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SP6659 Datasheet(PDF) 7 Page - Sipex Corporation |
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SP6659 Datasheet(HTML) 7 Page - Sipex Corporation |
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7 / 10 page  7 Date: 07/20/05 SP6659 1.5MHz, 600mA Synchronous Buck Regulator © Copyright 2005 Sipex Corporation _______________________ DROPOUT OPERATION When the input voltage decreases toward the value of the output voltage, the SP6659 allows the main switch to remain on for more than one switching cycle and increases the duty cycle until it reaches 100%. The duty cycle D of a step-down converter is defined as: D = T ON X ƒOSC X 100% ≈ VOUT X 100% V IN THEORY OF OPERATION ________________________ MAXIMUM LOAD CURRENT The SP6659 will operate with an input sup- ply voltage as low as 2.5V, however, the maximum load current decreases at lower inputs due to a large IR drop on the main switch and synchronous rectifier. The slope compensation signal reduces the peak in- ductor current as a function of the duty cycle to prevent sub-harmonic oscillations at duty cycles greater than 50%. Conversely, the current limit increases as the duty cycle decreases. _______________________ INDUCTOR SELECTION For most designs, the SP6659 operates with inductors of 1µH to 4.7µH. Low inductance values are physically smaller but require faster switching, which can result in some efficiency loss. The inductor value can be derived from the following equation: L = VOUT X (VIN - VOUT) VIN X ∆IL X ƒ OSC Where ∆IL is the inductor Ripple Current. Large value inductors lower ripple current and small value inductors result in higher ripple current. Choose inductors at ripple current of approximately 35% of the maxi- mum load current 600mA, or ~ 210mA. For output voltages above 2.0V, when light- load efficiency is important, the minimum recommended inductor is 2.2µH. For opti- mum voltage-positioning load transients, choose an inductor with DC series resis- tance in the 50mΩ to 150mΩ range. For higher efficiency at heavy loads (above where TON is the main switch on time, and ƒ OSC is the oscillator frequency (1.5Mhz). The output voltage then is the input voltage minus the voltage drop across the main switch and the inductor. At low input supply voltage, the RDSON of the P-Channel MOSFET increases, and the efficiency of the converter decreases. Caution must be exercised to ensure that the heat dissipated does not exceed the maximum junction tem- perature of the IC. Figure 8. SP6659 Idle Mode Operation |
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