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RT8800GS Datasheet(PDF) 19 Page - Richtek Technology Corporation |
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RT8800GS Datasheet(HTML) 19 Page - Richtek Technology Corporation |
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19 / 26 page  RT8800/B 19 DS8800/B-06 March 2007 www.richtek.com All brand name or trademark belong to their owner respectively DAC Offset Voltage Tuning The Intel specification requires that at no load the nominal output voltage of the regulator be offset to a value lower than the nominal voltage corresponding to the VID code. The offset is tuning from RG in the DAC generator as Figure 18. If VID0~6 is set at VSS (Ground), and to suppose that shunt resistance is Rs. From below equation, we can tune the value of RG to increase or decrease the base voltage of VDACQ. REF S F REF G F DACQ x V R R x V ) R R (1 V + + = + - V REF (0.8V) VDACFB OP VDACQ RF RG VID0 VID1 VID2 VID3 VID4 VID5 R R R R R R Figure 18. The Structure of Discrete DAC Generator Over Current Protection OCP comparator co\mpares each inductor current sensed & sample/hold by current sense circuit with this reference current(150uA). RT8800/B uses hiccup mode to eliminate fault detection of OCP or reduce output current when output is shorted to ground. Figure 19. The Over Current Protection in the interval CH1:(5V/Div) CH2:(5V/Div) Over Current Protection Time (25ms/Div) PWM IL Figure 20. Over Current Protection at steady state CH1:(5V/Div) CH2:(5V/Div) Over Current Protection Time (25ms/Div) PWM VSS Fault Detection The “hiccup mode” operation of over current protection is adopted to reduce the short circuit current. The in-rush current at the start up is suppressed by the soft start circuit through clamping the pulse width and output voltage by an internal slow rising ramp. Figure 17. Load Droop Circuit EA FB PI +- + - V DACQ V DRP 2xI X1 2xI X2 2xI X3 I SINK |
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