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FAN6520AIM Arkusz danych(PDF) 6 Page - Fairchild Semiconductor |
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FAN6520AIM Arkusz danych(HTML) 6 Page - Fairchild Semiconductor |
6 / 15 page ©2005 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6520A Rev. 1.0.5 6 Circuit Description Initialization The FAN6520A automatically initializes upon receipt of power. The Power-On Reset (POR) function continually monitors the bias voltage at the VCC pin. When the sup- ply voltage exceeds its POR threshold, the IC initiates the Over-Current Protection (OCP) sample-and-hold operation. Upon completion of the OCP sample-and-hold operation, the POR function initiates soft-start operation. Over-Current Protection The over-current function protects the converter from a shorted output by using the upper MOSFET’s on-resis- tance, RDS(ON), to monitor the current. This method enhances the converter’s efficiency and reduces cost by eliminating the need for a current-sensing resistor. The over-current function cycles the soft-start function in a hiccup mode to provide fault protection. A resistor (ROC- SET) programs the over-current trip level (see Typical Application diagram). Immediately following POR, the FAN6520A initiates the Over-Current Protection sam- pling-and-hold operation. First, the internal error ampli- fier is disabled. This allows an internal 20µA current sink to develop a voltage across ROCSET. The FAN6520A then samples this voltage at the COMP pin. This sam- pled voltage, which is referenced to the VCC pin, is held internally as the over-current set point. When the voltage across the upper MOSFET, which is also referenced to the VCC pin, exceeds the over-current set point, the over-current function initiates a soft-start sequence. Fig- ure 3 shows the inductor current after a fault is intro- duced while running at 15A. The continuous fault causes the FAN6520A to go into a hiccup mode with a typical period of 25ms. The inductor current increases to 18A during the soft-start interval and causes an over-current trip. The converter dissipates very little power with this method. The measured input power for the conditions shown in Figure 3 is 1.5W. Figure 3. Over-Current Operation The over-current function trips at a peak inductor current (IPEAK) determined by: where IOCSET is the internal OCSET current source (20µA typical). The OC trip point varies mainly due to the MOSFET’s RDS(ON) variations. To avoid over-current tripping in the normal operating load range, find the ROC- SET resistor from the equation above with: • The maximum RDS(ON) at the highest junction temperature • The minimum IOCSET from the specification table • Determine IPEAK for where ΔI is the output inductor ripple current. For an equation for the ripple current, see “Output Induc- tor (Lout)” under Component Selection. Internal circuitry of the FAN6520A does not recognize a voltage drop across ROCSET larger than 0.5V. Any volt- age drop across ROCSET greater than 0.5V sets the over- current trip point to: An over-current trip cycles the soft-start function. Soft-Start The POR function initiates the soft-start sequence after the over-current set point has been sampled. Soft-start clamps the error amplifier output (COMP pin) and refer- ence input (noninverting terminal of the error amp) to the internally generated soft-start voltage. Figure 4 shows a typical start-up interval where the COMP/OCSET pin has been released from a grounded (system shutdown) state. Initially, the COMP/OCSET is used to sample the over-current set point by disabling the error amplifier and drawing 20µA through ROCSET. Once the over-current level has been sampled, the soft-start function is initi- ated. The clamp on the error amplifier (COMP/OCSET pin) initially controls the converter’s output voltage during soft-start. The oscillator’s triangular waveform is com- pared to the ramping error amplifier voltage. This gener- ates SW pulses of increasing width that charge the output capacitor(s). When the internally generated soft- start voltage exceeds the feedback (FB pin) voltage, the output voltage is in regulation. This method provides a rapid and controlled output voltage rise. The entire start- up sequence typically takes about 11ms. OUTPUT INDUCTOR CURRENT 5A/DIV. I PEAK I OCSET R OCSET × R DS ON () ----------------------------------------------- = (1) I PEAK I OUT MAX () ΔI 2 ----- + > I PEAK 0.5V R DS ON () ---------------------- = (2) |
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