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FAN6520AMX Datenblatt(PDF) 6 Page - Fairchild Semiconductor |
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FAN6520AMX Datenblatt(HTML) 6 Page - Fairchild Semiconductor |
6 / 15 page 6 REV. 1.0.2 8/26/04 FAN6520A PRODUCT SPECIFICATION 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 supply voltage exceeds its POR threshold, the IC initiates the Over-current Protection (OCP) sample and hold operation. Upon completion of the OCP sampling and hold operation, the POR function initiates the soft-start operation. Over-Current Protection The over-current function protects the converter from a shorted output by using the upper MOSFET’s on-resistance, 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 (ROCSET) programs the over- current trip level (see Typical Application diagram). Immediately following POR, the FAN6520A initiates the Over-current Protection sampling and hold operation. First, the internal error amplifier is disabled. This allows an inter- nal 20µA current sink to develop a voltage across ROCSET. The FAN6520A then samples this voltage at the COMP pin. This sampled 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. Figure 3 shows the inductor current after a fault is introduced 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 inter- val 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 only 1.5W. Figure 3. Over-Current Operation The over-current function will trip 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 ROCSET resistor from the equation above with: 1. The maximum RDS(ON) at the highest junction temperature. 2. The minimum IOCSET from the specification table. 3. Determine IPEAK for , where ∆I is the output inductor ripple current. For an equation for the ripple current see “Output Inductor (Lout)” under Component Selection. Internal circuitry of the FAN6520A will not recognize a volt- age drop across ROCSET larger than 0.5V. Any voltage drop across ROCSET that is greater than 0.5V will set the overcur- rent trip point to: An overcurrent 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 reference 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 setpoint by disabling the error amplifier and drawing 20µA through ROCSET. Once the over-current level has been sampled, the soft-start function is initiated. 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 compared to the ramping error amplifier voltage. This generates SW pulses of increasing width that charge the output capacitor(s). When the inter- nally 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 startup 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 () ∆I 2 ----- + > I PEAK 0.5V R DS ON () ---------------------- = |
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