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FAN5350 Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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FAN5350 Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 15 page © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5350 Rev. 1.0.3 7 Applications Information Selecting the Inductor The output inductor must meet both the required inductance and the energy handling capability of the application. The inductor value affects the average current limit, the PWM-to-PFM transition point, the output voltage ripple, and the efficiency. The ripple current (∆I) of the regulator is: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ • − • ≈ Δ SW OUT IN IN OUT F L V V V V I (1) The maximum average load current, IMAX(LOAD) is related to the peak current limit, ILIM(PK) (see figure 17) by the ripple current: 2 I I I ) PK ( LIM ) LOAD ( MAX Δ − = (2) The transition between PFM and PWM operation is determined by the point at which the inductor valley current crosses zero. The regulator DC current when the inductor current crosses zero, IDCM, is: 2 I IDCM Δ = (3) The FAN5350 is optimized for operation with L=1 μH, but is stable with inductances ranging from 700nH to 3.0 μH. The inductor should be rated to maintain at least 80% of its value at ILIM(PK). Efficiency is affected by the inductor DCR and inductance value. Decreasing the inductor value for a given physical size typically decreases the DCR; but since ∆I increases, the RMS current increases, as do the core and skin effect losses. 12 I I I 2 2 ) DC ( OUT RMS Δ + = (4) The increased RMS current produces higher losses through the RDS(ON) of the IC MOSFETs as well as the inductor ESR. Increasing the inductor value produces lower RMS currents, but degrades transient response. For a given physical inductor size, increased inductance usually results in an inductor with lower saturation current. Table 1 shows the effects of inductance higher or lower than the recommended 1 μH on regulator performance. Output Capacitor Table 2 suggests 0603 capacitors. 0805 capacitors may further improve performance in that the effective capacitance is higher and ESL is lower than 0603. This improves the transient response and output ripple. Increasing COUT has no effect on loop stability and can therefore be increased to reduce output voltage ripple or to improve transient response. Output voltage ripple, ∆VOUT, is: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + • • • Δ = Δ ESR F C 8 1 I V SW OUT OUT (5) Input Capacitor The 4.7 μF ceramic input capacitor should be placed as close as possible between the VIN pin and GND to minimize the parasitic inductance. If a long wire is used to bring power to the IC, additional “bulk” capacitance (electrolytic or tantalum) should be placed between CIN and the power source lead to reduce ringing that can occur between the inductance of the power source leads and CIN. Table 1. Effects of changes in inductor value (from 1µH recommended value) on regulator performance Inductor Value IMAX(LOAD) EQ. 2 ILIM(PK) ∆VOUT EQ. 5 Transient Response Increase Increase Decrease Decrease Degraded Decrease Decrease Increase Increase Improved |
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