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The MAX16840 has an EXT pin for driving an external transistor when the voltages on the IN pin are less than 5 V. The internal MOSFET will be in the off state. The EXT pin turns on Q5 and the 5 Ω load is added to the output of the bridge rectifier. Once the voltage on the IN pin exceeds 5 V, this load is turned off. This approach is useful when the electronic transformer is operated with trailing-edge dimmers. With some electronic transformer and dimmer combinations, the transformer does not switch properly when the dimmer is set at the minimum light output. This occurs when AC power is applied to the transformer with the dimmer set at minimum. The circuit of Q5, R20, and the EXT pin of the MAX16840 overcomes this problem by adding the 5 Ω load to the electronic transformer. This load is removed as soon as the LEDs turn on and provide light because the IN pin is directly connected to the boost output voltage. The dimming performance (Figures 2 through 11) was tested with an LET75 and a Lutron® SELV-303P dimmer.
The correct MR16 LED Driver/Transformer is an investment for your MR16 LED bulb ensuring maximum lifespan and perfect operation. With LED lighting power supplies you are guaranteed our MR16 LED drivers are designed to perform with the latest LED technology. A low-frequency AC transformer is bulky, heavy, and occupies a lot of space. In comparison, an electronic transformer is small and compact, designed to power a resistive load with a typical power demand exceeding 20 W. When an electronic transformer is powered by 120 VAC/230 VAC, most will not work if the resistive load on the output is set to draw less than 20 W.Figure 7: LED current waveform when powered by an LET75 with a trailing-edge dimmer at 120 VAC. The dimmer is set at maximum light output.
The driver circuitry of the MR16 LED lamp can be adjusted so that it draws a constant current load from the output of the electronic transformer. No capacitance can be added to the electronic transformer's output, since this can prevent the MR16 LED lamp from acting as a constant current load. Moreover, the current drawn by the MR16 LED lamp needs to ramp up to the programmed current at a very fast rate. Specifically, it needs to jump to the programmed value within 3µs or 4µs. If it ramps up slower than this, then the electronic transformer may stop switching.By using an HB LED driver optimized for MR16 and other 12VAC lamps, you can make MR16 LEDs compatible with electronic transformers. The LED driver demonstrated here was the MAX16840. One should note, however, that the performance of each, distinct electronic transformer and dimmer combination must be tested. If you’re switching to LED MR16s from halogen MR16s, then LED drivers are pretty important. They’re used to power low voltage (12v) LED bulbs and strip lighting, providing a constant current to make sure they work properly. They’re mainly used for strip lighting, and MR16, MR11 and G4 spotlight bulbs. In some cases your existing (normal) 12v transformer will work. However, this can reduce the lifespan of the bulb. Different LED drivers are required for different environments for example indoor, outdoor use or kitchen or bathroom. If the driver is to be used outside we recommend that a waterproof driver is used with a minimim IP 65 rating. We would also recomend this for kitchens and bathrooms a waterproof driver with a IP67 minimum rating is used. The driver you need depends on how many bulbs you want it to power multiplied by the wattage. It’s also advised the total wattage of all bulbs combined doesn’t exceed 80% of the transformer’s wattage. So if you choose to power 4 x 4W MR16 bulbs (16W total) from one driver, you’ll need a 20W driver. If you want to power 4 x 6W MR16 bulbs, you’ll need a 30W transformer, and so on. Performance data are shown here for a 6 W boost, MR16 LED lamp when powered by different electronic transformers. The boost MR16 was tested with seven LEDs on the output. Tables 1 and 2 summarize performance with different transformers.
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