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SC339 Datasheet(PDF) 9 Page - Semtech Corporation |
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SC339 Datasheet(HTML) 9 Page - Semtech Corporation |
9 / 15 page 9 © 2006 Semtech Corp. www.semtech.com SC339 POWER MANAGEMENT This is a very severe example, since the output voltage is so low, therefore the allowable window is ver y small. See Solution 2 for an alternate circuit. For higher output voltages the components required will be less stringent. The input capacitance needs to be large enough to stop the input supply from collapsing below -5% (i.e., the design minimum) during output load steps. If the input to the pass MOSFET is not local to the supply bulk capacitance then additional bulk capacitance may be required. MOSFET selection: since the input voltage to the SC339 is 5V±5%, the minimum available gate drive is: V GS = (4.4 ― 1.1025) = 3.3V So a MOSFET rated for V GS = 2.7V will be required, with an R DS(ON)(MAX) (over-temperature) given by: (VIN(MIN) ― VOUT ) ( 1.14 ― 1.05) R DSON(MAX) = —————— = —————— = 36m Ω I OUT(MAX) 2.5 MOSFET SELECTION SO-8 Footprint Vgs Rds-on Imax FDS6682 1.7V 9m Ω 14A IRF7456 1.5V 6.5m Ω 16A 1206 Footprint Vgs Rds-on Imax Si5406DC 0.6V 20m Ω 9.5A NTHS5404 0.6V 25m Ω 7.2A SOT-23 Footprint Vgs Rds-on Imax IRLMS2002 1V 30m Ω 5.2A FDN337N (when Input is > 1.2V for 1.05V output) 0.7V 65m Ω 2.2A Solution 2: Using Passive Droop Passive droop allows us to use almost the full output tolerance window for transients, therefore making the output capacitor selection simpler and less expensive. The trade-offs are the cost of the droop resistor versus the reduction in output capacitor cost, and the reduction in headroom which impacts MOSFET selection. The top of the feedback chain connects to the input side of R DROOP, and the output is set for 1.075V. Thus at no load, V OUT will be 1.075V (or 1.05V + 2.4%) and at IOUT = 2.5A, V OUT will be 1.025V (or 1.05V - 2.4%). If 1% set resistors are used, the total DC error will be ±2.25% = 24mV. Thus, at no load, the minimum output voltage will be given by: V OUT(MIN_NO_LOAD) = 1.075 ― 0.024 = 1.051V This leaves 53.5mV for transient response, giving: 53.5mV = 21.4mΩ and, R ESR(MAX) = ———— 2.5A 2.5 • 1 • 10¯6 = 47μF C BULK(MIN) = ——————— 53.5 •10¯3 Instead of 2 x 100μF, 12m Ω capacitors, we can use 1 x 47μF, 15m Ω capacitor. Applications Information (Cont.) 1.075V 1.2V +/-5% IN 1.05V @ 2.5A C1 0.1uF R1 11.0k C3 100uF, 25mOhm POSCAP 4 5 6 R2 10.0k RDROOP 20mOhm Q1 IN GND EN DRV ADJ PGD 1 3 2 U1 SC339 |
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