	RC	Oscillator timing resistor input
5	GND	Ground
6	OUT	Output of the PWM latch and input to the output driver
7	VCC	Power supply voltage input
8	REF	Internal 5V reference voltage output

1. COMP (Pin 1)

The COMP pin is the output of the error amplifier and the input to the PWM comparator. It is used to compensate the feedback loop by connecting an external RC network between this pin and ground. The compensation network helps to stabilize the feedback loop and optimize the transient response of the power supply.

2. FB (Pin 2)

The FB pin is the inverting input of the error amplifier. It is used to sense the output voltage of the power supply through a feedback network. The feedback voltage is compared with the internal reference voltage to generate an error signal, which is then amplified by the error amplifier and fed to the PWM comparator.

3. CS (Pin 3)

The CS pin is the current sense input to the PWM comparator. It is used to sense the primary current of the power transformer through a current sense resistor. When the voltage across the current sense resistor exceeds the threshold voltage set by the error amplifier output, the PWM latch is reset, turning off the output driver.

4. RC (Pin 4)

The RC pin is the oscillator timing resistor input. It is used to set the switching frequency of the PWM controller by connecting an external timing resistor between this pin and ground. The switching frequency can be calculated using the following formula:

f = 1.72 / (R × C)

where f is the switching frequency, R is the timing resistor value, and C is the timing capacitor value (typically 1nF).

5. GND (Pin 5)

The GND pin is the ground reference for the IC. It should be connected to a solid ground plane on the PCB.

6. OUT (Pin 6)

The OUT pin is the output of the PWM latch and the input to the output driver. It provides a high-current drive signal to the gate of an external MOSFET or the base of a bipolar transistor. The maximum output current capability of the OUT pin is 1A peak and 200mA average.

7. VCC (Pin 7)

The VCC pin is the power supply voltage input to the IC. It should be bypassed with a low ESR capacitor to ground, as close to the IC as possible. The UC3843S can operate with a supply voltage range of 8V to 35V.

8. REF (Pin 8)

The REF pin is the internal 5V reference voltage output. It can be used to provide a stable reference voltage for the feedback network and other external circuitry. The REF pin can supply up to 50mA of current and should be bypassed with a ceramic capacitor to ground.

Key Features of UC3843S

The UC3843S offers several key features that make it a popular choice for SMPS applications:

1. Fixed-frequency operation (up to 500kHz)
2. Current-mode control for improved transient response and simplified loop compensation
3. Built-in current limiting and cycle-by-cycle overcurrent protection
4. Under-voltage lockout (UVLO) with hysteresis
5. Latching PWM comparator for improved noise immunity
6. High-current totem-pole output driver (1A peak, 200mA average)
7. Wide supply voltage range (8V to 35V)
8. Low startup current (<1mA)

These features enable designers to create efficient, reliable, and compact power supplies with fewer external components.

Applications of UC3843S

The UC3843S is widely used in various SMPS applications, including:

1. Off-line power supplies
2. DC-DC converters
3. Battery chargers
4. LED drivers
5. Automotive electronics
6. Industrial control systems

The IC’s versatility and robustness make it suitable for a wide range of power levels and output voltages.

Off-line Power Supplies

The UC3843S is commonly used in off-line power supplies, such as flyback and forward converters. These topologies are popular for low to medium power applications, ranging from a few watts to several hundred watts. The UC3843S’s current-mode control and built-in protection features simplify the design and improve the performance of these power supplies.

DC-DC Converters

In DC-DC converter applications, the UC3843S can be used in both isolated and non-isolated topologies. Isolated topologies, such as flyback and forward converters, provide galvanic isolation between the input and output, which is essential for safety-critical applications. Non-isolated topologies, such as buck, boost, and buck-boost converters, are used when isolation is not required and offer higher efficiency and lower cost.

Battery Chargers

The UC3843S is well-suited for battery charger applications, thanks to its current-mode control and built-in current limiting. By sensing the charging current and regulating the PWM duty cycle, the IC can implement constant-current (CC) and constant-voltage (CV) charging profiles. This ensures safe and efficient charging of various battery types, such as lead-acid, NiCd, NiMH, and Li-ion.

LED Drivers

The UC3843S can be used to design efficient and compact LED drivers for lighting applications. The IC’s current-mode control enables precise regulation of the LED current, ensuring consistent brightness and color temperature. The built-in overcurrent protection helps to prevent damage to the LEDs and extends their lifespan.

Automotive Electronics

In automotive applications, the UC3843S’s wide supply voltage range and robust design make it suitable for use in harsh environments. The IC can be used in various automotive power supply applications, such as headlight drivers, instrument cluster displays, and infotainment systems.

Industrial Control Systems

The UC3843S finds applications in industrial control systems, where reliable and efficient power supplies are crucial. The IC’s built-in protection features and wide operating temperature range (-40°C to +125°C) ensure reliable operation in demanding industrial environments.

Designing with UC3843S

When designing a power supply using the UC3843S, several key considerations should be taken into account:

1. Selection of the appropriate power transformer and switching frequency
2. Design of the feedback network and compensation circuit
3. Selection of the current sense resistor and overcurrent protection threshold
4. Design of the output filter and snubber circuits
5. PCB layout and grounding techniques

By carefully addressing these aspects, designers can optimize the performance, efficiency, and reliability of their power supplies.

UC3843S vs. Other PWM Controllers

The UC3843S is one of many PWM controller ICs available in the market. Some popular alternatives include:

• TL494: A general-purpose PWM controller with a wider feature set, including adjustable deadtime control and pulse-by-pulse current limiting.
• UC3825: A high-speed PWM controller designed for use in resonant mode power supplies and high-frequency applications (up to 1MHz).
• MC34063: A low-cost, monolithic DC-DC converter IC with built-in switch and adjustable output voltage.

When choosing a PWM controller, designers should consider factors such as the specific application requirements, power level, switching frequency, and cost.

Conclusion

The UC3843S is a versatile and reliable PWM controller IC that simplifies the design of switch mode power supplies. Its current-mode control, built-in protection features, and wide operating range make it suitable for a variety of applications, from off-line power supplies to automotive electronics. By understanding the Uc3843s Configuration, its features, and design considerations, engineers can create efficient and robust power solutions for their projects.

FAQ

1. Q: What is the maximum switching frequency of the UC3843S?
   A: The UC3843S can operate at switching frequencies up to 500kHz.

2. Q: Can the UC3843S be used in both isolated and non-isolated topologies?
   A: Yes, the UC3843S is suitable for use in both isolated topologies, such as flyback and forward converters, and non-isolated topologies, such as buck, boost, and buck-boost converters.

3. Q: What is the purpose of the current sense (CS) pin in the UC3843S?
   A: The CS pin is used to sense the primary current of the power transformer through a current sense resistor. It is connected to the PWM comparator to provide cycle-by-cycle current limiting and overcurrent protection.

4. Q: How do I set the switching frequency of the UC3843S?
   A: The switching frequency of the UC3843S is set by connecting an external timing resistor between the RC pin and ground. The frequency can be calculated using the formula: f = 1.72 / (R × C), where R is the timing resistor value and C is the timing capacitor value (typically 1nF).

5. Q: What is the maximum output current capability of the UC3843S’s OUT pin?
   A: The maximum output current capability of the OUT pin is 1A peak and 200mA average. This is sufficient to drive the gate of an external MOSFET or the base of a bipolar transistor in most SMPS applications.