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RT8800AGQV Datasheet(PDF) 13 Page - Richtek Technology Corporation |
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RT8800AGQV Datasheet(HTML) 13 Page - Richtek Technology Corporation |
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13 / 21 page  RT8800A 13 DS8800A-04 August 2007 www.richtek.com All brand name or trademark belong to their owner respectively Figure 9 COMM LX ADJ CORE CORE R x 3 R x R ΔI ΔV Line Load − = = Dead Zone Elimination and Output Voltage Offset Function RT8800A samples and holds inductor valley current by time-sharing sourcing a current IX to RCOMM. At light load condition the inductor valley current and consequently the voltage VX across the sensing capacitor may be negative. A negative IX is required to correctly sense the negative voltage. However, the RT8800ACANNOT provide a negative IX and consequently cannot sense negative inductor current. This results in dead zone of load line performance as shown in Figure 10. Therefore a technique as shown in Figure 11 is required to eliminate the dead zone of load line at light load condition. Figure 10 Figure 11 Referring to Figure 11, the Schottky diode provides a constant voltage drop VF with enough bias current. IX is expressed as: COMM LX LX_Valley CSN LX LX_Valley F X R R I R R I V I × + × + = To make sure RT8800A could sense the inductor current, right hand side of Equation (1) should always be positive: 0 R R I R R I V COMM LX LX_Valley CSN LX LX_Valley F ≥ × + × + Since VF >> (ILX_Valley x RLX) in practical application, Equation (2) could be simplified as: COMM LX LX_Valley CSN F R R I R V × ≥ (1) (2) (3) Droop and Load Lind Setting The sampled and held phase current IX are summed to get sum(IX). RT8800A then sinks a current that is 1/3 sum(IX) and produces a droop voltage that is proportional to the average phase voltage. VADJ = 1/3 sum(IX) x RADJ VADJ is then subtracted form DAC generator output as the real reference voltage at non-inverting input of the error amplifier. Consequently, load line slope is calculated as: + - 0.8V VREF OVP 500mV SUM/N + - VID125 VID125 RT9401 VID [0 : 4] VCORE RADJ PI FB COMP IX DAC_OUT ) sum(I 3 1 X VCORE ICORE DAC_OUT VOFFSET RCSN < RCSN = RCSN > Load Line Spec_High Spec_Low + - RCOMM RCSN DSKY RCS VCORE - VF CX + VX - RX LX RL_X IX IL VCORE |
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