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FAN5234MTCX датащи(PDF) 7 Page - Fairchild Semiconductor |
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FAN5234MTCX датащи(HTML) 7 Page - Fairchild Semiconductor |
7 / 15 page © 2004 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5234 • Rev. 2.0.0 7 Functional Description Overview The FAN5234 is a PWM controller intended for low- voltage power applications in notebook, desktop, and sub-notebook PCs. The output voltage of the controller can be set in the range of 0.9V to 5.5V by an external resistor divider. The synchronous buck converter can operate from an unregulated DC source (such as a notebook battery), with voltage ranging from 2V to 24V, or from a regulated system rail. In either case, the IC is biased from a +5V source. The PWM modulator uses an average-current- mode control with input voltage feed-forward for simplified feedback loop compensation and improved line regulation. The controller includes integrated feedback loop compensation that dramatically reduces the number of external components. Depending on the load level, the converter can operate in fixed-frequency PWM Mode or in Hysteretic Mode. Switch-over from PWM to Hysteretic Mode improves the converters' efficiency at light loads and prolongs battery run time. In Hysteretic Mode, a comparator is synchronized to the main clock to allow seamless transition between the operational modes and reduced channel-to-channel interaction. The Hysteretic Mode of operation can be inhibited independently using the FPWM pin if variable frequency operation is not desired. Oscillator Table 1. Converter Operating Modes Mode fSW Converter Power VIN Pin Battery 300 2 to 24V Battery (>5V) Fixed 300 300 <5.5V Fixed 100K Ω to GND Fixed 600 600 <5.5V Fixed GND When VIN is from the battery, the oscillator ramp amplitude is proportional to VIN, providing voltage feed- forward control for improved loop response. When in either of the fixed modes, oscillator ramp amplitude is fixed. The operating frequency is determined according to the connection on the VIN pin (see Table 1). Initialization and Soft Start Assuming EN is HIGH, FAN5234 is initialized when VCC exceeds the rising UVLO threshold. Should VCC drop below the UVLO threshold, an internal power-on reset function disables the chip. The voltage at the positive input of the error amplifier is limited by the voltage at the SS pin, which is charged with 5mA current source. Once CSS has charged to VREF (0.9V), the output voltage is in regulation. The time it takes SS to reach 0.9V is: 5 xC 9 . 0 t SS 9 . 0 = (1) where t0.9 is in seconds if CSS is in µF. When SS reaches 1.5V, the power-good outputs are enabled and Hysteretic Mode is allowed. The converter is forced into PWM Mode during soft-start. Operation Mode Control The mode-control circuit changes the converter’s mode from PWM to Hysteretic and vice versa based on the voltage polarity of the SW node when the lower MOSFET is conducting and just before the upper MOSFET turns on. For continuous inductor current, the SW node is negative when the lower MOSFET is conducting and the converters operate in fixed- frequency PWM Mode, as shown in Figure 4. This mode achieves high efficiency at nominal load. When the load current decreases to the point where the inductor current flows through the lower MOSFET in the “reverse” direction, the SW node becomes positive and the mode is changed to Hysteretic, which achieves higher efficiency at low currents by decreasing the effective switching frequency. To prevent accidental mode change or "mode chatter," the transition from PWM to Hysteretic Mode occurs when the SW node is positive for eight consecutive clock cycles (see Figure 4). The polarity of the SW node is sampled at the end of the lower MOSFET conduction time. At the transition between PWM and Hysteretic Mode, both the upper and lower MOSFETs are turned off. The SW node “rings” based on the output inductor and the parasitic capacitance on the SW node and settles out at the value of the output voltage. The boundary value of inductor current, where current becomes discontinuous, is estimated by the following: ⎟⎟ ⎟ ⎠ ⎞ ⎜⎜ ⎜ ⎝ ⎛ − = IN OUT SW OUT OUT IN ) DIS ( LOAD V L f 2 V ) V V ( I (2) Hysteretic Mode Conversely, the transition from Hysteretic Mode to PWM Mode occurs when the SW node is negative for eight consecutive cycles. A sudden increase in the output current causes a change from Hysteretic to PWM Mode. This load increase causes an instantaneous decrease in the output voltage due to the voltage drop on the output capacitor ESR. If the load causes the output voltage (as presented at VSEN) to drop below the hysteretic regulation level (20mV below VREF), the mode is changed to PWM on the next clock cycle. In Hysteretic Mode, the PWM comparator and the error amplifier that provide control in PWM Mode are inhibited and the hysteretic comparator is activated. In Hysteretic Mode the low-side MOSFET is operated as a synchronous rectifier, where the voltage across (VDS(ON)) is monitored and it is switched off when VDS(ON) goes positive (current flowing back from the load), allowing the diode to block reverse conduction. |
Аналогичный номер детали - FAN5234MTCX |
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Аналогичное описание - FAN5234MTCX |
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