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ADP3207 Datasheet(PDF) 20 Page  ON Semiconductor 

ADP3207 Datasheet(HTML) 20 Page  ON Semiconductor 
20 / 29 page ADP3207 Rev. 1  Page 20 of 29  www.onsemi.com Solving Equation 6 for a 20 mV peaktopeak output ripple voltage yields (( ) ) () nH 356 mV 20 kHz 280 061 . 0 1 061 . 0 2 1 m 1 . 2 V 150 . 1 = × − × × − × Ω × ≥ L If the ripple voltage ends up being less than the initially selected value, then the inductor can be changed to a smaller value until the ripple value is met. This iteration allows optimal transient response and minimum output decoupling. The smallest possible inductor should be used to minimize the number of output capacitors. For this example, choosing a 360 nH inductor is a good starting point, and gives a calculated ripple current of 10.7 A. The inductor should not saturate at the peak current of 27.4 A, and should be able to handle the sum of the power dissipation caused by the average current of 16 A in the winding and core loss. Another important factor in the inductor design is the DCR, which is used to measure phase currents. A large DCR causes excessive power losses, though too small a value leads to increased measurement error. This example uses an inductor with a DCR of 0.89 m Ω. Selecting a Standard Inductor Once the inductance and DCR are known, the next step is to either design an inductor or select a standard inductor that comes as close as possible to meeting the overall design goals. It is also important to have the inductance and DCR tolerance specified to keep the accuracy of the system controlled; 20% inductance and 15% DCR (at room temperature) are reasonable assumptions that most manufacturers can meet. Power Inductor Manufacturers The following companies provide surface mount power inductors optimized for high power applications upon request: • Vishay Dale Electronics, Inc. http://www.vishay.com • Panasonic http://www.panasonic.com • Sumida Corporation http://www.sumida.com • NEC Tokin Corporation http://www.nectokin.com Output Droop Resistance The inductor design requires that the regulator output voltage measured at the CPU pins drops when the output current increases. The specified voltage drop corresponds to a dc output resistance (RO). The output current is measured by summing the currents of the resistors monitoring the voltage across each inductor and by passing the signal through a lowpass filter. This summerfilter is implemented by the CS amplifier that is configured with resistors RPH(X) (summer), and RCS and CCS (filter). The output resistance of the regulator is set by the following equations, where RL is the DCR of the output inductors: L X PH CS O R R R R × = ) ( (7) CS L CS R R L C × = (8) Users have the flexibility of choosing either RCS or RPH(X). Due to the current drive ability of the CSCOMP pin, the RCS resistance should be larger than 100 k Ω. For example, users should initially select RCS to be equal to 220 kΩ, then use Equation 8 to solve for CCS nF 84 . 1 k 220 m 89 . 0 nH 360 = Ω × Ω = CS C Because CCS is not the standard capacitance, it is implemented with two standard capacitors in parallel: 1.8 nF and 47 pF. For the best accuracy, CCS should be a 5% NPO capacitor. Next, solve RPH(X) by rearranging Equation 7. Ω = Ω × Ω Ω ≥ k 2 . 93 k 220 m 1 . 2 m 89 . 0 ) ( X PH R The standard 1% resistor for RPH(X) is 93.1 kΩ. Inductor DCR Temperature Correction With the inductor DCR used as a sense element, and copper wire being the source of the DCR, users need to compensate for temperature changes in the inductor’s winding. Fortunately, copper has a wellknown temperature coefficient (TC) of 0.39%/°C. If RCS is designed to have an opposite sign but equal percentage change in resistance, then it cancels the temperature variation of the inductor DCR. Due to the nonlinear nature of NTC thermistors, series resistors, RCS1 and RCS2 (see Figure 11) are needed to linearize the NTC and produce the desired temperature coefficient tracking. PLACE AS CLOSE AS POSSIBLE TO NEAREST INDUCTOR OR LOW–SIDE MOSFET KEEP THIS PATH AS SHORT AS POSSIBLE AND WELL AWAY FROM SWITCH NODE LINES TO SWITCH NODES TO VOUT SENSE CSREF CSSUM CSCOMP ADP3207 18 17 16 RPH1 RTH RCS1 CCS RCS2 RPH2 RPH3 Figure 11. Temperature Compensation Circuit Values 
