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7447714101 Scheda tecnica(PDF) 10 Page - Union Semiconductor, Inc. |
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7447714101 Scheda tecnica(HTML) 10 Page - Union Semiconductor, Inc. |
10 / 13 page ________________________________________________________________________ http://www.union-ic.com Rev.05 Apr.2016 10/13 UM1360 IOUT=(0.1*D)/RS (0≤D≤100%, 2.5V<Vpulse<5V) IOUT=(Vpulse*0.1*D)/(2.5*RS) (0≤D≤100%, 0.5V<Vpulse<2.5V) D1 Rs 0.13Ω L1 68 μH 10 μF C1 VIN (6V-40V) VIN VSET ISENSE LX UM1360 LED 3W PWM dimming provides reduced brightness by modulating the LED’s forward current between 0% and 100%. The LED brightness is controlled by adjusting the relative ratios of the on time to the off time. A 25% brightness level is achieved by turning the LED on at full current for 25% of one cycle. To ensure this switching process between on and off state is invisible by human eyes, the switching frequency must be greater than 100Hz. When above 100Hz, the human eyes average the on and off times, seeing only an effective brightness that is proportional to the LED’s on-time duty cycle. The advantage of PWM dimming is that the forward current is always constant, therefore the LED color does not vary with brightness as it does with analog dimming. Pulsing the current provides precise brightness control while preserving the color purity. Capacitor Selection A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the supply source impedance and lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. A minimum value of 4.7μF is acceptable if the input source is close to the device, but higher values will improve performance at lower input voltages, especially when the source impedance is high. The input capacitor should be placed as close as possible to the IC. For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this application and should NOT be used. Inductor Selection Recommended inductor values for the UM1360 are in the range 27μH to 220μH. Higher values of inductance are recommended at higher supply voltages in order to minimize errors due to switching delays, which result in increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current over the supply voltage range. The inductor should be mounted as close to the device as possible with low resistance connections to the LX and VIN pins. The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current. Suitable coils for the UM1360 are listed in the table below: |
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