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SM8122A Datenblatt(PDF) 9 Page - Nippon Precision Circuits Inc |
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SM8122A Datenblatt(HTML) 9 Page - Nippon Precision Circuits Inc |
9 / 13 page SM8122A NIPPON PRECISION CIRCUITS INC.—9 Brightness adjustment using FB pin (PWM signal input) When the brightness is controlled by PWM signal (VPWM × Duty), the LED current (ILED) is given by equa- tion 4. If the values R1 = 30 Ω, R2 = 20kΩ, R3 = 50kΩ, R4 = 50kΩ, VFB = 0.5V, VPWM = 3V, and Duty = 0% are inserted in equation 4, the LED current ILED = 20mA, as shown in equation 5. If the values R1 = 30 Ω, R2 = 20kΩ, R3 = 50kΩ, R4 = 50kΩ, VFB = 0.5V, VPWM = 3V, and Duty = 100% are inserted in equation 4, the LED current ILED = 0mA, as shown in equation 6. Taking the above diagram as an example, inserting the values R1 = 30 Ω, R2 = 20kΩ, R3 = 50kΩ, R4 = 50kΩ, VFB = 0.5V, VPWM = 3V, and Duty = 0 to 100% into equation 4 gives the maximum LED current ILED of 20mA when Duty = 0% (equation 5) and the minimum LED current ILED of 0mA when Duty = 100% (equa- tion 6). Brightness adjustment circuit using FB pin (PWM signal input) SW VOUT FB SM8122 R1 30 Ω SBD R3 50k Ω R2 20k Ω VDD CE COUT 1.0 µF CIN 4.7 µF VIN 3.6V LED R4 50k Ω C1 0.1 µF PWM signal Duty [ %] VPWM [V] VSS L 4.7 µH PWM signal vs. LED current 0 5 10 15 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 VPWM × Duty [V] R2 × (VPWM × Duty − VFB) R3 +R4 R1 ILED = VFB − ... (4) 30 30 ILED == = 20,000 × (3 × 0 − 0.5) 50,000 + 50,000 0.5 − 0.6 20mA ... (5) 30 30 ILED == = 20,000 × (3 × 1 − 0.5) 50,000 + 50,000 0.5 − 0 0mA ... (6) |
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