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A8586 Datasheet(PDF) 9 Page - Allegro MicroSystems |
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A8586 Datasheet(HTML) 9 Page - Allegro MicroSystems |
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9 / 19 page  Wide Input Voltage, Adjustable Frequency, 3.5 Amp Buck Regulator A8586, A8586-1, A8586-2 9 Allegro MicroSystems, LLC 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com Freewheeling Diode Protection If the freewheeling diode is missing or damaged (open), the SW pin is subject to unusually high negative voltages. This nega- tive voltage may cause the device to malfunction and could lead to damage. The A8586 includes protection circuitry to detect when the freewheeling diode is missing. If the SW pin is below −1.25 V typically, for more than 50 ns typically, the device enters hiccup mode after detecting 1 missing diode fault. Also, if the freewheeling diode is shorted, the device experiences extremely high currents through the high-side MOSFET. If this occurs, the device enters hiccup mode after detecting 1 shorted diode fault. Overvoltage Protection (OVP) The A8586 provides an always-on overvoltage protection that monitors VOUT, to protect against VOUT rising up at light loads due to high side switch leakage. In this case, the high-side switch is forced off and the low-side switch continues to operate and can correct the OVP condition provided only a few milliamperes of pull-down current are required. When the condition causing the overvoltage is corrected, the regulator automatically recovers. Pin-to-Ground and Pin-to-Pin Short Protec- tions The A8586 is designed to satisfy the most demanding automotive applications. For example, the device is carefully designed funda- mentally to withstand a short circuit to ground at each pin without suffering damage. In addition, care was taken when defining the device pin-out to optimize protection against adjacent pin-to- pin short circuits. For example, logic pins and high-voltage pins are separated as much as possible. Inevitably, some low-voltage pins had to be located adjacent to high-voltage pins, but in these instances the low-voltage pins are designed to withstand increased voltages, with clamps and/or series input resistance, to prevent damage to the device. Startup and Shutdown If both VIN and VEN are higher than their appropriate thresholds, the chip starts. The reference block starts first, generating stable reference voltage and currents, and then the internal regulator is enabled. The regulator provides stable supply for the remaining circuits. When the internal soft-start block is enabled, it first holds its SS output low to ensure the remaining circuits are ready and then slowly ramps up. Three events can shut down the chip: EN low, VIN low and ther- mal shutdown. In the shutdown procedure, power MOSFET is turned off first to avoid any fault triggering. The COMP voltage and the internal supply rail are then pulled down. Programmable Oscillator A resistor (RFREQ) from FREQ to ground sets the operation frequency of the A8586. The Applications Information section details the selection of this resistor. Frequency Dithering (A8586, A8586-2) The A8586 and A8586-2 include a dithering function, which changes the switching frequency within a certain frequency range. By shifting the switching frequency of the regulator in a triangle fashion around the programmed switching frequency, the overall system noise magnitude can be greatly reduced. The dithering sweep is internally set at ±8%. The switching frequency will ramp from a low of 0.92 times the programmed frequency to a high of 1.08 times the programmed frequency. The rate or modulation at which the frequency sweeps is governed by an internal 12 kHz triangle pattern. |
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