Introduction to the UC3844
The UC3844 is a fixed-frequency, current-mode PWM controller IC manufactured by Texas Instruments. It is designed to control offline switch-mode power supplies with minimal external components, making it a popular choice for power supply designers.
Key Features of the UC3844
- Wide operating voltage range: 7.5V to 40V
- Programmable oscillator frequency up to 500kHz
- Built-in current limiting and soft-start capabilities
- Under-voltage lockout protection
- Low startup current (< 1mA)
- Totem-pole output driver
- Internally trimmed bandgap reference
UC3844 Pin Configuration and Functions
The UC3844 comes in a 14-pin DIP or SOIC package. The pin configuration and functions are as follows:
| Pin Number | Pin Name | Function |
|---|---|---|
| 1 | COMP | Error amplifier output and PWM comparator input |
| 2 | FB | Feedback voltage input |
| 3 | ISENSE | Current sense input |
| 4 | RT/CT | Oscillator timing resistor and capacitor connection |
| 5 | GND | Ground |
| 6 | OUT | Output driver |
| 7 | VCC | Positive supply voltage |
| 8 | REF | Internal 5V reference voltage output |
| 9 | VREF | Reference voltage input |
| 10 | PWRGND | Power ground |
| 11 | VC | Error amplifier output and compensation node |
| 12 | NC | No connection |
| 13 | SS | Soft-start input |
| 14 | VFB | Feedback voltage input for voltage-mode control |
Functional Block Diagram
Applications of the UC3844
The UC3844 is commonly used in various switch-mode power supply applications, such as:
- Off-line power supplies
- Flyback Converters
- Forward converters
- Boost converters
- Battery Chargers
- LED drivers
Designing with the UC3844
When designing a power supply using the UC3844, there are several key considerations to keep in mind:
Oscillator Frequency Selection
The oscillator frequency is set by the RT and CT components connected to pin 4. The frequency can be calculated using the following formula:
$f_{osc} = \frac{1.72}{R_T \times C_T}$
where $R_T$ is in ohms and $C_T$ is in farads.
Current Sensing
The UC3844 uses a current sense resistor connected to the ISENSE pin to monitor the primary current of the transformer. The value of the current sense resistor can be calculated based on the desired maximum current limit and the current sense threshold voltage (typically 1V).
Feedback Loop Compensation
Proper compensation of the feedback loop is crucial for stable operation of the power supply. The compensation network is connected to the COMP and VC pins of the UC3844. The values of the compensation components can be calculated based on the power supply’s specifications and the desired loop response.
Soft-Start
The UC3844 features a soft-start function to limit the inrush current during startup. The soft-start time is controlled by a capacitor connected to the SS pin. The soft-start capacitor value can be calculated using the following formula:
$C_{SS} = \frac{t_{SS} \times I_{SS}}{V_{SS}}$
where $t_{SS}$ is the desired soft-start time, $I_{SS}$ is the soft-start charging current (typically 10µA), and $V_{SS}$ is the soft-start threshold voltage (typically 1V).
UC3844 Protection Features
The UC3844 includes several built-in protection features to ensure safe and reliable operation of the power supply:
- Under-voltage Lockout (UVLO): Prevents the IC from operating when the supply voltage is below a certain threshold.
- Current Limiting: Limits the peak primary current to protect the power supply components from overcurrent conditions.
- Soft-Start: Gradually increases the output voltage during startup to minimize inrush current and stress on components.
UC3844 vs. Other PWM Controllers
There are several other PWM controller ICs available on the market, such as the TL494, SG3525, and UC3842. The UC3844 stands out due to its simplicity, wide operating voltage range, and built-in protection features. However, the choice of PWM controller depends on the specific requirements of the power supply design, such as input voltage range, output power, and available board space.
Frequently Asked Questions (FAQ)
-
Q: What is the maximum operating frequency of the UC3844?
A: The maximum operating frequency of the UC3844 is 500kHz. -
Q: Can the UC3844 be used in a voltage-mode control scheme?
A: Yes, the UC3844 can be used in both current-mode and voltage-mode control schemes. For voltage-mode control, the VFB pin is used instead of the ISENSE pin. -
Q: What is the purpose of the COMP pin on the UC3844?
A: The COMP pin is the output of the error amplifier and is used to compensate the feedback loop for stable operation of the power supply. -
Q: How does the soft-start function work in the UC3844?
A: The soft-start function gradually increases the output voltage during startup by charging a capacitor connected to the SS pin. This limits the inrush current and reduces stress on the power supply components. -
Q: Can the UC3844 be used in a boost converter topology?
A: Yes, the UC3844 can be used in various switch-mode power supply topologies, including boost converters, flyback converters, and forward converters.
Conclusion
The UC3844 is a versatile and reliable PWM controller IC that simplifies the design of switch-mode power supplies. By understanding its features, pin configuration, and design considerations, engineers can effectively utilize the UC3844 to create efficient and robust power supply solutions. With its wide operating voltage range, built-in protection features, and flexibility in control schemes, the UC3844 remains a popular choice for power supply designers across various applications.
