Using the ubiquitous 555 timer to replace the uP control of LED drivers

LEDs are used in more applications today than just a few years ago. These applications range from high-end video displays to low-end lighting applications. Designers often require only a subset of the functionality of dedicated LED drivers but cannot afford the associated costs of the microprocessors required to control them.

Dedicated LED drivers are often designed to be microprocessor controlled to implement features such as analog or pulse width modulation (PWM) LED current control, independent control of each LED, LED status and fault information reading, etc. For some applications that only require constant LED current (such as LED lighting or luminescence), these advanced features may not be needed. In these applications, a 555 timer such as the TLC555 can replace the microprocessor, thereby reducing system cost while achieving precise control of LED current, which is independent of input voltage, temperature and LED forward voltage drop.

For example, the TLC5917 is a dedicated LED driver that controls eight independent constant current sinks. Normally, it requires a microprocessor to drive four digital input signals. Command /OE (output enable) activates and deactivates the IC. Serial Data Input (SDI) Data is clocked into the IC's input shift register on the rising edge of the clock (CLK). The data in the shift register is transferred to the internal on/off latch on the falling edge of LE (latched). When simple LED on/off control of the LED current is required, the following circuit uses the ubiquitous 555 timer instead of microprocessor control.

Figure 1. TLC555 timer replaces the microprocessor of LED driver

The TLC5917 outputs can drive eight individual LEDs, or their outputs can be paralleled to increase current capability to drive a single higher power LED. Its internal current setting registers have default startup values. These values, together with Rext, set the LED current. In this application, Rext sets the current for each output to IOUT = 18.75A / Rext = 18.75A / 178 ohm = 0.105A. Connecting all outputs in parallel gives an LED current of 0.842 A.

At power-up, the internal on/off latches default to turning all outputs on or off to "0", so these latches must be set to "1" before the outputs turn on. The 555 timer implements this function instead of the microprocessor. Both CLK and LED are connected to the square wave output of the 555 timer. At each rising edge of CLK, the SDI data is shifted into the TLC5917 input shift register. At the falling edge of LE, the data is latched into the on/off latch. Since the transfer and latching of data occur on different clock edges, the CLK and LE pins can be connected to the same input clock signal. By hardwiring/OE to ground, the IC is permanently activated. SDI can be connected to Vcc to automatically turn on the LEDs at power-up. This connection "1s" are continuously timed to turn on all outputs. We can also connect SDI to a switch or digital input to achieve LED on/off control. SDI can then be pulled to Vcc and all "1s" are continuously timed to turn on the outputs. Otherwise it will be pulled to ground and all “0s” will be timed continuously to turn the output off.

The clock speed of the 555 timer determines how fast the LEDs are switched on and off. As each LE falling edge latches the SDI data into another of the eight internal on/off latches, the LED current ramps from 0-100% during the eight clock pulses, turning another of the eight outputs on and off. Figure 2 shows the resulting staircase LED current, which increases and decreases with each successive LE falling edge. Even a relatively slow 10 kHz clock rate produces an off-on and on-off transition of only 0.8 mS, which our eyes perceive as a split second. A very slow clock rate can be used to achieve gradual on and off. Setting the clock rate to 0.1 Hz will gradually turn the LED on and off in 0.8 seconds.

Figure 2 LED on and off at 10 kHz clock frequency

For more information about this article, or to download the TLC5917 product manual or other technical documents, please visit: www.ti.com.cn/product/cn/tlc5917.

For more information on the 555 timer, visit www.ti.com/product/tlc555.

About the Author

Michael Day is the Director of Power Management Applications at TI Power Products Division. He has 16 years of design experience in the field of power conversion. Currently, Michael is responsible for managing TI's DC/DC Power Applications Division. He graduated from Texas Tech University with a Bachelor of Science in Electrical Engineering and a Master of Science in Electrical Engineering, with research focus on pulsed power supplies. Michael is a member of IEEE and has published more than 60 papers on power supplies, portable power supplies, and lighting.