World-Semi WS2801 LED Driver

Application Note

Document Review and Accuracy: The information in this document has been carefully reviewed and is believed to be accurate. However, this document is subject to change without notice. World-Semi assumes no responsibility for any inaccuracies that may be contained in this document, and makes no commitment to update or to keep current the contained information, or to notify a person or organization of any update. World-Semi reserves the right to make changes at any time to improve reliability, function, or design and to attempt to supply the best product possible.

1. Introduction

The WS2801 is a programmable constant current LED driver with three channels and PWM output. It is designed for indoor/outdoor LED display screens and decorative LED lighting systems, ideal for use in relay-connected LED lighting systems.

2. Features and Applications

Supports both constant current and constant voltage driving modes.
Programmable LED constant driving current with a wide range: 2~200mA.
Each output channel has a built-in PWM grayscale control circuit, suitable for low-cost control solutions.
PWM dimming free run capability with an internal oscillator (1.2MHz) for a refresh frequency of approximately 2.5KHz.
Built-in clock data regeneration circuit for outputting input data and clock signals, suitable for cascading applications.
Output polarity reversal function.
Maximum input signal clock frequency of 25MHz.
Power supply voltage range of 3.3~5.5V.
Applications:

LED decorative lighting systems.
Indoor/outdoor information and video display systems.
PWM signal generators.
LCD TV LED RGB backlight control.
3. Functional Description

The WS2801 is a constant current LED driver designed for indoor/outdoor LED displays and decorative LED lighting systems. It features three independent output channels, each capable of 256-level PWM grayscale control, allowing for 256-level LED grayscale control without changing the LED color. The WS2801 can output a constant LED driving current up to 200mA and includes a serial shift register, data latch, output register, bandgap reference voltage source, internal oscillator, and programmable constant current driver.

4. Internal Functional Block Diagram

The WS2801 integrates a serial shift register, data latch, output register, bandgap reference voltage source, internal oscillator, and programmable constant current driver. It samples the serial data input on the rising edge of the input clock at the SDI pin and stores it in the internal shift register. When WS2801 does not detect the clock signal at the CKI pin for 1ms, it latches the previously sampled grayscale data to the data latch and refreshes the internal status register for the next data transmission.

5. Constant Current Drive Mode

A high-performance operational amplifier in the WS2801 controls transistor M0 to generate a constant current for driving the external LED strings.

6. Constant Voltage Drive Mode

The WS2801 is a flexible LED driving chip that provides both constant current and constant voltage driving modes. In constant voltage mode (POL=“HIGH” or floating), the driving output voltage is up to 50mA.

7. Driving High-Power High-Brightness LEDs

The WS2801 can use an external switch to drive high-brightness LEDs in constant voltage mode, achieving 256-level grayscale control of high-power LEDs.

8. Using External Switches for High-Voltage Constant Current Drive

The WS2801 can use an external switch to achieve LED series driving at high input voltage with very low cost.

9. Grayscale Data Format

When the WS2801 maintains a low level on the clock PIN for 500us, the IC resets, locking the grayscale data transmission and preparing for the next grayscale data transmission.

10. WS2801 Relay Connection and Application

To meet the needs of long-distance data and clock transmission for cascading applications, WS2801 integrates a push-pull output level with strong driving capability and low EMI characteristics.

11. Typical Application Circuits

Typical Application Circuit One
Typical Application Circuit Two (Using external switches for high-voltage constant current drive LED strings)
Typical Application Circuit Three (Using external switches for constant voltage drive high-power LED strings)