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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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SP7662 Arkusz danych(PDF) 7 Page - Sipex Corporation |
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SP7662 Arkusz danych(HTML) 7 Page - Sipex Corporation |
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7 / 19 page  7 Nov3-06 Rev AE SP7662 Wide Input Voltage Range 2A, 300kHz 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 • (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 SP7662 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 SP7662 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 SP7662 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 the op-amp used to sense the voltage across the inductor. Over-current is de- tected by monitoring a differential voltage across the output inductor as shown in the nextfigure. ISN SP7662 SWN ISP VOUT L=2.7uH,DCR=4.1mΩ RS 5.11KΩ RS2 5.11KΩ CS 0.uF 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 com- parator is equal to the product of inductor current and inductor DC resistance (DCR), then IMAx = 60mV / DCR. Solving this equa- tionforthespecificinductorincircuit1,IMAx = 4.6A. When IMAx is reached, a 220 ms time-out is initiated, during which top and bottom drivers are turned off. Following the time-out, a restart is attempted. If the fault condition persists, then the time-out is re- peated (referred to as hiccup). Increasing the Current Limit If it is desired to set IMAx > {60mV / DCR} (in this case larger than 4.6A), then a resistor RS3 should be added as shown in the next figure.RS3 forms a resistor divider and re- duces the voltage seen by the comparator. Since:60mV (IMAx • DCR) RS3 = {RS + RS2 + RS3} |
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