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FAN9611 Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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FAN9611 Datasheet(HTML) 8 Page - Fairchild Semiconductor |
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8 / 18 page  AN-6086 © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.4 • 4/22/10 8 TON VCOMP 4.3 0.73 0.93 TON MAX 13% of POUT MAX (T ON MAX) 18% of POUT MAX (T ON MAX) POUT MAX POUT 0.2 Figure 13. VCOMP vs. On Time of Gate Drive Signal FAN9611/12 determines the phase management according to the COMP pin voltage (0.73V and 0.93V). Since the COMP pin voltage is proportional to the output power, the power levels for phase shedding and adding are given as percentages of limited maximum power, as shown in Figure 13. Thus, the maximum power limiting factor affects the actual phase management thresholds as percentages of nominal output power, as shown in Figure 14, which shows an example where the maximum power limit is 170% of the nominal output power. In that case, the actual phase management thresholds are 22% and 31% of nominal output power, which can maximize the efficiency at 20% of nominal load for 80-plus efficiency requirement. In this way, the phase management thresholds can be adjusted upward by adjusting the maximum on time (through RMOT). Output Power Normalized to PO MAX # of phase operating 2 1 13% 18% 100% 50% Output Power Normalized to PO NOMINAL # of phase operating 2 1 22% 31% 100% 50% PO MAX=1.7 P O NOMINAL 170% Figure 14. Phase Management Threshold When the resulting maximum power limit to obtain the desired phase management threshold is too high compared to the nominal output power, the maximum power limit level can be reduced using clamping circuit on COMP pin, as shown in Figure 15. The COMP pin voltage is clamped at 3.3V and the resulting maximum power limit level is reduced from 170% to 130% of nominal output power. Figure 15. COMP Voltage Clamping Circuit and Phase Management Threshold After determining the limited maximum output power, the boost inductor maximum flux density should be examined to make sure that the inductor is not saturated during overload condition. The maximum flux density of boost inductor during overload condition is given by: , max LPK MAX e BOOST I KL B AN ⋅ ⋅ = ⋅ (20) |
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