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EC9223NNQ1R Datasheet(PDF) 9 Page - E-CMOS Corporation |
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EC9223NNQ1R Datasheet(HTML) 9 Page - E-CMOS Corporation |
9 / 13 page EC9223 Multi-Channel TFT LCD Supply E-CMOS Corp. (www.ecmos.com.tw) 5E26N-Rev. F001 9/13 Compensation This current mode boost converter has a current sense loop and a voltage feedback loop. The current sense loop does not need any compensation. The voltage feedback loop is compensated by an external series R-C network RCOMP and CCOMP from COMP pin to ground. RCOMP sets the high-frequency loop gain and the unity gain bandwidth of the loop which determines the transient response. CCOMP together with RCOMP determine the phase margin which relates to loop stability. Output Capacitor Selection The output voltage ripple due to converter switching is determined by the output capacitor total capacitance, COUT, and the output capacitor total effective series resistance, ESR. The first ripple component can be reduced by increasing COUT. Changing COUT may require adjustment of compensation R and C in order to provide adequate phase margin and loop bandwidth. The second ripple component can be reduced by selecting low-ESR ceramic capacitors and using several smaller capacitors in parallel instead of just one large capacitor. Inductor Selection To prevent magnetic saturation of the inductor core the inductor has to be rated for a maximum current larger than IPK in a given application. Since the chip provides current limit protection of 1.8A (typ) it is generally recommended that the inductor be rated at least for 1.8A. Selection of the inductor requires trade-off between the physical size (footprint x height) and its electrical properties (current rating, inductance, resistance). Within a given footprint and height, an inductor with larger inductance typically comes with lower current rating and often larger series resistance. Larger inductance typically requires more turns on the winding, a smaller core gap or a core material with a larger relative permeability. An inductor with a larger physical size has better electrical properties than a smaller inductor. It is desirable to reduce the ripple current ∆ILPP in order to reduce voltage noise on the input and output capacitors. In practice, the inductor is often much larger than the capacitors and it is easier and cheaper to increase the size of the capacitors. The ripple current ∆ILPP is then chosen the largest possible while at the same time not degrading the maximum input and output current that the converter can operate with before reaching the current limit of the chip or the rated current of the inductor. For example, ∆ILPP could be set to 20% of IMAX. |
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