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ML4830 Datasheet(PDF) 7 Page - Micro Linear Corporation |
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ML4830 Datasheet(HTML) 7 Page - Micro Linear Corporation |
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7 / 15 page  ML4830 7 Mode Pin 8 Definition VCO GND Frequency Modulation VCO-O OPEN Overlapping VCO F.M. PWM VREF Pulse Width Modulation Table 1. ML4830 Operating Modes OSCILLATOR In Table 1 above, the two VCO frequency ranges are controlled by the output of the LFB amplifier (Pin 6). As lamp current decreases, Pin 6 rises in voltage, causing the C(T) charging current to decrease, thereby causing the oscillator frequency to decrease. Since the ballast output network attenuates high frequencies, the power to the lamp will be increased. In PWM Mode, ICHG is 0 so the oscillator’s frequency is set per (1) below. 18 + – 1.25/3.75 9 C(T) R(T) VREF ICHG VREF CONTROL R(T)/C(T) 5 mA CLOCK C(T) VTH VTL tDIS tCHG Figure 2. Oscillator Block Diagram and Timing Also, in both VCO modes, the when LFB OUT is high, ICHG = 0 and the minimum frequency occurs. The charging current varies according to two control inputs to the oscillator: 1. The output of the preheat timer 2. The voltage at Pin 6 (lamp current output) In preheat condition, charging current is fixed at I R SET CHG PREHEAT () . () = 25 (1) In running mode, charging current decreases as the VPIN7 rises from 0V to VOH of trhe LAMP FB amplifier. The highest frequency will be attained when ICHG is highest, which is attained when VPIN6 is at 0V: I R SET CHG( ) () 0 5 = (2) The oscillator frequency is determined by the following equations: F tt OSC CHG DIS = + 1 (3) and tR C In IR IR CHG T T CHG T CHG T = + + 625 375 . . (4) The oscillator’s minimum frequency is set when ICHG = 0 where: F RC OSC TT ≅ × 1 051 . (5) This assumes that tCHG >> tDIS. Highest lamp power, and lowest output frequency are attained when VPIN6 is at its maximum output voltage (VOH). In this condition, the minimum operating frequency of the ballast is set per (5) above. For the IC to be used effectively in dimming ballasts with higher Q output networks a larger CT value and lower RT value can be used, to yield a smaller frequency excursion over the control range (VPIN6). The discharge current is set to 5mA. Assuming that IDIS >> IRT: tC DIS VCO T () ≅× 490 (6) |
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