Introduction to the UC3843S

The UC3843S is a popular fixed-frequency current-mode PWM controller integrated circuit (IC) used in power supply applications. Manufactured by Texas Instruments and other semiconductor companies, the UC3843S provides essential features for designing efficient and reliable switched-mode power supplies (SMPS). This article will explore the pin configuration, key features, and typical applications of the UC3843S controller.

UC3843S Pin Configuration

The UC3843S comes in a 14-pin SOIC or 8-pin PDIP package. Here is a table showing the pin configuration for the 14-pin SOIC package:

Pin No.	Pin Name	Description
1	COMP	Compensation pin for error amplifier
2	FB	Feedback pin for error amplifier
3	CS	Current sense input
4	RC	Oscillator timing resistor pin
5	GND	Ground
6	OUT	Output pin to drive external MOSFET
7	VCC	Supply voltage input
8	REF	5V reference voltage output
9	PGND	Power ground
10	ISENSE	Current sense input for cycle-by-cycle current limiting
11	RAMP	Oscillator ramp waveform output
12	SS	Soft-start pin
13	SYNC	Synchronization input for multiple ICs
14	VFB	Voltage feedback input

Key Features of the UC3843S

The UC3843S offers several key features that make it a versatile choice for designing SMPS:

1. Fixed-Frequency Current-Mode Operation: The UC3843S operates in current-mode control, which provides inherent cycle-by-cycle current limiting and improved transient response compared to voltage-mode control. The controller maintains a fixed switching frequency, simplifying the design of magnetic components and input/output filters.

2. Under-Voltage Lockout (UVLO): The UC3843S has a built-in UVLO circuit that disables the output when the supply voltage falls below a preset threshold, typically around 8.5V. This feature ensures proper operation of the controller and prevents damage to the power supply components during low-voltage conditions.

3. Soft-Start Capability: The soft-start feature allows for a gradual increase in the output voltage during power-up, reducing inrush current and minimizing stress on the power supply components. The soft-start time can be programmed using an external capacitor connected to the SS pin.

4. Programmable Oscillator Frequency: The switching frequency of the UC3843S can be programmed using an external resistor connected to the RC pin. The oscillator frequency can be set between 20kHz and 500kHz, allowing for design flexibility and optimization of the power supply performance.

5. Cycle-by-Cycle Current Limiting: The UC3843S includes a cycle-by-cycle current limiting feature that protects the power supply from overload and short-circuit conditions. The current sense input (ISENSE) monitors the peak current through the power switch and terminates the output pulse if the current exceeds a predetermined limit.

6. Synchronization Capability: Multiple UC3843S controllers can be synchronized to operate at the same switching frequency using the SYNC pin. This feature is useful in multi-phase power supply designs, where interleaved operation can reduce input and output ripple currents and improve overall efficiency.

7. Voltage Reference: The UC3843S provides a 5V±1% voltage reference (REF) that can be used for various purposes, such as setting the output voltage or providing a stable reference for external circuitry.

Applications of the UC3843S

The UC3843S is widely used in a variety of SMPS applications, including:

1. AC-DC Power Supplies: The UC3843S is commonly used in offline AC-DC power supplies, such as those found in consumer electronics, home appliances, and industrial equipment. The controller’s current-mode control and fixed-frequency operation make it well-suited for flyback, forward, and push-pull converter topologies.

2. DC-DC Converters: The UC3843S can be used in isolated and non-isolated DC-DC converter applications, such as buck, boost, and buck-boost converters. These converters are used in battery-powered devices, automotive electronics, and distributed power systems.

3. LED Drivers: The UC3843S is popular in LED driver applications, where constant current control is required to maintain consistent brightness and color temperature. The controller’s cycle-by-cycle current limiting and programmable oscillator frequency make it easy to design efficient and reliable LED drivers.

4. Chargers: The UC3843S is used in various charging applications, such as battery chargers for portable devices, electric vehicles, and UPS systems. The controller’s soft-start and current limiting features help protect the batteries and charging circuitry from damage.

Typical Application Circuit

Here is a typical application circuit for the UC3843S in a flyback converter:

In this circuit, the UC3843S controls the switching of the primary-side MOSFET (Q1) based on the feedback from the optocoupler (U2) and current sense resistor (Rs). The transformer (T1) provides isolation and voltage scaling, while the secondary-side diode (D1) and capacitor (C4) form the output rectifier and filter. The resistors R1 and R2 set the output voltage, and the compensation network (R3, C2, C3) stabilizes the feedback loop.

UC3843S vs. Other PWM Controllers

The UC3843S is one of many PWM controllers available in the market. Here is a comparison of the UC3843S with some other popular controllers:

Controller	Control Mode	Max. Frequency	Packages	Key Features
UC3843S	Current	500kHz	SOIC-14, PDIP-8	UVLO, soft-start, sync
UC3844	Current	500kHz	SOIC-14, PDIP-8	UVLO, soft-start, sync, frequency foldback
UC3845	Current	500kHz	SOIC-16, PDIP-16	UVLO, soft-start, sync, overcurrent protection
TL494	Voltage	300kHz	SOIC-16, PDIP-16	Dual output, deadtime control, soft-start
SG3525	Voltage	400kHz	SOIC-16, PDIP-16	Dual output, deadtime control, soft-start, shutdown

While these controllers share some common features, the UC3843S stands out for its current-mode control, fixed-frequency operation, and synchronization capability, making it a popular choice for many SMPS applications.

Frequently Asked Questions (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 power supply designs?
   A: Yes, the UC3843S is suitable for both isolated and non-isolated power supply topologies, such as flyback, forward, buck, and boost converters.

3. Q: How does the soft-start feature work in the UC3843S?
   A: The soft-start feature gradually increases the duty cycle of the UC3843S output during power-up, limiting the inrush current and reducing stress on the power supply components. The soft-start time is determined by an external capacitor connected to the SS pin.

4. Q: What is the purpose of the SYNC pin in the UC3843S?
   A: The SYNC pin allows multiple UC3843S controllers to be synchronized to operate at the same switching frequency. This is useful in multi-phase power supply designs, where interleaved operation can reduce input and output ripple currents and improve overall efficiency.

5. Q: What is the advantage of current-mode control in the UC3843S compared to voltage-mode control?
   A: Current-mode control, as used in the UC3843S, provides inherent cycle-by-cycle current limiting and improved transient response compared to voltage-mode control. This makes the UC3843S well-suited for applications that require fast load transient response and robust protection against overload and short-circuit conditions.

Conclusion

The UC3843S is a versatile and widely-used PWM controller that offers a range of features essential for designing efficient and reliable switched-mode power supplies. Its fixed-frequency current-mode control, under-voltage lockout, soft-start capability, and programmable oscillator frequency make it a popular choice for various applications, including AC-DC power supplies, DC-DC converters, LED drivers, and chargers.

By understanding the pin configuration, key features, and typical applications of the UC3843S, designers can leverage its capabilities to create optimized power supply solutions that meet the specific requirements of their projects. With its robust protection features and synchronization capability, the UC3843S is well-equipped to handle the demands of modern power supply designs.