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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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SP7656EN2-L/TR 데이터시트(PDF) 5 Page - Exar Corporation |
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SP7656EN2-L/TR 데이터시트(HTML) 5 Page - Exar Corporation |
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5 / 10 page  11/07/08 SP7656 PowerBlox Copyright 2008 EXAR Corporation 5 Overcurrent Protection Figure 2 – Overcurrent Protection Circuit The overcurrent protection circuit functions by monitoring the voltage across the internal P- Channel FET. When this voltage exceeds 0.3V (nominal), the overcurrent comparator triggers and the controller enters hiccup mode. Since the FET has nominal Rds(on) of 0.06Ω, assuming no temperature rise, the overcurrent will trigger at Iocp = 0.3V/0.06Ω=5A. To program a lower overcurrent use a resistor Rs between ISET and LX pins. Calculate Rs from: uA Kt Iocp Rs 30 06 . 0 15 . 1 3 . 0 Where: - 1.15 is a multiplier to calculate peak current - Iocp is the desired overcurrent protection, - 0.06Ω is nominal FET Rsd(on) at 25°C - Kt is a multiplier that calculates increase in Rds(on) for a given temperature Kt is 1.0 and 1.4 for the FET operating at 25°C and 150°C respectively. This accounts for 40% increase in FET Rds(on) as temperature is increased from 25°C to 150°C. Example: Calculate Rs for Iocp of 3.5A. FET operating temperature is 88°C. uA A Rs 30 2 . 1 06 . 0 5 . 3 15 . 1 3 . 0 Rs=340Ω Using the above equation there is good agreement between calculated and test results for Rs up to 3k. For Rs larger than 3k test results are lower than those predicted by the Rs equation due to circuit parasitics. Therefore the maximum value of Rs should be limited to 3kΩ. Using the ON/OFF Function via VFB The feedback pin serves a dual role of On/Off control. The MOSFET driver is disabled when a voltage greater than 1V is applied at FB pin. Maximum voltage rating of this pin is 5.5V. The controlling signal should be applied through a small signal diode to FB pin. Please note that an optional 10k bleeding resistor across the output helps keep the output capacitor discharged under no load condition. Programming the Output Voltage To program the output voltage, calculate R2 using the following equation: 1 6 . 0 200 2 Vout k R 0.6 is used as it is the reference voltage of the SP7656. 200k is a fixed-value and is the top resistor in the output set point resistor pair. In addition to being part of the voltage divider, it is part of the compensation network. R1 of the output voltage divider should always be 200kΩ. Soft Start Soft Start is preset internally to 5ms (nominal). Internal Soft Start eliminates the need for the external capacitor CSS that is commonly used to program this function. Input Capacitance Selection Select the input capacitor for Voltage, Capacitance, ripple current, ESR and ESL. Voltage rating is nominally selected to be approximately twice the input voltage. The RMS value of input capacitor current, assuming a low inductor ripple current (IRIP), can be calculated from: ) 1 ( D D Iout Icin In general total input voltage ripple should be maintained below 1.5% of VIN (not to exceed 180mV). Input voltage ripple has three components: ESR and ESL cause a step voltage drop upon turn on of the MOSFET. During the on time |
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