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## ISL85012 Datasheet(PDF) 16 Page - Renesas Technology Corp

 Part No. ISL85012 Description 12A, 3.8V to 18V Input, Synchronous Buck Regulator Download 19 Pages Scroll/Zoom 100% Maker RENESAS [Renesas Technology Corp] Homepage http://www.renesas.com Logo

## ISL85012 Datasheet(HTML) 16 Page - Renesas Technology Corp

 16 / 19 page ISL85012FN8677 Rev.2.00Page 16 of 19Mar 17, 2017The simplified transfer function is derived in Equation 14.where:Note that Co is the actual capacitance seen by the regulator,which may include ceramic high frequency decoupling and bulkoutput capacitors. Ceramic may have to be derated byapproximately 40% depending on dielectric, voltage stress, andtemperature.Usually, a type II compensation network is used to compensatethe peak current mode control converter. Figure 38 shows atypical type II compensation network and its transfer function isexpressed in Equation 16. The frequency response is shown inFigure 39.where:Design example: VIN = 12V, VO = 1.8V, IO = 10A, fSW = 600kHz,R1 = 200kΩ, R2 = 100kΩ, Co = 3x100µF/3mΩ 6.3V ceramic(actually ~150µF), L = 0.68µH.Select fc = 80kHz. The gain of the Gp(s)xAv(s) should has a unitygain at crossover frequency. Thus, R3 can be derived as:Select 800kΩ for R3. Place the zero fz1 around the pole fp toachieve -20db/dec roll off.where Rc is the ESR of the output capacitor.Select 30pF for C2. Zero fz2 is a phase boost zero to increase thephase margin. Place it between fc and 1/2 switching frequency.In this case, 4.7pF capacitor is selected and the zero is placed atfz2:The calculated values for R1, R2, C1, and R3, C2 match with the1.8V output application in the recommended design with internalcompensation shown in Table 1 on page 2. Do not selectresistance higher than 370kΩ for R1 in real applications to avoidparasitic impaction.In practice, it is recommended to select lower resistance forR1/R2 and R3 in the external compensation applications.Usually, 10 times lower compared with the internalcompensation is a good start.FIGURE 38. TYPE II COMPENSATION NETWORKFIGURE 37. POWER TRAIN SMALL FREQUENCY RESPONSEGdcfpfzfcGp Svˆovˆcomp-----------------Gdc1Sz------+1Sp-------+-----------------==(EQ. 14)GdcRoRt-------z;=2fz1RcxCo------------------p;2fp1Ro Rc+xCo-------------------------------------====(EQ. 15)VrefVfbVoR1R2C1R3C2VcompAv Svˆcompvˆo-----------------1Scz1-------------+ 1Scz2-------------+SC2R1---------------------------------------------------------==(EQ. 16)cz12fz11R3C2---------------cz22fz2=1R1C1--------------- fpc12R1C2----------------------====FIGURE 39. POWER TRAIN FREQUENCY RESPONSEfcfz1fpcfz2R3/R1R32fcCoRtR1829k==(EQ. 17)C2Ro Rc+xCoR3--------------------------------------29pF==(EQ. 18)fz212R1C1----------------------169kHz==(EQ. 19)