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LP3390 Datasheet(PDF) 5 Page - Lowpower Semiconductor inc |
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LP3390 Datasheet(HTML) 5 Page - Lowpower Semiconductor inc |
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5 / 8 page  Preliminary Datasheet LP3390 LP3390 Version 1.0 Datasheet NOV.-2013 www.lowpowersemi.com Page 5 of 8 Application Information LED Current Setting The loop of Boost structure will keep the FB pin voltage equal to the reference voltage VREF. FB voltage is 200mV,Therefore, when RSET connects FB pin and GND, the current flows from VOUT through LED and RSET to GND will be decided by the current on RSET, which is equal to following equation: Dimming Control a. Using a PWM Signal to EN Pin For the brightness dimming control of the LP3390, the IC provides typically 200mV feedback voltage when the EN pin is pulled constantly high. However, EN pin allows a PWM signal to reduce this regulation voltage by changing the PWM duty cycle to achieve LED brightness dimming control. The relationship between the duty cycle and FB voltage can be calculated as following equation: VFB = Duty x 200mV Where Duty = duty cycle of the PWM signal 200mV = internal reference voltage The duty cycle of the PWM signal is used to cut the internal 200mV reference voltage. An internal low pass filter is used to filter the pulse signal. And then the reference voltage can be made by connecting the output of the filter to the error amplifier for the FB pin voltage regulation. However, the internal low pass filter 3db frequency is 500Hz. When the dimming frequency is lower then 500Hz, VA is also a PWM signal and the LED current is controlled directly by this signal. When the frequency is higher than 500Hz, PWM is filtered by the internal low pass filter and the VA approach a DC signal. And the LED current is a DC current which elimate the audio noise. Two figures of PWM Dimming from EN are shown in Typical Operating Characteristics section and the PWM dimming frequency is 100Hz and 1MHz respectively. But there is an offset in error amplifier which will cause the VA variation. In low PWM duty signal situation, the filtered reference voltage is low and the offset can cause bigger variation of the output current. So the LP3390 is not recommend to be dimming by the EN pin. For the LP3390, the minimum duty vs frequency is listed in following table: b. Using a DC Voltage Using a variable DC voltage to adjust the brightness is a popular method in some applications. The dimming control using a DC voltage circuit. The output voltage can be calculated by the following Equations. c. Using a Filtered PWM signal Another common application is using a filtered PWM signal as an adjustable DC voltage for LED dimming control. A filtered PWM signal acts as the DC voltage to regulate the output current. In this circuit, the output ripple depends on the frequency of PWM signal. For smaller output voltage ripple (<100mV), the recommended frequency of 2.8V PWM signal should be above 2kHz. To fix the frequency of PWM signal and change the duty cycle of PWM signal can |
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