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AME5286 Datasheet(PDF) 7 Page - Analog Microelectronics |
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AME5286 Datasheet(HTML) 7 Page - Analog Microelectronics |
7 / 20 page AME 7 AME5286 Rev. A.02 3A, 300KHz ~ 2MHz Synchronous Rectified Step-Down Converter Normal Operation The AME5286 uses a user adjustable frequency, cur- rent mode step-down architecture with internal MOSFET switch. During normal operation, the internal high-side (PMOS) switch is turned on each cycle when the oscilla for sets the SR latch, and turned off when the comparator resets the SR latch. The peak inductor current at which comparator resets the SR latch is controlled by the out- put of error amplifier EA. While the high-side switch is off, the low-side switch turns on until either the inductor current starts to reverse or the beginning of the next switching cycle. Dropout Operation The output voltage is dropped from the input supply for the voltage which across the high-side switch. As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply volt- age forces the high-side switch to remain on for more than one cycle until it reaches 100% duty cycle. Power Good Output The AME5286 power good output is an open-drain out- put and requires a pull up resistor. When the output volt- age is 12.5% above or 12.5% below its set voltage, PGOOD will be pulled low. It is held low until the output voltage returns to within the allowed tolerances once more. During soft-start, PGOOD is actively held low and is only allowed to transition high when soft-start is over and the output voltage reaches 87.5% of its set voltage. Soft-Start The AME5286 contains an external soft-start clamp that gradually raises the output voltage. The soft-start timing is programmed by the external capacitor between SS pin and GND. The chip provides an internal charge current for the external capacitor. If 10nF capacitor is used to set the soft-start, the period will be 800uS(typ.). n Detailed Description Hiccup Mode During hiccup mode, the AME5286 disables the high- side MOSFET and begins a cool down period of 8000uS . At the conclusion of this cool down period, the regula- tor performs an external 800uS identical to the soft start at turn-on. ( If 10nF capacitor is used to set the soft-start) Under Voltage Protection Under Voltage Protection will activate once the feed- back voltage falls below 0.4V, the operating frequency is switched to 1/10 of normal switching frequency and after four-times hiccup mode counted, the internal high-side power switch will be turned off, and latched, Unless Re- start the power supply. Over Current Protection The AME5286 cycle-by-cycle limits the peak inductor current to protect embedded switch from damage. Hence the maximum output current (the average of inductor cur- rent) is also limited. In case the load increases, the in- ductor current is also increase. Whenever the current limit level is reached, the output voltage can not be regu- lated and starting to drop. Over Temperature Protectiion The in most applications the AME5286 does not dissi- pate much heat due to high efficiency. But, in applica- tions where the AME5286 is running at high ambient tem- perature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junc- tion temperature reaches approximately 160oC, the inter- nal high-side power switch will be turned off and the SW switch will become high impedance. |
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