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SC122EVB Datasheet(PDF) 10 Page - Semtech Corporation |
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SC122EVB Datasheet(HTML) 10 Page - Semtech Corporation |
10 / 13 page SC122 10 Applications Information (continued) A suggested very low duty cycle refresh oscillator circuit is included on the SC122 EVB-RM, the SC122 Evaluation Board with Refresh Modulation. Regulator Startup, Short Circuit Protection, and Current Limits The SC122 permits power up at input voltages from 0.85V to 1.6V. Startup current limiting of the internal switching n-channel and p-channel FET power devices protects them from damage in the event of a short between OUT and GND. This protection prevents startup into an exces- sive load. At the beginning of the cycle, the p-channel FET between the LX and OUT pins turns on with its current limited to approximately 100mA, the short-circuit output current. When V OUT approaches V IN (still below 1.7V), the n-channel current limit is set to 350mA (the p-channel limit is dis- abled), an internal oscillator turns on (approximately 200kHz), and a fixed 75% duty cycle PWM-type operation begins. When the output voltage exceeds 1.7V, fixed fre- quency PSAVE operation begins, with the duty cycle deter- mined by an n-channel FET peak current limit of 350mA. Note that startup with a regulated active load is not the same as startup with a resistive load. The resistive load output current increases proportionately as the output voltage rises until it reaches V OUT /R LOAD , while a regulated active load presents a constant load as the output voltage rises from 0V to V OUT . Note also that if the load applied to the output exceeds the startup current limit, the criterion to advance to the next startup stage may not be achieved. In this situation startup may pause at a reduced output voltage until the load is reduced further. Output Overload and Recovery As the output load increases, the duration of each burst increases, and the time between bursts decreases. The output load reaches its maximum when the burst dura- tion becomes indefinite (and the time between bursts becomes zero). At this time, all the energy stored in the inductor during the on-time portion of each burst cycle is discharged into the output during off-time. The inductor current reduces to zero just as the next on-time begins. Above this critical maximum load, the output voltage will decrease rapidly, and the startup current and switching limits will be invoked in reverse order as the output voltage falls through its various startup voltage thresh- olds. How far the output voltage drops depends on the load voltage vs. current characteristic. A reduction in input voltage, such as a discharging battery, will lower the load current at which overload occurs. At the overload threshold, the energy stored in the inductor at the end of each on-time is the same for all V IN . But since the voltage increase above the input voltage is greater, the available output current, I OUT = P/(V OUT - V IN ), must decrease. When an overload has occurred, the load must be decreased to permit recovery. The conditions required for overload recovery are identical to those required for successful initial startup. Anti-ringing Circuitry When both FET switches are simultaneously turned off, an internal switch between the IN and LX pins is closed. This provides a moderate resistance path across the inductor to dampen the oscillations at the LX pin. This effectively reduces EMI that can develop from the reso- nant circuit formed by the inductor and the drain capaci- tance at LX. The anti-ringing circuitry is disabled between PSAVE bursts. Inductor Selection The inductance value primarily affects the amplitude of inductor current ripple ( ΔI L ). The inductor peak current I L-max = I L-avg + ΔI L /2, where I L-avg is the inductor current aver- aged over a full on/off cycle, is subject to the n-channel FET peak current limit I LIM(N) . The inductor average current is equal to the output load current. Increasing inductance reduces ΔI L and therefore increases the maximum sup- portable output current. The performance plots of this datasheet were obtained with L = 4.7μH. Larger values of inductance can provide higher maximum output currents. Any chosen inductor should have low DCR, compared to the R DS-ON of the FET switches, to maintain efficiency. For DCR << R DS-ON , further reduction in DCR will provide diminishing benefit. The inductor I SAT value must exceed I LIM(N) . The inductor self-resonant frequency should exceed |
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