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FAN5230 датащи(PDF) 11 Page - Fairchild Semiconductor |
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FAN5230 датащи(HTML) 11 Page - Fairchild Semiconductor |
11 / 16 page FAN5230 REV. 2.8.5 10/17/01 11 3.3V and 5V Under-voltage Protection When the output voltage of either the 3.3V or 5V falls below 75% of the nominal value, both converters, go into under-voltage (UV) protection, after a 2µsec delay. In under- voltage protection, the high and low side MOSFETs are turned off. Once under-voltage protection is triggered, it remains on until power is recycled or the SDWN pin is reset. 12V Architecture The 12V converter is a traditional non-isolated fly-back (also known as a "boost" converter). The converter’s input voltage is the +5V switcher output, so that +12V can only be present if +5V is present. Also, if the external MOSFET is off, the output of the +12V converter is +5V, not zero. This in turn will provide non-zero output for the 12V regulator. For complete turn-off of the 12V regulator an external P-channel MOSFET or an LDO regulator with on/off control may be used. If an LDO is used for 12V then the boost converter should be set to 13.2V using the external resistor divider network. If the 12V “boost” converter is not used, connect VFB12 (pin 15) to 5V-ALWAYS (pin 6). 12V Loop Compensation The 12V converter should be run in discontinuous conduc- tion mode. In this mode, the converter will be stable if a capacitor with suitable ESR value is selected. A 68uF tantalum with 500mA ripple current rating and 95m Ω is recommended here. 12V Protection The 12V converter is protected against overvoltage. If the 12V feedback is more than 10–15% above the nominal set voltage, a comparator forces the MOSFET off until the volt- age falls below the comparator threshold. The 12V converter is also protected against over-current. If a short circuit pulls the output below 9V, all of the switching converters go into UV protection, after a 2µs delay. In UV protection, all MOSFETs are turned off. Once UV protection is triggered, it remains on until the input power is recycled or the SDWN is reset. 12V Softstart and Sequencing The 12V output is started at the same time as the 5V output. The softly rising 5V output automatically generates a softly rising 12V output. The duty cycle of the 12V PWM is lim- ited to prevent excessive current draw. The 12V supply must build up a voltage higher than the UVLO limit (9V) by the time the 5V is above its UVLO (3.75V) in order to avoid triggering of UV protection during soft start. 5V/3.3V-ALWAYS Operation The 5V-ALWAYS supply is generated from either the on-chip linear regulator or through an internal switch from the VFB pin of the 5V switching supply. The 5V-ALWAYS supply should be decoupled to ground with a 10µF capacitor. When the 5V switching supply is off, or if its output voltage is not within tolerance, the 5V-ALWAYS switch is open, and the linear regulator is on. When the 5V switching supply is running and has an output voltage within specification, the linear regulator is off, and the switch is on. The switch has sufficiently low resistance that at maximum current draw on the 5V-ALWAYS supply, the output voltage is regulated within specifications. The 3.3V-ALWAYS is generated from a linear regulator attached internally to the 5V-ALWAYS. The 3.3V-ALWAYS supply should be decoupled to ground with a 10µF capacitor. The purpose of the two ALWAYS supplies (combined cur- rent is specified to never exceed 50mA) is to provide power to the system micro-controller (8051 class) as well as other IC’s needing a stand-by power. The micro-controller as well as the other IC’s could be operated from either 5V or 3.3V ALWAYS, so the FAN5230 provides both. 5V/3.3V-ALWAYS Protections The two internal linear regulators are current limited and under-voltage protected. Once protection is triggered all outputs are turned off until power is cycled or the SDWN is reset. Power good Power good is asserted when both PWM Buck converters are above specified threshold. No other regulators are monitored by Power good. When PGOOD goes low it will stay low for at least 10µsec (Tw). See fig. 5. Figure 5. PGOOD Timing Diagram Vmain t PGOOD t Vth Tw |
Аналогичный номер детали - FAN5230 |
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Аналогичное описание - FAN5230 |
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