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SM8122AD датащи(PDF) 10 Page - Nippon Precision Circuits Inc |
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SM8122AD датащи(HTML) 10 Page - Nippon Precision Circuits Inc |
10 / 13 page SM8122A NIPPON PRECISION CIRCUITS INC.—10 Brightness Adjustment using CE Pin The LED average current can be adjusted by controlling the duty of a PWM signal input on the CE pin. When CE goes from LOW to HIGH, the soft start function operates (with 500 µs constant soft start time) and, there- fore, the LED average current ratio for a given PWM signal duty falls with increasing PWM signal frequency. Taking this into consideration, the recommended PWM control signal has a frequency range of 100 to 400Hz with duty cycle range of 10 to 90%. When adjusting the brightness using the CE pin, a ripple voltage synchronized to the PWM signal is generated across the output capacitor COUT. The amplitude of the ripple voltage is determined by the number of LEDs and their forward-bias voltage drop characteristics. If a ceramic capacitor is used for the output capacitor COUT, an audible noise may be generated due to the ceramic capacitor’s piezoelectric effect. The audible noise level depends on the ceramic capacitor (capacitance, bias dependency, withstand voltage etc.), LEDs (number, forward-bias voltage drop etc.), and mounting board (thickness, mounting conditions etc.), and thus should be verified under actual conditions. Brightness adjustment circuit using CE pin VOUT SM8122 R1 25 Ω SBD PWM signal VDD SW CE FB COUT 1.0 µF VIN 3.6V CIN 4.7 µF LED VSS L 4.7 µH PWM signal duty vs. LED average current 10 20 0.0 5.0 10.0 15.0 20.0 0 30405060708090 100 PWM signal duty [%] 400 [Hz] 100 [Hz] 1000 [Hz] 1400 [Hz] Alternatively, a tantalum capacitor or film capacitor with low piezoelectric effect can be used as the output capacitor COUT to minimize the noise level, or the brightness can be adjusted using the FB pin as described earlier. The audible noise generated when using the CE pin is not an inherent phenomena of the SM8122A device, but of the brightness adjustment method employed. Output voltage with LEDs ON Output voltage with LEDs OFF CE input signal and output ripple voltage 20mA COUT 3.5V 3.5V 3.5V 0.5V 11.0V 0mA COUT 2.7V 2.7V 2.7V 0V 8.1V |
Аналогичный номер детали - SM8122AD |
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Аналогичное описание - SM8122AD |
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