555 timer eliminates LED driver’s need for microprocessor control

-September 03, 2009

LEDs find their way into applications that range from high-end video displays to low-end lighting applications. Designers often need only some of the functions of a dedicated LED driver but can’t afford the cost of the microprocessor to control them. Microprocessors typically control dedicated LED drivers, enabling features such as analog or PWM (pulse-width modulation) for LED-current control, independent control of each LED, and reading LED status and faults. If your design requires a constant-current LED, such as those in LED lighting or luminaires, then you may not need these advanced features. In these applications, a 555 timer can replace the microprocessor and still allow accurate control of LED current independently of input voltage, temperature, and LED forward-voltage drops.

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).

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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.

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.

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