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AT9933 Datasheet(PDF) 8 Page - Supertex, Inc |
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AT9933 Datasheet(HTML) 8 Page - Supertex, Inc |
8 / 9 page 8 AT9933 Assuming a 30% peak-to-peak ripple when the converter is in input current limit mode, the minimum value of the input current will be: I LIM,MIN = 0.85 • IIN,LIM (5) Setting I LIM,MIN = 1.05 • IIN,PK (6) The current level to limit the converter can then be com- puted. (7) Using IO = 350mA and ΔI O = 87.5mA in (1) and (2), Before the design of the output side is complete, over voltage protection has to be included in the design. For this applica- tion, choose a 33V zener diode. This is the voltage at which the output will clamp in case of an open LED condition. For a 350mW diode, the maximum current rating at 33V works out to about 10mA. Using a 2.5mA current level during open LED conditions, and assuming the same R S2/RREF2 ratio, R CS2 + RS2A = 120Ω (8) Choose the following values for the resistors: R CS2 = 1.65Ω, 1/4W, 1% R REF2 = 10kΩ, 1/8W, 1% R S2A = 100Ω, 1/8W, 1% R S2B = 5.23kΩ, 1/8W, 1% The current sense resistor needs to be at least a 1/4W, 1% resistor. Similarly, using I IN = 2.1A and ΔIIN = 0.3xIIN = 0.63 in (1) and (2): R CS1 = 0.228Ω P RCS1 = I 2 IN,LIM • RCS1 = 1W Choose the following values for the resistors: R CS1 = parallel combination of three 0.68Ω, 1/2W, 5% R REF1 = 10kΩ, 1/8W, 1% R S1 = 4.42kΩ, 1/8W, 1% IIN LIM = • IIN PK 0.85 = 2.1A 1.05 RCS2 RS2 RREF2 = 1.78Ω = 0.5625 RS1 RREF1 = 0.442 |
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