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How to simplify LED driver development

Posted: 03 Jan 2014 ?? ?Print Version ?Bookmark and Share

Keywords:LEDs? drivers? TPS9255x? dimming frequency? TPS54062?

LEDs are increasingly taking the place of various kinds of incandescent light bulbs. It means that electronic engineers who are not necessarily familiar with power management techniques have to develop LED drivers. Starting from scratch to develop a solution based on discrete components can be challenging and also takes plenty of time which equates to money. An easier and much faster way is to use highly integrated LED driver modules, which includes almost all of the passive components already.

LED driver module
One main advantages of modules is that most of the passive components are integrated which simplifies the design process and also saves space on the PCB. For the TPS9255x series the only additional external passive component is a single ceramic capacitor to stabilise the input voltage if the standard output current of 350 mA is needed (figure 1). For a higher or lower LED current just one or two (depending on the current range) additional resistors are requred. There is no need for complicated calculations for dimensioning the inductor, the compensation network and everything else which is usually associated with a LED driver controller.

The module also provides a dimming input which works with a wide frequency and duty cycle range and offers very fast dimming capability. The switching frequency is fixed to 400kHz so the dimming frequency should not be higher than 40kHz and the pulse width not smaller than 16?s.

If the maximum output current (450 mA) of one module is not enough then several modules can be simply paralleled for the desired total output current.

Figure 1: TPS92551 with typical configuration.

In this example, the LED driver modules are used to drive three separate LED strings with red, green and blue colour. This configuration allows dimming of each string independently and not only adjusts the brightness but also mixes the colour.

Figure 2 shows the block diagram of this typical setup where each colour can be controlled individually. On the input there is active reverse polarity protection which provides much lower losses compared to a diode and protects the circuit from damage due to wrong cabling. The LED driver modules are directly connected to the input voltage which can be as high as 50 V. The minimum input voltage is given by the maximum forward voltage of the LED string attached.

To generate the dimming signals, a small low-cost microcontroller MPS430G2332 is utilised. The three PWM signals are buffered, which is not necessary for this application, but for the master-slave configuration shown later.

Figure 2: Single configuration.

An auxiliary power supply based on a tiny TPS54062 synchronous buck converter supplies the microcontroller as well as the buffer at high input voltages. As the supply current for these two parts is quite low a linear regulator can also be utilised if the input voltage is not too high.

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