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CAT37TDI-T3 Datasheet(PDF) 8 Page - Texas Instruments |
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CAT37TDI-T3 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 13 page CAT37 Doc. No. MD-5005 Rev. T 8 © Catalyst Semiconductor, Inc. Characteristics subject to change without notice LED DIMMING WITH DC SIGNAL Dimming the LEDs can be done by applying a variable DC voltage as shown on Figure 3. As the VDC increase the voltage across R1 decreases and therefore lower the LED current. The resistors R2 and R3 must be large enough so that their current (tens of µA) is much smaller than the LED current but much larger than the FB leakage current (IFB). When adjusting VDC between 0V and 2V, the resistors shown on Figure 3 will set the LED current between 0mA and 15mA. LED DIMMING WITH PWM SIGNAL PWM brightness control provides the widest dimming range (greater than 20:1). By turning the LEDs ON and OFF using the control signal, the LEDs operate at either zero or full current, but their average current changes with the PWM signal duty cycle. Typically, a 5kHz to 40kHz PWM signal is used. A filtered PWM signal is used to dim the LEDs, as shown in Figure 4. In addition to providing the widest dimming range, PWM brightness control also ensures the “purest” white LED color over the entire dimming range. The true color of a white LED changes with operating current, and is the “purest” white at a specific forward current, usually 15mA or 20mA. If the LED current is less than or more than this value, the emitted light becomes more blue. Applications involving color LCDs can find the blue tint objectionable. LED DIMMING WITH A LOGIC SIGNAL For applications that need to adjust the LED brightness in discrete steps, a logic signal can be used as shown in Figure 5. R1 sets the minimum LED current value (when the NMOS switch is OFF): RINCR determines how much LED current increases when the external NMOS switch is turned ON. PCB LAYOUT GUIDELINES The CAT37 is a high-frequency switching regulator where proper PCB board layout and component placement can minimize noise and radiation and increase efficiency. To maximize efficiency, the CAT37 design has fast switch rise and fall times. To prevent radiation and high frequency resonance problems minimize the length and area of all traces connected to the SW pin and use a ground plane under the switching regulator. The SW pin, schottky diode and capacitor C2 signal path should be kept as short as possible. The ground connection for the R1 resistor should be tied directly to the GND pin and not be shared with other components. Figure 3. Dimming with a DC Voltage Figure 4. Dimming with a Filtered PWM Signal Figure 5. Dimming with a Logic Signal VDC R3 18kΩ R2 1kΩ R1 6.3Ω CAT37 FB 2N7002 RINCR R1 6.3Ω CAT37 FB Logic Signal PWM R3 18kΩ 0.1µF 10kΩ R2 1kΩ R1 6.3Ω CAT37 FB ) MIN ( LED I V 095 . 0 = 1 R ) INCREASE ( LED INCR I V 095 . 0 = R |
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