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FAN4860UC5X Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN4860UC5X Datasheet(HTML) 11 Page - Fairchild Semiconductor |
11 / 15 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN4860 • Rev. 1.0.3 11 Application Information External Component Selection Table 2 shows the recommended external components for the FAN4860: Table 2. External Components REF Description Manufacturer L1 1.0µH, 0.8A, 190m Ω, 0805 Murata LQM21PN1R0MC0, or equivalent CIN 2.2µF, 6.3V, X5R, 0402 Murata GRM155R60J225M TDK C1005X5R0J225M COUT 4.7µF, 10V, X5R, 0603 (3) Kemet C0603C475K8PAC TDK C1608X5R1A475K Note: 3. A 6.3V-rated 0603 capacitor may be used for COUT, such as Murata GRM188R60J225M. All datasheet parameters are valid with the 6.3V-rated capacitor. Due to DC bias effects, the 10V capacitor offers a performance enhancement; particularly output ripple and transient response, without any size increase. Output Capacitance (COUT) Stability The effective capacitance (CEFF) of small, high-value, ceramic capacitors decrease as their bias voltage increases, as shown in Figure 30. Figure 30. CEFF for 4.7μF, 0603, X5R, 6.3V (Murata GRM188R60J475K) FAN4860 is guaranteed for stable operation with the minimum value of CEFF (CEFF(MIN)) outlined in Table 3. Table 3. Minimum CEFF Required for Stability Operating Conditions CEFF(MIN) (μF) VIN (V) ILOAD (mA) 2.3 to 4.5 0 to 200 1.5 2.7 to 4.5 0 to 200 1.0 2.3 to 4.5 0 to 150 1.0 CEFF varies with manufacturer, dielectric material, case size, and temperature. Some manufacturers may be able to provide an X5R capacitor in 0402 case size that retains CEFF >1.5 μF with 5V bias; others may not. If this CEFF cannot be economically obtained and 0402 case size is required, the IC can work with the 0402 capacitor as long as the minimum VIN is restricted to >2.7V. For best performance, a 10V-rated 0603 output capacitor is recommended (Kemet C0603C475K8PAC, or equivalent). Since it retains greater CEFF under bias and over temperature, ouptut ripple can is reduced and transient capability enhanced. Output Voltage Ripple Output voltage ripple is inversely proportional to COUT. During tON, when the boost switch is on, all load current is supplied by COUT. OUT LOAD ON ) P P ( RIPPLE C I t V • = − and EQ. 2 ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − • = • = OUT IN SW SW ON V V 1 t D t t Therefore: EQ. 3 OUT LOAD OUT IN SW ) P P ( RIPPLE C I V V 1 t V • ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − • = − EQ. 4 where: SW SW f 1 t = EQ. 5 As can be seen from EQ. 4, the maximum VRIPPLE occurs when VIN is minimum and ILOAD is maximum. Startup Input current limiting is in effect during soft-start, which limits the current available to charge COUT. If the output fails to achieve regulation within the time period described in the soft-start section above; a FAULT occurs, causing the circuit to shut down, then restart after a significant time period. If COUT is a very high value, the circuit may not start on the first attempt, but eventually achieves regulation if no load is present. If a high-current load and high capacitance are both present during soft-start, the circuit may fail to achieve |
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