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LM3489-Q1 Datasheet(PDF) 10 Page - Texas Instruments |
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LM3489-Q1 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 25 page LM3489 LM3489-Q1 SNVS443B – MAY 2006 – REVISED FEBRUARY 2013 www.ti.com The LM3489 operates in discontinuous conduction mode at light load current or continuous conduction mode at heavy load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up to the peak, then ramps down to zero. The next cycle starts when the FB voltage reaches the reference voltage. Until then, the inductor current remains zero and the output capacitor supplies the load. The operating frequency is lower and switching losses reduced. In continuous conduction mode, current always flows through the inductor and never ramps down to zero. The output voltage (VOUT) can be programmed by 2 external resistors. It can be calculated as follows: VOUT = 1.239 x (R1 + R2) / R2 (1) Figure 24. Hysteretic Window The minimum output voltage ripple (VOUT_PP) can be calculated in the same way. VOUT_PP = VHYST (R1 + R2) / R2 (2) For example, with VOUT set to 3.3V, VOUT_PP is 26.6mV VOUT_PP = 0.01 x (33K + 20k) / 20k = 0.0266V (3) Operating frequency (F) is determined by knowing the input voltage, output voltage, inductor, VHYST, ESR (Equivalent Series Resistance) of output capacitor, and the delay. It can be approximately calculated using the formula: (4) where: α: (R1 + R2) / R2 delay: It includes the LM3489 propagation delay time and the PFET delay time. The propagation delay is 90ns typically (see Figure 25). 10 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated Product Folder Links: LM3489 LM3489-Q1 |
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