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ADP3166 Datasheet(PDF) 11 Page - Analog Devices |
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ADP3166 Datasheet(HTML) 11 Page - Analog Devices |
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11 / 20 page  REV. 0 ADP3166 –11– Soft Start and Current Limit Latch-Off Delay Times Because the soft start and current limit latch-off delay functions share the DELAY pin, these two parameters must be considered together. The first step is to set CDLY for the soft start ramp. This ramp is generated with a 20 µA internal current source. The value of RDLY will have a second order impact on the soft- start time because it sinks part of the current source to ground. However, as long as RDLY is kept greater than 200 k Ω, this effect is minor. The value for CDLY can be approximated using C= A – V R t V DLY VID DLY SS VID 20 2 µ × × (2) where tSS is the desired soft start time. Assuming an RDLY of 390 k Ω and a desired a soft start time of 3 ms, CDLY is 36 nF. The closest standard value for CCS is 39 nF. Once CDLY has been chosen, RDLY can be calculated for the current limit latch off time using R= .t C DLY DLY DLY 196 × (3) If the result for RDLY is less than 200 k Ω , then a smaller soft start time should be considered by recalculating the equation for CDLY or a longer latch-off time should be used. In no case should RDLY be less than 200 k Ω. In this example, a delay time of 8 ms makes RDLY = 402 k Ω. The closest standard 5% value is 390 kΩ. Inductor Selection The choice of inductance for the inductor determines the ripple current in the inductor. Less inductance leads to more ripple current, which increases the output ripple voltage and conduc- tion losses in the MOSFETs but allows using smaller-size inductors and, for a specified peak-to-peak transient deviation, less total output capacitance. Conversely, a higher inductance means lower ripple current and reduced conduction losses, but requires larger-size inductors and more output capacitance for the same peak-to-peak transient deviation. In any multiphase converter, a practical value for the peak-to-peak inductor ripple current is less than 50% of the maximum dc current in the same inductor. Equation 4 shows the relationship between the induc- tance, oscillator frequency, and peak-to-peak ripple current in the inductor. Equation 5 can be used to determine the mini- mum inductance based on a given output ripple voltage: I= V– D f R VID SW × () × 1 L (4) L ≥ ×× × () () × VR – n D fV VID OD SW RIPPLE 1 (5) Solving Equation 5 for a 10 mV p-p output ripple voltage yields L ≥ ×Ω × () × 15 19 1 0 375 330 10 540 .V .m – . kHz mV =nH If the ripple voltage is less than that designed for, the inductor can be made smaller until the ripple value is met. This will allow opti- mal transient response and minimum output decoupling. The smallest possible inductor should be used to minimize the number of output capacitors. A 600 nH inductor is a good choice for a starting point, and it gives a calculated ripple cur- rent of 6.6 A. The inductor should not saturate at the peak current of 22 A, and should be able to handle the sum of the power dissipation caused by the average current of 18.7 A in the winding and the core loss. Another important factor in the inductor design is the DCR, which is used for measuring the phase currents. A large DCR will cause excessive power losses, while too small a value will lead to increased measurement error. A good rule is to have the DCR be about 1 to 1 1/2 times the static droop resistance (RO). For our example, we are using an inductor with a DCR of 1.6 m Ω. Designing an Inductor Once the inductance and DCR are known, the next step is either to design an inductor or to find 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. Using 20% for the inductance and 8% for the DCR (at room temperature) are rea- sonable tolerances that most manufacturers can meet. The first decision in designing the inductor is to choose the core material. There are several possibilities for providing low core loss at high frequencies. Two examples are the powder cores (e.g., Kool-M µ® from Magnetics, Inc. or Micrometals) and the gapped soft ferrite cores (e.g., 3F3 or 3F4 from Philips). Low frequency powdered iron cores should be avoided due to their high core loss, especially when the inductor value is relatively low and the ripple current is high. The best choices for a core geometry are closed-loop types, such as pot cores, PQ, U, and E cores, or toroids. A good compromise between price and performance are cores with a toroidal shape. There are many useful references for quickly designing a power inductor, such as • Magnetic Designer Software Intusoft (http://www.intusoft.com) • Designing Magnetic Components for High-Frequency DC-DC Converters McLyman, Kg Magnetics ISBN 1-883107-00-8 |
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