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FPF2195 Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FPF2195 Datasheet(HTML) 11 Page - Fairchild Semiconductor |
11 / 14 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FPF2193/4/5 • Rev. 1.0.5 11 Application Information Figure 28. Typical Application Setting Current Limit The FPF2193, FPF2194, and FPF2195 current limit is set with an external resistor connected between ISET and GND. This resistor is selected using the following equation: LIM SET I 6 . 551 R = (1) RSET is in Ω and ISET is in Amps. Table 1 can also be used to select RSET. A typical application would be the 500 mA current required by a single USB port. Using Table 1, an appropriate selection for the RSET resistor would be 788 Ω. This ensures that the port load could draw 525 mA, but not more than 875 mA. Likewise for a dual-port system; an RSET of 368 Ω always delivers at least 1125 mA and never more than 1875 mA. Table 1. Current Limit Various RSET Values RSET( Ω) Min. Current Limit (mA) Typ. Current Limit (mA) Max. Current Limit (mA) 368 1125 1500 1875 441 928 1250 1562 552 750 1000 1250 613 675 900 1125 690 600 800 1000 788 525 700 875 919 450 600 750 1103 375 500 625 1226 338 450 563 1379 300 400 500 1576 263 350 438 1839 225 300 375 2206 188 250 313 2758 150 200 250 3677 113 150 188 5516 75 100 125 Input Capacitor To limit the voltage drop on the input supply caused by transient inrush currents when the switch is turned on into a discharged load capacitor or a short-circuit, a capacitor needs to be placed between VIN and GND. A 0.1 µF ceramic capacitor, CIN, placed close to the pins is usually sufficient. Higher values of CIN can be used to further reduce the voltage drop. Output Capacitor A 0.1 µF capacitor, COUT, should be placed between VOUT and GND. This capacitor prevents parasitic board inductances from forcing VOUT below GND when the switch turns off. For the FPF2193 and FPF2194, the total output capacitance needs to be kept below a maximum value, COUT(max), to prevent the part from registering an over-current condition and turning off the switch. The maximum output capacitance can be determined from the following formula: IN BLANK LIM OUT V (min) t (max) I (max) C × = (2) Power Dissipation During normal operation as a switch, the power dissipated depends upon the level at which the current limit is set. The maximum allowed setting for the current limit is 1.5 A and results in a power dissipation of: mW 75 . 123 055 . 0 ) 5 . 1 ( R ) I ( P 2 ON 2 LIM = × = × = (3) If the part goes into current limit, the maximum power dissipation occurs when the output is shorted to ground. For the FPF2193, the power dissipation scales by the auto-restart time, tRSTRT, and the over-current blanking time, tBLANK, so that the maximum power dissipated is: mW 6 . 515 5 . 1 5 . 5 450 30 30 (max) I (max) V t t t (max) P LIM IN RSTRT BLANK BLANK = × × + = × × + = (4) |
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