Accelerate the design of LED drivers
As more and more applications that use incandescent light bulbs of any kind are being replaced by LEDs it means that electronic engineers who are not necessarily familiar with power management techniques are having 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 like Texas Instruments TPS9255x series, 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 stabilize 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 400 kHz so the dimming frequency should not be higher than 40 kHz and the pulse width not smaller than 16 µs.
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 color. This configuration allows dimming of each string independently and not only adjusts the brightness but also mixes the color.
Figure 2 – Single Configuration
Figure 2 shows the block diagram of this typical setup where each color 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 utilized. The three PWM signals are buffered, which is not necessary for this application, but for the master-slave configuration shown later.
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 utilized if the input voltage is not too high.
This reference design was made for developing and testing different setups which means that all unused pins of the microcontroller are accessible by multi-pin connectors to connect it to other circuitry. The higher output current capability of the buck converter compared to a linear regulator makes an additional power supply for an extended test setup needless.
Master-Slave Configuration
The maximum input voltage of the TPS9255x limits the number of LEDs per string. If more LEDs are needed, several strings with separate driver modules have to be used as shown in Figure 3.
Figure 3 – Master-Slave Configuration
The master is populated with the DC/DC converter and the microcontroller as shown before. The input voltage and the three PWM signals for dimming are distributed to slave 1. The slave is populated with an LDO instead of a buck converter as it only supplies the buffer for the PWM signals.
The slave setup is always the same and any number of them can be interconnected to each other.
The cabling between the master and a slave or between slaves can be in the range of several meters. These lines act as antennas picking up noise which is injected into the circuit. Also the high input impedance of the buffers can cause ringing and spikes on ling lines causing malfunction. Therefore it is important to protect the PWM inputs of each slave and to shape the rectangular waveforms before applying them to the LED driver.
Figure 4 – PWM Input Protection
Figure 4 shows the protection of the PWM inputs and also the buffering which is implemented for each slave circuit. Over- and under-voltage protection is basically done by clamping the input to the supply voltage of the buffer/microcontroller (3.45 V) and to GND respectively using schottky diodes. In series to each input a 1kΩ resistor limits the current to prevent damaging the diodes due to high current caused by large spikes.
A buffer (SN74HC125) shapes the PWM signal to get a clean waveform for the dimming inputs of the driver module. A 10 kΩ pull-down resistor on the input of each buffer stage ensures a defined input level if no waveform is applied.
The output of the buffer is also the source for the next slave and needs to be protected as well. 330 Ω resistors in series to each output limit the output current if a short is applied accidently.
LED Strip and Software
The LED strip is built of five Avago ASMT-MT00 ‘Moonstone Tri-Color’ LEDs in series. Each package contains three single LEDs (red, green, blue) with the anodes and cathodes accessible.
The two-layer PCB is made with two ounces of copper to achieve good power dissipation. Additional small heatsinks are attached to the bottom side of the PCB to reduce the heating of the LEDs further.
The firmware is done with Code Composer Studio and contains basically a software-PWM to show the dimming capability and color mixing. It enables an easy start when developing your applications.
Conclusion
Designing a LED driver using a controller with an external power stage and additional circuitry like a compensation network not only requires a certain amount of time but also knowledge of power management techniques. With Texas Instruments TPS9255x series of LED driver modules the design is much easier and faster to achieve because only one external component is needed for the standard configuration. Another benefit is that engineers with only basic or no power management knowledge can create a working solution immediately.
The article introduces a reference design with PWM dimming for RGB-LEDs and shows how several driver boards can be interconnected. All information for this reference design (schematic, layout, bill of material, software) is available on www.ti.com under the keyword ‘PMP7247’.
About the author
Matthias Ulmann was born in Ulm, Germany, in 1980. He was awarded a degree in electrical engineering from the University of Ulm in 2006. After working for several years in the field of motor control and solar inverters (specialized in IGBT-drivers), Ulmann joined TI’s Analog Academy for a one year trainee program. Since 2010 he has worked in the EMEA Design Services Group as a Reference Design Engineer in Freising, Germany. Ulmann’s design activity includes isolated and non-isolated DC/DC converters for all application segments.
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