Simple, Configurable LED Driver Application Design for MR16 LED Lamps

MR16 lamps are a type of multi-faceted reflector lamps, widely used in commercial retail and home decorative lighting. Since they usually use halogen filaments as light sources, they have many disadvantages such as low efficiency, high heat generation and halogen capsule handling. However, current LED technology provides MR16-compatible, highly reliable, and cost-effective alternatives to halogen lamps. For example, ON Semiconductor's driver CAT4201, which can drive three series-connected LEDs, provides a new option for engineers designing MR16 LED lamps. The following will introduce a high-efficiency MR16 LED lamp driver solution that replaces MR16 halogen lamps in combination with the characteristics of MR16 lamps.

 

MR16 halogen lamps are used for creative lighting in many retail and consumer applications due to their unique size, configurability, focusing ability, aesthetics, and practicality. Common MR16 halogen lamps have a luminous flux range of 150 lumens (lm) to 800 lm, with a luminous efficacy of about 15 lm/W, or 15% luminous efficiency. Low efficiency, heat generation, and halogen capsule handling issues are disadvantages of this technology. In addition, the typical halogen bulb life is about 2,000 hours. It is also possible that the filament breaks due to violent vibrations, causing the bulb to fail prematurely.

 

In comparison, LED has the advantages of energy saving and environmental protection, and its power consumption is quite low. The general working voltage of LED is 2 to 3.6 V. LED is a cold light technology, and the heat generation is much lower than that of ordinary lighting sources; LED is completely encapsulated in epoxy resin, and there are no loose parts in the lamp body, so it is not easy to be damaged. Under the appropriate current and voltage, the service life of LED can reach 50,000 to 100,000 hours; LED is made of non-toxic materials and does not pollute. The current disadvantage of LED is that it is relatively expensive and it is difficult to handle the heat, so the driving power supply is a key factor in the overall life of LED lamps.

 

ON Semiconductor's CAT4201 is a 300 mA LED driver that can drive three LEDs in series and can be used in MR-16 LED lamp designs. Because the device has been optimized, it is more perfect in terms of external component count and thermal performance.

 

CAT4201 adopts buck topology, does not require input/output (I/O) electrical isolation, has an input voltage of 12 Vac, and an output power of 2.7 W; other specifications include: output voltage of 9.1 V, rated input power of 3.4 W, rated average current of 300 mA, maximum average current of 310 mA, minimum average current of 295 mA, and typical energy efficiency of 83%. Figure 1 is an MR16 lamp circuit using CAT4201.

 

 

The CAT4201 is a step-down, easy-to-configure, dimmable LED driver. The device is available in a 5-pin SOT-23 package; with less than 10 discrete components (most of which are surface-mount components), the entire circuit board can be small enough to fit into the lamp holder of an MR16 lamp. The VBAT, CTRL, and SW pins are rated for 40 V, so there is enough voltage to drive 12 LEDs. The RSET pin determines the output DC current; CTRL is used for the dimming signal input. SW is the output pin of the internal MOSFET.

 

The working principle of this application circuit is not complicated. It is a simple buck converter. In AC applications, the input is rectified by a rectifier bridge. VBAT and GND are directly connected to the two ends of the large-capacity capacitor C1, which reduces the current ripple. During the first switching phase, the internal MOSFET charges the inductor with a linearly rising current until the MOSFET is turned off; during the second phase, the MOSFET is turned off, and the current stored in the inductor will be discharged through the Schottky diode (D5) while the current decays until the next switching cycle. The output capacitor is used to reduce the current ripple in the LED. The voltage of the RSET pin is regulated at 1.2 V, so the resistor connected to RSET determines the RSET current. The RSET current is roughly proportional to the constant current output of the CAT4201.

