Application of 555 timer in LED driver

This article introduced how to use the cheapest 555 timer, you don't need a LED some drives fully functional applications, on special LED drive instead of a microprocessor control. Doing so allows users to reduce the total system cost at the same time, maintain the LED constant current drive.

compared to a few years ago, now use the LED application more and more. These applications from high-end video display to low-end lighting applications, and so on. Designers are usually only need special LED drive part of the function, but can't afford the control they need the related cost of the microprocessor.

special LED drive type is often design for the microprocessor control, to achieve such as analog or pulse width modulation ( 脉宽调制) The LED current control, each LED an independent control, LED state and fault information read. For some only requires the application of the constant LED current ( For example: LED lighting or light) , may not need these advanced features. In these applications, such as TLC555 555 timer can be instead of the microprocessor, the accurate control to realize the LED current at the same time reduce the system cost, with the input voltage, temperature and pressure drop LED forward.

TLC5917, for example, is a dedicated LED drive, its control eight independent constant current current well. Under normal circumstances, it requires a microprocessor, to drive the four digital input signal. Instructions/OE ( Allow output) Activation and close the IC. Serial data input, SDI) Data on the clock, CLK) Rising along the clock input to the IC input shift register. In the shift register data in the falling edge of LE ( Locking) Into the internal open/shut in the store. When in need of LED current simple LED on/off control, use the following circuit can be seen everywhere in the 555 timer, instead of a microprocessor control. Figure 1

TLC555 timer instead of the LED drive microprocessor

TLC5917 output can drive eight independent leds, or parallel to its output in order to improve the current ability to drive a single higher power LED. Its internal current set up a register have enabled by default values. These values are set with the Rext LED current. In this application, Rext set each output current to IOUT = 18. 75 / Rext = 18。 75A / 178 ohm=0. 105 a。 All output in parallel connection, 0. 842 a LED current.

access to electricity, internal open/shut all output on or off by default to register '0', so before the output on the latch must be set to '1'. 555 timer instead of the microprocessor to implement this function. CLK and LED to connect to square wave output of 555 timer. In each CLK rise along, SDI to TLC5917 input data has been shifted in the shift register. On the falling edge of the LE, the data is latched to open/shut in the store. Due to the transfer of data and latches on a different clock along, so the CLK and LE pin can be connected to the same input clock signal. Through the hardwired/OE grounding, IC is permanently activated. SDI can be connected to Vcc to open automatically when power on the LED. This kind of connection '1 s' continuous timing to open all the output. SDI we can also be connected to a switch or digital input, in order to realize the LED on/off control. After, can pull SDI to Vcc, continuous time, all the '1 s' to open the output. Otherwise it will be pulled to the ground, all the '0' s continuous time to shut down output.

555 timer clock speed determines the speed of the LED switch. Each falling edge LE SDI data latch to another eight internal open/shut store, LED current in 0 - during the eight clock pulse 100% slope change, thus opening or closing another eight output. Figure 2 shows the ladder-like LED current, it varies with each successive LE down along the increase and decrease. Even the relatively slow 10 kHz clock frequency, also can produce a is only 0. 8 level - ms And open - Closed transition, our eyes to this feeling is only for an instant. Use very slow clock frequency can be realized and off gradually. The clock frequency is set to 0. 1 hz, can be in 0. 8 seconds time gradually turned on and off the LED.

figure 2 to 10 kHz clock frequency case LED turned on and off when

the author profile Michael Day current power management applications TI power products division director, has 16 years of design experience in the field of power conversion. The current Michael main products division is responsible for managing the TI DC/DC power applications. He graduated from Texas tech university ( 德州理工大学) And got a bachelor of science degree in electronic engineering, and then with a master's degree in electronic engineering, the research direction for the pulse power supply. Michael is now a member of IEEE, published more than 60 papers about power supply, portable power and lighting.