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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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SP7663 Datenblatt(PDF) 7 Page - Sipex Corporation |
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SP7663 Datenblatt(HTML) 7 Page - Sipex Corporation |
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7 / 19 page  7 Nov3-06 Rev L SP7663 Wide Input Voltage Range 6A, 600kHz Buck Regulator © 2006 Sipex Corporation THEORY OF OPERATION Therefore without external biasing the ViN start threshold is 9.5V. A small capacitor mayberequiredbetweenUVINandGNDto filteroutnoise.ForapplicationswithViN of 5V or 3.3V, connect UVIN directly to ViN. To program the ViN start threshold, use a pair of external resistors as shown. If external resistors are an order of magnitude smaller than internal resistors, then the ViN start thresholdisgivenby: ViN(start) = 2.5 V• (R4+R5)/R5 For example, if it is required to have a ViN startthresholdof7V,thenletR5=5KΩand using the ViN start threshold equation we get R4=9.09KΩ. Thermal and Short-Circuit Protection Because the SP7663 is designed to drive largeoutputcurrent,thereisachancethatthe power converter will become too hot. There- fore, an internal thermal shutdown (45°C) has been included to prevent the IC from malfunctioning at extreme temperatures. A short-circuit detection comparator has also been included in the SP7663 to protect against an accidental short at the output of the power converter. This comparator con- stantly monitors the positive and negative terminals of the error amplifier, and if the VfB pin falls more than 250mV (typical) below the positive reference, a short-circuit fault is set. Because the SS pin overrides the internal 0.8V reference during soft start, the SP7663 is capable of detecting short-circuit faults throughout the duration of soft start as well as in regular operation. Over-Current Protection The Over-current protection feature can onlybeusedonoutputvoltages≤3.3volts. It is limited by the common mode rating of theoperationalamplifierusedtosensethe voltage across the inductor. Over-current is detected by monitoring a differential voltage across the output inductor as shown in the nextfigure. ISN SP7663 SWN ISP VOUT L = 1.5uH, DCR = 9.2mΩ RS1 5.11KΩ RS2 5.11KΩ CS 0.1uF CSP 6.8nF Over-current detection circuit Inputs to an over-current detection com- parator, set to trigger at 60 mV nominal, are connected to the inductor as shown. Since the average voltage sensed by the comparator is equal to the product of in- ductor current and inductor DC resistance (DCR), then IMAx = 60mV / DCR. Solving thisequationforthespecificinductorincir- cuit , IMAx = 9.2A. When IMAx is reached, a 20 ms time-out is initiated, during which top and bottom drivers are turned off. Fol- lowing the time-out, a restart is attempted. If the fault condition persists, then the time- out is repeated (referred to as hiccup). Increasing the Current Limit If it is desired to set IMAx > {60mV / DCR} (in thiscaselargerthan9.6A),thenaresistor RS3 shouldbeaddedasshowninthenextfigure. RS3 forms a resistor divider and reduces the voltage seen by the comparator. Since:60mV (IMAx • DCR) RS3 = {RS + RS2 + RS3} SolvingforRS3weget: RS3 = [60mV • (RS1 + RS2)] [(IMAx • DCR) – 60mV] |
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