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SP6137 Datasheet(PDF) 11 Page - Exar Corporation |
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SP6137 Datasheet(HTML) 11 Page - Exar Corporation |
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11 / 15 page  S SP P6 61 13 37 7 W Wiid de e IIn np pu utt,, 9 90 00 0k kH Hz z S Sy yn nc ch hrro on no ou us s P PW WM M S Stte ep p D Do ow wn n C Co on nttrro olllle err © 2008 Exar Corporation 11/15 Rev. 2.0.0 copper pad directly under the package, without occupying additional board space, can increase the maximum power from approximately 1 to 1.2W. For DPAK package, enlarging the tap mounting pad to 1 square inches reduces the RΘJA from 96°C/W to 40°C/W. LOOP COMPENSATION DESIGN The open loop gain of the whole system can be divided into the gain of the error amplifier, PWM modulator, buck converter output stage, and feedback resistor divider. In order to crossover at the selected frequency FCO, the gain of the error amplifier has to compensate for the attenuation caused by the rest of the loop at this frequency. The goal of loop compensation is to manipulate loop frequency response such that its gain crosses over 0db at a slope of -20db/dec. The first step of compensation design is to pick the loop crossover frequency. High crossover frequency is desirable for fast transient response, but often jeopardizes the system stability. Crossover frequency should be higher than the ESR zero but less than 1/5 of the switching frequency. The ESR zero is contributed by the ESR associated with the output capacitors and can be determined by: SCHOTTKY DIODE SELECTION When paralleled with the bottom MOSFET, an optional Schottky diode can improve efficiency and reduce noises. Without this Schottky diode, the body diode of the bottom MOSFET conducts the current during the non-overlap time when both MOSFETs are turned off. Unfortunately, the body diode has high forward voltage and reverse recovery problem. The reverse recovery of the body diode causes additional switching noises when the diode turns off. The Schottky diode alleviates these noises and additionally improves efficiency thanks to its low forward voltage. The reverse voltage across the diode is equal to input voltage, and the diode must be able to handle the peak current equal to the maximum load current. () ESR OUT ESR Z R C f π 2 1 = The next step is to calculated the complex conjugate poles contributed by the LC output filter, The power dissipation of the Schottky diode is determined by () LC f LC P π 2 1 = S NOL OUT F Diode F T I V P 2 = where When the output capacitors are of a Ceramic Type, the SP6137CU Evaluation Board requires a Type III compensation circuit to give a phase boost of 180° in order to counteract the effects of an under damped resonance of the output filter at the double pole frequency. TNOL = non-overlap time between GH and GL. VF = forward voltage of the Schottky diode. |
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