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ADP3208D Datasheet(PDF) 28 Page  ON Semiconductor 

ADP3208D Datasheet(HTML) 28 Page  ON Semiconductor 
28 / 37 page ADP3208D http://onsemi.com 28 a calculated ripple current of 9.0 A. The inductor should not saturate at the peak current of 24.5 A, and it should be able to handle the sum of the power dissipation caused by the winding’s average current (20 A) plus the ac core loss. In this example, 330 nH is used. Another important factor in the inductor design is the DCR, which is used for measuring the phase currents. Too large of a DCR causes excessive power losses, whereas too small of a value leads to increased measurement error. For this example, an inductor with a DCR of 0.8 m W is used. Selecting a Standard Inductor After the inductance and DCR are known, select a standard inductor that best meets the overall design goals. It is also important to specify the inductance and DCR tolerance to maintain the accuracy of the system. Using 20% tolerance for the inductance and 15% for the DCR at room temperature are reasonable values 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. • Panasonic • Sumida Corporation • NEC Tokin Corporation Output Droop Resistance The design requires that the regulator output voltage measured at the CPU pins decreases when the output current increases. The specified voltage drop corresponds to the droop 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 low−pass filter. The summing is implemented by the CS amplifier that is configured with resistor RPH(x) (summer) and resistors RCS and CCS (filters). The output resistance of the regulator is set by the following equations: RO + RCS RPH(x) RSENSE (eq. 8) CCS + L RSENSE RCS (eq. 9) where RSENSE is the DCR of the output inductors. Either RCS or RPH(x) can be chosen for added flexibility. Due to the current drive ability of the CSCOMP pin, the RCS resistance should be greater than 100 k W. For example, initially select RCS to be equal to 200 kW, and then use Equation 9 to solve for CCS: CCS + 330 nH 0.8 mW 200 kW + 2.1 nF (eq. 10) If CCS is not a standard capacitance, RCS can be tuned. For example, if the optimal CCS capacitance is 1.5 nF, adjust RCS to 280 k W. For best accuracy, CCS should be a 5% NPO capacitor. In this example, a 220 k W is used for RCS to achieve optimal results. Next, solve for RPH(x) by rearranging Equation 8 as follows: (eq. 11) RPH(X) w 0.8 mW 2.1 mW @ 220 kW + 83.8 kW The standard 1% resistor for RPH(x) is 86.6 kW. Inductor DCR Temperature Correction If the DCR of the inductor is used as a sense element and copper wire is the source of the DCR, the temperature changes associated with the inductor’s winding must be compensated for. Fortunately, copper has a well−known temperature coefficient (TC) of 0.39%/ °C. If RCS is designed to have an opposite but equal percentage of change in resistance, it cancels the temperature variation of the inductor’s DCR. Due to the nonlinear nature of NTC thermistors, series resistors RCS1 and RCS2 (see Figure 42) are needed to linearize the NTC and produce the desired temperature coefficient tracking. Figure 42. Temperature−Compensation Circuit Values ADP3208D 17 19 18 CSCOMP CSSUM CSREF +  CCS2 R R R TH Place as close as possible to nearest inductor RR R To Switch Nodes To V Sense OUT Keep This Path As Short As Possible And Well Away From Switch Node Lines CCS1 CS2 CS1 PH2 PH1 PH3 The following procedure and expressions yield values for RCS1, RCS2, and RTH (the thermistor value at 25°C) for a given RCS value. 1. Select an NTC to be used based on its type and value. Because the value needed is not yet determined, start with a thermistor with a value close to RCS and an NTC with an initial tolerance of better than 5%. 2. Find the relative resistance value of the NTC at two temperatures. The appropriate temperatures will depend on the type of NTC, but 50 °C and 90 °C have been shown to work well for most types of NTCs. The resistance values are called A (A is RTH(50°C)/RTH(25°C)) and B (B is RTH(90°C)/RTH(25°C)). Note that the relative value of the NTC is always 1 at 25 °C. 3. Find the relative value of RCS required for each of the two temperatures. The relative value of RCS is based on the percentage of change needed, which is initially assumed to be 0.39%/ °C in this example. The relative values are called r1 (r1 is 1/(1+ TC × (T1 − 25))) and r2 (r2 is 1/(1 + TC × (T2 − 25))), where TC is 0.0039, T1 is 50°C, and T2 is 90°C. 
