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SC4501MSETRT Datasheet(PDF) 9 Page - Semtech Corporation |
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SC4501MSETRT Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 21 page  9 © 2005 Semtech Corp. www.semtech.com POWER MANAGEMENT SC4501 Duty Cycle The duty cycle D of a boost converter is: D OUT CESAT D OUT IN V V V 1 V V V 1 D + − + − = (3) where V CESAT is the switch saturation voltage and VD is voltage drop across the rectifying diode. Maximum Output Current In a boost switching regulator the inductor is connected to the input. The DC inductor current is the input current. When the power switch is turned on, the inductor current flows into the switch. When the power switch is off, the inductor current flows through the rectifying diode to the output. The output current is the average diode current. The diode current waveform is trapezoidal with pulse width (1 – D)T (Figure 5). The output current available from a boost converter therefore depends on the converter operating duty cycle. The power switch current in the SC4501 is internally limited to 2A. This is also the maximum inductor or the input current. By estimating the conduction losses in both the switch and the diode, an expression of the maximum available output current of a boost converter can be derived: ( ) − − − − = IN CESAT D D OUT IN LIM OUTMAX V V V D V 45 D 1 V V I I (4) where I LIM is the switch current limit. It is worth noting that I OUTMAX is directly proportional to the ratio OUT IN V V . Equation (4) over-estimates the maximum output current at high frequencies (>1MHz) since switching losses are neglected in its derivation. Nevertheless it is a useful first-order approximation. Using V CESAT = 0.3V, VD = 0.5V and ILIM = 2A in (3) and (4), the maximum output currents for three V IN and VOUT combinations are shown in Table 1. V IN ( V ) V OUT ( V ) D I OUTMAX ( A ) 2.5 12 0.820 0.35 3.3 5 0.423 1.14 5 12 0.615 0.76 Considerations for High Frequency Operation The operating duty cycle of a boost converter decreases as V IN approaches VOUT. The PWM modulating ramp in a current-mode switching regulator is the sensed current ramp of the control switch. This current ramp is absent unless the switch is turned on. The intersection of this ramp with the output of the voltage feedback error amplifier determines the switch pulse width. The propagation delay time required to immediately turn off the switch after it is turned on is the minimum switch on time. Regulator closed-loop measurement shows that the SC4501 has a minimum on time of about 150ns at room temperature. The power switch in the SC4501 is either not turned on at all or for at least 150ns. If the required switch on time is shorter than the minimum on time, the regulator will either skip cycles or it will start to jitter. Example: Determine the maximum operating frequency of a Li-ion cell to 5V converter using the SC4501. Assuming that V D=0.5V, VCESAT=0.3V and VIN=2.6 - 4.2V, the minimum duty ratio can be found using (3). 25 . 0 5 . 0 5 3 . 0 1 5 . 0 5 2 . 4 1 D MIN = + − + − = Application Information Figure 5. Current Waveforms in a Boost Regulator Inductor Current Switch Current Diode Current ON ON OFF ON ON OFF ON OFF IOUT IIN (1-D)T DT Table 1. Calculated Maximum Output Current [ Equation (4)] |
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