Question

What is an LED constant current chip? LED constant current chip is used for

Answer 1

LED constant current chips are chips that require constant current drive.

LED constant current chips are integrated circuits used to implement constant current drive in lighting equipment and backlights. They are primarily used in LED lighting (wall washers/linear lights), automotive lighting, high bay lights, and backlight power supply drivers. These chips achieve stable drive through boost conversion and constant current control, supporting two main drive modes: boost (Boost) and buck (Buck).

This product has been developed based on different technical solutions: the third generation uses cycle-by-cycle output current detection to maintain constant current despite changes in input voltage and inductance.

Typical chips, such as the AP2402, offer a wide input voltage range of 5-100V and a maximum current of 1.5A. Some models, such as the SM2083EGL, integrate overcurrent protection and PWM dimming functions. The SM7581PB supports isolated drive with a full input voltage of 85-265Vac.

Answer 2

History of LED Constant Current Chips

First-Generation LED Constant Current Chips:

The main technical feature of these chips is that they are based on fixed-frequency PWM chips (such as the UC384X). They achieve constant current by reducing inductor ripple current and maintaining a constant inductor peak current. However, due to the fixed switching frequency, their maximum duty cycle is limited to 50% to avoid subharmonic oscillation, significantly limiting their application range. Furthermore, the minimal inductor ripple requires a relatively large inductor, which also presents drawbacks such as difficulty in reducing EMI and low efficiency. These chips include the HV9910 (American Supertech), PT4107 (China Resources Silicon Microelectronics), SMD802 (Taiwan Xinrui), FT870 (Huimang Microelectronics), and LNK506 (PI).

Second-Generation LED Constant Current Chips:

The innovation of these chips over the first generation is fixed Toff technology. By fixing the off-time of the switch, they can tolerate larger inductor ripple currents and achieve a duty cycle close to 100%. Their application range is also relatively broad. However, this generation of products has several drawbacks. One is that they cannot maintain constant current when the output voltage or inductance changes. This type of chip primarily includes the HB9910B (Supertech), SN3910 (Silicon), and LM3445 (National Semiconductor).

Third-generation LED constant current chips:

This type of product can be called a true constant current source because it detects and controls the actual output current cycle by cycle in real time. Ultimately, constant current can be achieved regardless of changes in output voltage, inductance, or input voltage.

Answer 3

What is the UC384X? The UC384X is a commonly used pulse-width modulation (PWM) controller integrated circuit widely used in switching power supply designs. The following is a brief introduction to the UC384X:

1. Series Overview

Models: UC3842, UC3843, UC3844, UC3845, etc. (different models have slightly different parameters and features).

Manufacturer: Texas Instruments (TI, formerly Unitrode).

Applications: Primarily used in switching power supply circuits such as isolated or non-isolated DC-DC converters, flyback, and forward converters.

2. Core Functions

PWM Control: Controls the on-time of the switching diode by adjusting the duty cycle to achieve output voltage stability.

Built-in Oscillator: Generates a fixed-frequency triangle wave for PWM comparison.

Error Amplifier: Compares the output voltage with a reference voltage through a feedback circuit and adjusts the duty cycle to stabilize the output. Current Sense: Supports overcurrent protection (such as the current sense pin of the UC3842/3843).

Soft Start: Some models (such as the UC3844) support a soft start function to reduce inrush current during startup.

3. Typical Application Circuit

Flyback Switching Power Supply: The UC384X is commonly used to drive MOSFET or BJT switching transistors, working with a transformer for isolation and voltage conversion.

Feedback Control: The output voltage is sampled via an optocoupler or directly fed back to the error amplifier to adjust the PWM signal.

Protection Function: Protection mechanisms such as overvoltage, overcurrent, and undervoltage lockout (UVLO) can be integrated.

4. Key Pins

COMP (Compensation): Connects the error amplifier output and the feedback network.

FB (Feedback): Inputs the feedback voltage for comparison and regulation.

ISENSE (Current Sense): Senses the switching transistor current for overcurrent protection.

RT/CT (Timing Resistor/Capacitor): Sets the oscillator frequency.

Vref (Reference Voltage): Provides an internal reference voltage (typically 5V). GND (ground) and Vcc (power): Power supply pins.

OUT (output): PWM signal output for driving the switching transistor.

5. Model Differences

UC3842/3843: Support current-mode control, suitable for high-frequency switching power supplies (e.g., 50kHz-1MHz).

UC3844/3845: Add a soft-start function, suitable for circuits requiring slow startup.

UC3846: Dual output, suitable for push-pull or full-bridge topologies.

6. Design Considerations

Frequency Selection: When adjusting the oscillation frequency using the RT/CT resistors, consider switching losses and efficiency.

Compensation Network: Properly design the error amplifier compensation circuit to ensure system stability.

Layout: High-frequency circuits require careful attention to parasitic parameters to reduce noise interference.

7. Alternative Solutions

If the UC384X is out of stock, other PWM controllers (such as the SG3525, TL494, or compatible models recommended by the manufacturer) may be considered, but this requires redesigning the peripheral circuitry. For more detailed technical parameters or application cases, please refer to the data sheet of the specific model or the switching power supply design manual.