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SM8121AD Datasheet(PDF) 10 Page - Nippon Precision Circuits Inc |
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SM8121AD Datasheet(HTML) 10 Page - Nippon Precision Circuits Inc |
10 / 19 page SM8121A NIPPON PRECISION CIRCUITS INC.—10 BRIGHTNESS ADJUSTMENT Brightness Adjustment using FB Pin The LED brightness can be adjusted using an input DC control voltage connected through resistor R3 to the FB pin. Alternatively, the brightness can be controlled by a PWM signal by adding a low-pass filter comprising resistor R4 and capacitor C1. The PWM signal frequency range is determined by the low-pass filter coeffi- cients. For example, the recommended values for resistor R4 (50k Ω) and capacitor C1 (0.1µF) provide a PWM signal frequency range of 1kHz to 1MHz. Brightness adjustment using FB pin (DC voltage input) When the brightness is controlled by DC voltage (VDC) connected to resistor R3, the LED current (ILED) is given by equation 1. If the values R1 = 30 Ω, R2 = 20kΩ, R3 = 100kΩ, VFB = 0.5V, and VDC = 0V are inserted in equation 1, the LED current ILED = 20mA, as shown in equation 2. If the values R1 = 30 Ω, R2 = 20kΩ, R3 = 100kΩ, VFB = 0.5V, and VDC = 3V are inserted in equation 1, the LED current ILED = 0mA, as shown in equation 3. Taking the above diagram as an example, inserting the values R1 = 30 Ω, R2 = 20kΩ, R3 = 100kΩ, VFB = 0.5V, and VDC = 0 to 3V into equation 1 gives the maximum LED current ILED of 20mA when VDC = 0V (equation 2) and the minimum LED current ILED of 0mA when VDC = 3V (equation 3). Brightness adjustment circuit using FB pin (DC voltage input) SW VSS FB SM8121 R1 30 Ω SBD DC Voltage 0 to 3V R3 100k Ω R2 20k Ω VDD CE COUT 1.0 µF L 10 µH CIN 4.7 µF VIN 3.6V LED DC voltage vs. LED current 0 5 10 15 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 DC voltage [V] R1 R2 × (VDC − VFB) R3 ILED = VFB − ... (1) 20,000 × (0 − 0.5) 100,000 30 30 ILED == = 0.5 − 0.6 20mA ... (2) 20,000 × (3 − 0.5) 100,000 30 30 ILED == = 0.5 − 0 0mA ... (3) |
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