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SC2612E Datasheet(PDF) 5 Page - Semtech Corporation |
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SC2612E Datasheet(HTML) 5 Page - Semtech Corporation |
5 / 11 page 5 2004 Semtech Corp. www.semtech.com POWER MANAGEMENT SC2612E Theory of Operation The SC2612E is a step down DC/DC controller designed for minimum cost and size without sacrificing accuracy and protection. Overcurrent protection is implemented by a simple undervoltage detection scheme and is dis- abled until soft start has been completed to eliminate false trips due to output capacitor charging. The SS/EN pin is held low, as are the DH and DL pins, until the undervoltage lockout points are exceeded. Once the VCC and BST pins both rise above their undervoltage lockout points, the SS capacitor begins to charge, controlling the duty cycle of the switcher, and therefore slowly ramping up the switcher output voltage. Once the SS capacitor is charged, the current limit circuitry is enabled. If a short circuit is applied , the output will be pulled down below it’s trip point and shut down. The device may be restarted by either cycling power, or momentarily pulling SS/EN low. Component Selection OUTPUT INDUCTOR OUTPUT INDUCTOR OUTPUT INDUCTOR OUTPUT INDUCTOR OUTPUT INDUCTOR - A good starting point for output filter component selection is to choose an inductor value that will give an inductor ripple current of approximately 20% of max. output current. Inductor ripple current is given by:- OSC IN O O RIPPLE L f L V V V I × ÷÷ø ö ççè æ - × = 1 So choose inductor value from:- OSC O IN O O f I V V V L × ÷÷ø ö ççè æ - × × = 1 5 OUTPUT CAP OUTPUT CAP OUTPUT CAP OUTPUT CAP OUTPUT CAPAAAAACIT CIT CIT CIT CITOR(S) OR(S) OR(S) OR(S) OR(S) - The output capacitors should be selected to meet output ripple and transient response criteria. Output ripple voltage is caused by the inductor ripple current flowing in the output capacitor’s ESR (There is also a component due to the inductor ripple current charging and discharging the output capacitor itself, but this component is usually small and can often be ignored). Given a maximum output voltage ripple requirement, ESR is given by:- OSC IN O RIPPLE O ESR f L V V V V R × ÷÷ø ö ççè æ - × × < 1 Output voltage transient excursions are a function of load current transient levels, input and output voltages and inductor and capacitor values. Capacitance and R ESR values to meet a required tran- sient condition can be calculated from:- release) (load transients positive for V V and n) applicatio (load transients negative for V V V where V V I L C I V R O A O IN A A T T T T ESR = - = × × × > < 2 2 values for positive and negative transients must be cal- culated seperately and the worst case value chosen. For Capacitor values, the calculated value should be doubled to allow for duty cycle limitation and voltage drop issues. |
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