 

In terms of circuit configuration, the VBAT voltage rating is -0.3 to +40 V, so it is safe to have an input voltage of up to 24 Vac on the bridge rectifier. A large capacitance C1 is required to maintain a high input voltage level. The large fluctuations in the rectified AC current will pull the output current to zero at double the AC line frequency, thus reducing the output current. A 220 μF capacitor is sufficient for this design. For a 12 Vac input, the C1 voltage rating should be 25 V. Due to the need for a lower forward voltage, the MBR0520L surface mount Schottky diode was selected for the bridge rectifier.

 

The output capacitor C2 is recommended to be at least 4.7 μF to reduce the output ripple. A larger value of C2 will effectively suppress the output ripple while increasing the output current by a few mA. However, since its contribution to the overall energy efficiency is negligible and the human eye cannot perceive high-frequency fluctuations, it is not necessary to use a capacitor larger than 10 μF. The value of L is recommended to be 22 μH to set the switching frequency at about 150 kHz. The LED current range is 0 to 350 mA, so an effective rated current of the inductor of about 800 mA is sufficient.

 

The total continuous current through the rectifier is always less than 400 mA, even when the output current provided is 350 mA. Therefore, a continuous current rating of 0.5 A is sufficient for the rectifier. A closer look at the voltage across the freewheeling diode D5 shows that despite the 12 Vac input, voltage spikes exceeding 20 V occur. Higher input voltages (such as 15 V) further stress the diode and increase the risk of failure. Therefore, the rated voltage on D5 should be 30 V or 40 V. ON Semiconductor's Schottky rectifier MBR0540 can be used as D5.

 

The RSET pin is used to configure the output current value. The RSET voltage is adjusted at 1.2 V. Adding a resistor between RSET and GND can determine the RSET current, which has an approximate linear relationship with the output constant voltage:

It is calculated that for 300 mA output, I _RSET = 0.12 mA. The RSET resistor value should be 10 kΩ. A smaller RSET resistor may increase the output current, and a resistor of at least 8.0 kΩ is recommended for stable operation. If the input voltage is high enough (such as 15 Vac), RSET can be set to 8.2 kΩ, thus ensuring at least 350 mA of output current. CTRL obtains its voltage from the cathode of the LED through R2. The value of R2 is not critical.

 

The basic performance and characteristics of the circuit can be understood from the waveforms shown in Figures 2 to 4. The test equipment includes: Global Specialties 1506 AC isolated variable voltage AC line power supply; Voltech PM1000 power analyzer; Tektronix TDS754D digital phosphor oscilloscope; Tektronix TCP202 current probe; Agilent 34401A multimeter.

 

 

In the circuit configured according to the circuit diagram of Figure 1, Vin = 12 Vac 50Hz. Due to the AC line input, the LED current has very slight low-frequency (twice the AC frequency, 2 × 50HZ) fluctuations. Using a larger value C1 or a higher Vin will further smooth out the ripple current associated with the AC.

 

 

In a circuit configured according to the circuit diagram of FIG. 1 , the startup and power-down transients are shown in FIG. 4 .

 

 

The advantages of the CAT4201 driver are compact circuitry and high energy efficiency, making it ideal for replacing filament-based bulbs such as spotlight applications. This ultra-small design can fit into the space of a standard connector (lamp holder) for MR16-type LED spotlights, isolating these temperature-sensitive components from the heat-generating LEDs as much as possible.

 

The design is optimized for both component count and thermal performance. This design can typically use three 1 W LEDs in the mirror section and can be adjusted to meet the requirements of the lighting system designer. In the final solution, the LED current and capacitor size are traded off to achieve the best efficiency, accuracy, size and component count.

Figure 5: The micro LED driver board can be embedded in the lamp holder

 

This design provides the elements needed to design a 3-LED MR-16 lamp using the CAT4201. The CAT4201 helps achieve a smaller size and fewer components for the MR-16 compatible adapter.

 

Reducing the capacitor and inductor values, or eliminating the CTRL resistor may further reduce cost and PCB size. Using a small capacitor will result in a significant drop in average output current, so it is not recommended, but it is suitable for lower brightness (200 to 250 mA) applications. Selecting high-quality LEDs with a small forward voltage is very important for designs that achieve higher output currents.