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BL8587 データシート(PDF) 6 Page - SHANGHAI BELLING CO., LTD. |
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BL8587 データシート(HTML) 6 Page - SHANGHAI BELLING CO., LTD. |
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6 / 8 page  www.belling.com.cn 6 BL8587 DETAIL DESCRIPTION Startup sequence The BL8587 starts to work only when VIN powered on and the EN pin became high after a delay, the delay time is at least 4us. EN pin is not allowed to connect to VIN directly or keep it floating. The BL8587 starts with 1X mode, the output follows closely to the supply voltage. If the average voltage of pin D1 to D4 falls below 0.25V the IC switches to 1.5X mode, and it can automatically switch back to 1X mode when the input voltage is higher than 3.7V. PWM dimming control If the frequency of the PWM signal to the EN pin is less than 5KHz, the average LED current is proportional to the duty cycle of the PWM signal, and the EN works as a simple on/off control. A high level turns on and a low shuts down the LEDs. There is a delay time between the input PWM signal and the output LED current waveform, as shown in Fig.5, the TD is about 20uS. I_LED 20mA Td=20uS ON duty Td=20uS EN Fig. 5 PWM Dimming Control EN dimming control The LEDs’ brightness can also be controlled by the pulses applied to the EN pin, Refer to the Fig. 6. Inside the device there is a 4-bit counter connecting to the EN pin. The LED current can be programmed up to 16 levels depends on the number of rise edges of the input waveform. A high level input (“1”) with pulse width wider than 20uS is regarded as a signal to stop the build-in counter. The time between two wide high levels (more than 20uS) is a counting period. When some rise edges occur between two high level pulses of 20us, the equation as shown below can calculate the active pulse number, called Code_No. ) 16 , ( _ N Mod No Code = , In the equation, N is the number of rise edges of the input waveform and mod function returns the remainder of N divided by 16. For example, if N=0, 16, or other integral multiple of 16, the mod function returns a value of 0, if N=1, 17 or other integral multiple of 16 plus 1, the mod function returns a value of 1, and so on. The relationship between LEDs’ brightness and the Code_No is listed as the following table. Code No ISET/20mA Code No ISET/20mA 0 0 8 8/15 1 1 9 7/15 2 14/15 10 6/15 3 13/15 11 5/15 4 12/15 12 4/15 5 11/15 13 3/15 6 10/15 14 2/15 7 9/15 15 1/15 The frequency of the input pulse should be lower than 100KHz, and the width of the high level and low level larger than 4uS to prevent false trigger. Code_No=1 Code_No=4 Iset/20mA=15/15 20uS 20uS EN I_LE D Iset/20mA=12/15 Fig. 6 Linear current Dimming Control |
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