There are more and more applications using LEDs now compared to a few years ago. These applications range from high-end video Displays to low-end lighting applications. Designers often require only some of the functionality of dedicated LED drivers, but cannot afford the cost associated with the microprocessors needed to control them.

Dedicated LED drivers are often designed as 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 readout. For some applications requiring only constant LED current (eg LED lighting or lighting) these advanced features may not be required. In these applications, a 555 timer such as the TLC555 can replace a microprocessor, enabling precise control of LED current while reducing system cost, 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 current sinks. Normally, it requires a microprocessor to drive four digital input signals. The command /OE (Enable Output) 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 into the internal on/off latch on the falling edge (latch) of LE. When simple LED on/off control of LED current is required, the following circuit uses the ubiquitous 555 timer instead of microprocessor control.

Replacing the uP control of the LED driver with the ubiquitous 555 timer

Figure 1 TLC555 timer replaces LED driver microprocessor

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 start-up values. These values ​​work with Rext to set the LED current. In this application, Rext sets the current per output to IOUT = 18.75A / Rext = 18.75A / 178 ohm = 0.105A. Connect all the outputs in parallel for an LED current of 0.842 A.

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

The clock speed of the 555 timer determines how fast the LED switches. As each LE falling edge latches the SDI data into the other octal internal on/off latch, the LED current ramps between 0-100% during the octal clock pulse, turning the other octal output on or off. Figure 2 shows the resulting stepped 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 0.8mS, which our human eye perceives as only a split second. Gradual on and off can be achieved with very slow clock frequencies. Setting the clock frequency to 0.1Hz can gradually turn the LED on and off in 0.8 seconds.

Figure 2 LED turn-on and turn-off at 10 kHz clock frequency

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About the Author

Michael Day is the Director of Power Management Applications for TI’s Power Products Group and has 16 years of design experience in the power conversion field. Currently Michael is primarily responsible for managing TI’s DC/DC Power Applications Products Group. He graduated from Texas Tech University with a bachelor of science degree in electrical engineering and a master’s degree in electrical engineering with a focus on pulsed power supplies. Michael is an IEEE member and has published more than 60 papers on power supplies, portable power supplies, and lighting.

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