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ZLED7025 数据表(PDF) 21 Page - Integrated Device Technology |
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ZLED7025 数据表(HTML) 21 Page - Integrated Device Technology |
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21 / 26 page ![]() ZSLS7025 Datasheet © 2016 Integrated Device Technology, Inc. 21 April 20, 2016 ( ) FB 5 4 FB PWM PWM 3 FB OUT R R R V D V R V I + − × × − = ( ) 86 . 0 R R 3 . 0 % 100 5 R 3 . 0 0 5 4 3 + − × × − = PWM 1 5 f 2 50 C R π ≥ × ( ) ( ) A 35 . 0 R 10 400 3 . 0 % 0 5 2 . 26 3 . 0 R R R V D V R V I FB FB 5 4 FB PWM PWM 3 FB OUT = + − × × − = + − × × − = 4.4.4 Selecting R3, R4, R5 and C1 R3, R4, and R5 can be obtained by (17) Substitute DPWM=100%, VPWM = 5V, and IOUT =0 in the equation, and the result is (18) which can be simplified to 15.66 x R3 = R4 + R5 The low pass filter formed by R5 and C1 must have a corner frequency much lower than the PWM frequency. As the corner frequency of the filter decreases, the response time of the LED current to changes in PWM increases. Choose a corner frequency 50 times lower than fPWM. (19) Assume fPWM is 200Hz (or higher) and choose C1 = 0.1μF, and the result is R 5 ≥400kΩ. Choose C4 = 0.1μF, R 5 = 400kΩ. Choose a nominal value for R4, and then calculate R3. Choose R 4 = 10kΩ, then R 3 = 26.2kΩ. Substitute DPWM=0, VPWM = 5V and IOUT = 350mA in the equation, and the result is (20) So, RFB =0.91Ω. (With the RC filter PWM dimming, RFB will be different than in the no dimming application shown in Figure 2.1.) |
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