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SC1408IMSTRT Datasheet(PDF) 6 Page - Semtech Corporation |
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SC1408IMSTRT Datasheet(HTML) 6 Page - Semtech Corporation |
6 / 12 page 6 2005 Semtech Corp. www.semtech.com POWER MANAGEMENT SC1408 COMPONENT SELECTION SEPIC Converter RSENSE Again, with the SEPIC topology, the value of the sense resistor is the primary determining factor for maximum output current. The simplest approach to select RSENSE is to add Vin to Vo and use this value as the output voltage in the output current curves or in the equation for Boost converter. Output Voltage Output voltage setting works exactly the same in SEPIC topology as in Boost, including the ability to set to 5V by connecting the FB pin to GND. Care must be taken to ensure that the IC supply (pin2; BST) does not exceed its 16.5V rating. In the circuit of Fig.2: This requires maxi- mum output voltage to be limited to 16.5V-Vin. Higher output voltages are possible with different IC supply strat- egies. Inductor The SEPIC topology requires a coupled inductor. Again A good choice for most applications is 22µH. Smaller induc- tor values result in higher peak currents and increase out- put ripple, while larger values will result in slower loop re- sponse. Transistor selection The choice of FET can be critical, especially in battery pow- ered applications where the converter must be able to use all of the available energy in the battery. This requires that the converter be capable of starting up from very low input voltages. For example a two cell alkaline system’s Applications Information (Cont.) terminal voltage will drop to 1.8V as it approaches full discharge. For these demanding applications, a FET with low V GS(th) is required. A good rule of thumb is that VGS(th) should be at least 0.5V less than the minimum input volt- age. Diode For most applications, a Schottky diode should be used as the output rectifier. It will be subjected to reverse volt- ages of at least V O +VIN and average current will be some- what less than the Inductor peak current. Industry stan- dard 1N5817 series or an equivalent surface mount part would be suitable. Output Capacitors Output capacitors should be low ESR to minimize ripple voltage and maximize efficiency. Low ESR tantalums, OSCONs or the newer Polymer capacitors should be used. Input Capacitors Input capacitors on a SEPIC converter are less critical than the output capacitors, since there are no fast cur- rent pulses drawn from the input supply. A 100 µF tanta- lum will be adequate for most applications. Series Capacitors The Series capacitor(s) must be capable of handling an RMS current given by:- IN O O RMS V 5 . 0 V I I + = |
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