LED, STRIPS, AND CONTROLLERS: PART-8, WS2801 Continue

In the previous Part-7, we have told about a special ASIC – RGB PIXEL with a Serial controlling IC – WS2801.

We have begun to explain its principal.

With today tutorial, we will continue with the details and how The WS2801 is Working.

AMAZON.CO.UK Affiliate Link: Selection with Color LED Strips, based on IC W2801

WS2801 – OVERVIEW

The Integral Circuit WS2801 is an LED Driver with constant current mode capability. The WS2801 Supports and Constant Voltage Drive Mode.

RGB Controlling IC WS2801 is designed to use with a serial connection of type: SPI (Serial Peripheral Interface type). THAT allows the cascading type of connection between the different members of the “string”.

WS2801 has three independent Drivers/Channels one for Red, second for Green, and third for a Blue color. Each one of them has a capability driving a programmable and constant Currents (IC supports Current between 5 and 150 mA). Each channel has a different PWM Dimming Circuitry, supporting a Free-Run mode.

Into the IC also are integrated: Input Control block, Oscillator block, BAND-GAP Reference Block, PWM Counter Block, PWM Comparator Block, GRAY Scale Data-Shifting Register Block, and DATA & CLOCK Reconstructing Block. WS2801 have PWM, with free-run capability – refresh rate = 2.5kHz.

The Serial Data need to be Ready on the Rising EDGE of the External Clock – in this moment the device is reading the three Bytes Color data. Both Data and Clock are ready to cascading another IC WS-2801. There is an output polarity reversing feature.

Described IC uses a very reliable type of two wire ultra-fast SPI connection (up to 25MHz), supports a Hot-Swap, has the anti-interference abilities. WS 2801 is very attractive for low-cost applications for interior and decorative Lighting solutions

IC has variants with both DIP-14 & SOP-14 packages.

It has wide operating temperature range: –40℃ ÷ +85℃

Power Supply of the IC – WS2801 range: 3.3 ÷ 5.5V DC

WS2801 – Typical Applications:

The IC allows for indoor and Outdoor Usage. With Its help, can be produced :

many different types of decorative LED lighting solutions
Message Display,
Signal Generator (PWM),
Back-Lighting of Posters, TVs, etc.

DIAG. 1 – BLOCK DIAGRAM of WS2801 – Serial LED Constant Current, Full-color Controlling ASIC

LET see the WS2801’s ingredients in a Block Diagram 1 (DIAG. 1):

Input Control block
Oscillator block
BAND-GAP Reference
PWM Counter
PWM Comparator
GRAY Scale Data-SHIFT Register
DATA & CLOCK Reconstructing Block
RED Side Buffer Block : Precise Current Limiting schematic using OP-Amp with feed Back and Buffered MO Output.
GREEN Side Buffer Block : Precise Current Limiting schematic using OP-Amp with feed Back and Buffered MO Output.
BLUE Side Buffer Block : Precise Current Limiting schematic using OP-Amp with feed Back and Buffered MO Output.

PIC. 1 WS2801: Pin Configuration and the Typical Application Circuit

SCH. 1 Typical Example of Usage of the RGB PIXEL Controlling IC WS2801

A principal of working the Device WS2801:

This versatile LED driver has two driving modes related the principal of use:

Constant Current Mode,
Constant Voltage Mode.

SCH. 2 Constant Current & Constant Voltage MODE

1.) Constant Current Driving MODE :

The WS2801 uses high-quality Operational Amplifier OP-AMP and HIGH Current ability element MO to generate constant current to COLOR Equal Output Port. Here is a feedback point, where, with feedback adjusting resistor is set the LED’s Powering Constant current.

EQUATION 1. Relation between I_xOUTand R_XFB

The WS2801 internal V_REF = 0.6V ;

Using that equation (Equation 1) you can calculate R_XFB with known current trough the LED string I_COLOROUT (I_xOUT) or you can calculate I_XOUT with known feedback sensing resistor R_XFB.

Example: R_XFB = 20Ω to set I_XOUT = 30mA;

2.) Constant Voltage Driving MODE :

pin [POL] = HIGH or it is Floating;

When are Using only limiting resistor RL, The IC works in the constant voltage driving mode.

TABLE. 1 The 14-pin Package of WS2801 – Pin Description

Time Diagram 1. RGB Pixel WS2801 – COLOR Data Format and Timing Diagram

On the Time Diagram 1, are shown the three bytes data with the colors settings (Data Format of WS2801) and the timing of SPI (The timing of the WS2801 Strings).

Real Examples of usage – WS2801:

AMAZON Link: 16.4ft_160LEDs_DC5v_Diffused_Digital_RGB_LED_Pixels_Lights

SPARKFUN Link: RGB LED Strip – 32 LED/m Addressable – 1m