555 timer eliminates LED driver's need for MPU control22 Oct 2015 | Michael Day
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Figure 1: A 555 timer provides the necessary pulses for configuring an LED driver.
IC2, a TLC5917 dedicated LED driver, controls eight independent constant-current sinks (figure 1). It normally requires a microprocessor to drive four digital-input signals. The command (output enable) enables and disables the IC. Data on the SDI (serial-data-input) pin clocks into the IC's input shift registers on the rising edge of the clock. The data in the shift registers transfers into internal on/off latches on the falling edge of the LE (latch).
Either the TLC5917 outputs can drive eight independent LEDs, or you can parallel its outputs to increase the current to drive one higher-power LED. Its internal current-setting registers have default values at start-up. These values, along with external current-setting resistor R3, set the LED current. In this application, R3 sets each output's current to 105 mA: 18.75V/R3=18.75A/178Ω. Connecting all outputs in parallel yields 842 mA of LED current.
At power-up, the internal on/off latches that turn each output on or off default to zero, so you must set these latches to one before the outputs turn on. The 555 timer replaces the microprocessor for this function. The clock and latch lines both connect to the 555 timer's square-wave output. At each rising edge of the clock, the SDI shifts into the TLC5917's input shift register. This data latches into the on/off latch at the falling edge of the latch signal. Because shifting the data and latching the data occur at different clock edges, the clock and latch pins can connect to the same input clock signal. Hard-wiring to ground permanently enables the IC. You can connect SDI to the power-supply voltage to automatically turn on the LED at power-up. This connection continuously clocks in ones to turn on all outputs. You can also connect SDI to a switch or a digital input to allow for LED on/off control. Then, SDI can pull to the power-supply voltage, which continuously clocks in all ones to turn on the outputs. Alternatively, it can pull to ground, which continuously clocks in all zeros to turn off the outputs.
Figure 2: The LED current (lower trace) ramps up and down in eight steps.
The 555 timer's clock speed determines how fast the LEDs turn on and off. The LED current ramps from 0 to 100% in eight clock pulses as each falling edge of the latch pin latches the SDI data into another of the eight internal on/off latches, turning on or off another one of the eight outputs. Figure 2 shows the resulting stair-stepped LED current increasing and decreasing with each successive falling edge of the latch. Even a relatively low clock speed of 10 kHz results in an off/on and on/off transition of only 0.8 msec, which the human eye perceives as instantaneous. You can achieve gradual turn-on and turn-off with low clock speeds. Setting the clock to 0.1 Hz gradually turns the LED on and off in 0.8 sec.
About the author
Michael Day contributed this article.
This article is a Design Idea selected for re-publication by the editors. It was first published on September 3, 2009 in EDN.com.
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