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MAX15053 Datasheet(PDF) 18 Page - Maxim Integrated Products |
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MAX15053 Datasheet(HTML) 18 Page - Maxim Integrated Products |
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18 / 21 page  ( ) ( ) LOAD S SW C CO OUT MV MC LOAD S LOAD SW R K 1 D 0.5 1 Lf R1 R2 R 2f C R2 g g R 1 ESR K 1 D 0.5 1 R Lf −− + × + = × ×π × ×× + −− + × and where the ESR is much smaller than the parallel combination of the equivalent load resistance and the current loop impedance, e.g.,: ( ) S LOAD SW 1 ESR K 1 D 0.5 1 R Lf << −− + × RC becomes: CO OUT C MV MC 2f C R1 R2 R R2 g g π× + = × × 3) Determine CC by selecting the desired first sys- tem zero, fZ1, based on the desired phase margin. Typically, setting fZ1 below 1/5 of fCO provides suf- ficient phase margin. CO Z1 CC f 1 f 2 CR 5 = ≤ π× therefore: C CO C 5 C 2f R ≥ π× × 4) For low duty-cycle applications, the addition of a phase-leading capacitor (CFF in Figure 1) helps mitigate the phase lag of the damped half-frequency double pole. Adding a second zero near to but below the desired crossover frequency increases both the closed-loop phase margin and the regulator’s unity- gain bandwidth (crossover frequency). Select the capacitor as follows: ( ) FF CO 1 C 2 f R1|| R2 = π× × This guarantees the additional phase-leading zero occurs at a frequency lower than fCO from: PHASE_LEAD FF 1 f 2 C R1 = π× × Using CFF the zero-pole order is adjusted as follows: P1 P2 Z1 FF FF CO P3 Z2 11 ff f 2 C R1 2 C (R1|| R2) f ff < ≤ < < ≈ π π ≤< Confirm the desired operation of CFF empirically. The phase lead of CFF diminishes as the output voltage is a smaller multiple of the reference voltage, e.g., below about 1V. Do not use CFF when VOUT = VFB. Setting the Soft-Start Time The soft-start feature ramps up the output voltage slowly, reducing input inrush current during startup. Size the CSS capacitor to achieve the desired soft-start time, tSS, using: SS SS SS FB It C V × = ISS, the soft-start current, is 10FA (typ) and VFB, the out- put feedback voltage threshold, is 0.6V (typ). When using large COUT capacitance values, the high-side current limit can trigger during the soft-start period. To ensure the correct soft-start time, tSS, choose CSS large enough to satisfy: OUT SS SS OUT HSCL OUT FB VI CC (I I ) V × >> × − × IHSCL is the typicalhigh-sideMOSFETcurrent-limitvalue. An external tracking reference with steady-state value between 0V and VIN-1.8VcanbeappliedtoSS/REFIN. In this case, connect an RC network from external track- ing reference and SS/REFIN, as shown in Figure 4. The recommended value for RSS is approximately 1kI. RSS is needed to ensure that, during hiccup period, SS/REFINcanbeinternallypulleddown. WhenanexternalreferenceisconnectedtoSS/REFIN, the soft-start must be provided externally. Figure 4. RC Network for External Reference at SS/REFIN CSS RSS VREF_EXT SS/REFIN MAX15053 MAX15053 High-Efficiency, 2A, Current-Mode Synchronous, Step-Down Switching Regulator www.maximintegrated.com Maxim Integrated │ 18 |
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