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ISL97675 Datasheet(PDF) 13 Page - Renesas Technology Corp |
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ISL97675 Datasheet(HTML) 13 Page - Renesas Technology Corp |
13 / 21 page ISL97675 FN7630 Rev.2.00 Page 13 of 21 Sep 13, 2017 PWM Control The ISL97675 has a high speed 8-bit digitizer that decodes an incoming PWM signal and converts it into four channels of 8-bit PWM current with a phase shift function that will be described later. During the PWM On period, the LED peak current is defined by the value of RISET resistor, the average LED current of each channel is controlled by ILEDmax and the PWM duty cycle in percent as: When the PWM input = 0, all channels are disconnected and the ILED is guaranteed to be <10µA in this state. The PWM dimming frequency is adjusted by a resistor at the RFPWM pin, which will be described in “PWM Dimming Frequency Adjustment” on page 14. Phase Shift Control The ISL97675 is capable of delaying the phase of each current source. Conventional LED drivers pose the worst load transients to the boost circuit by turning on all channels simultaneously as shown in Figure 20. In contrast, the ISL97675 phase shifts each channel by turning them on once during each PWM dimming period as shown in Figure 21. At each dimming duty cycle except at 100%, the sum of the phase shifted channel currents will be less than a conventional LED driver as shown in Figure 21 and 23. Equal phase means there is fixed delay between channels and such delay can be calculated as: where (255/N) in Equation 6 and Equation 7 can only be integer because the PWM dimming is controlled by an internal 8-bit ILED ave ILEDmax PWM = (EQ. 5) FIGURE 20. CONVENTIONAL 4-Ch LED DRIVER WITH 10% PWM DIMMING CHANNEL CURRENT (UPPER) AND TOTAL CURRENT (LOWER) ILED1-20mA ILED_Total_80mA TIME (ms) ILED2-20mA ILED3-20mA ILED4-20mA 510 15 FIGURE 21. PHASE SHIFT 4-Ch LED DRIVER WITH 10% PWM DIMMING CHANNEL CURRENT (UPPER) AND TOTAL CURRENT (LOWER) ILED1-20mA ILED_Total_20mA TIME (ms) ILED2-20mA ILED3-20mA ILED4-20mA 5 10 FIGURE 22. CONVENTIONAL LED DRIVER PWM DIMMING CHANNEL AND TOTAL CURRENT AT 50% DUTY CYCLE ILED4-20mA ILED_Total_80mA TIME (ms) ILED3-20mA ILED2-20mA ILED1-20mA 5 10 FIGURE 23. EQUAL PHASE SHIFT LED DRIVERpwm DIMMING CHANNEL AT 50% DUTY CYCLE ILED4-20mA ILED_Total_40mA TIME (ms) ILED3-20mA ILED2-20mA ILED1-20mA 5 10 tD1 tFPWM 255 ------------------x 255 N ---------- = (EQ. 6) tD2 tFPWM 255 ------------------x 255 N1 – 255 N ---------- – = (EQ. 7) |
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