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SC1173 Datasheet(PDF) 11 Page - Semtech Corporation |
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SC1173 Datasheet(HTML) 11 Page - Semtech Corporation |
11 / 12 page PROGRAMMABLE SYNCHRONOUS DC/DC CONVERTER WITH LOW DROPOUT REGULATOR CONTROLLER SC1172/3 © 1999 SEMTECH CORP. October 25, 1999 652 MITCHELL ROAD NEWBURY PARK CA 91320 11 FET type R DS(on) (mΩ) P D (W ) Package BUK556H 22 2.48 TO220 IRL2203 7.0 0.79 D 2PAK Si4410 13.5 1.53 SO-8 ) 1 ( R I P ) on ( DS 2 O COND δ − ⋅ ⋅ = FET type R DS(on) (mΩ) P D (W ) Package BUK556H 22 1.95 TO220 IRL2203 7.0 0.62 D 2PAK Si4410 13.5 1.20 SO-8 BOTTOM FET - Bottom FET losses are almost entirely due to conduction. The body diode is forced into conduc- tion at the beginning and end of the bottom switch con- duction period, so when the FET turns on and off, there is very little voltage across it, resulting in low switching losses. Conduction losses for the FET can be deter- mined by: For the example above: Each of the package types has a characteristic thermal impedance, for the TO-220 package, thermal impedance is mostly determined by the heatsink used. For the sur- face mount packages on double sided FR4, 2 oz printed circuit board material, thermal impedances of 40 oC/W for the D 2PAK and 80oC/W for the SO-8 are readily achievable. The corresponding temperature rise is de- tailed below: Temperature rise ( oC) FET type Top FET Bottom FET BUK556H 49.6 (1) 39.0 (1) IRL2203 31.6 24.8 Si4410 122.4 96 (1) With 20 oC/W Heatsink It is apparent that single SO-8 Si4410 are not adequate for this application, but by using parallel pairs in each posi- tion, power dissipation will be approximately halved and temperature rise reduced by a factor of 4. INPUT CAPACITORS - since the RMS ripple current in the input capacitors may be as high as 50% of the out- put current, suitable capacitors must be chosen ac- cordingly. Also, during fast load transients, there may be restrictions on input di/dt. These restrictions require useable energy storage within the converter circuitry, either as extra output capacitance or, more usually, additional input capacitors. Choosing low ESR input capacitors will help maximize ripple rating for a given size. |
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