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RT9246APC Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT9246APC Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 16 page  RT9246A 10 DS9246A-04 March 2007 www.richtek.com Over Current Protection RT9246A uses an external resistor RIMAX connected to IMAX pin to generate a reference current IIMAX for over current protection: where VIMAX is 1.0V typical. OCP comparator compares each sensed phase current IX with this reference current as shown in Figure 3. Equivalently, the maximum phase current is calculated as: RT9246A uses hiccup mode to eliminate nuisance detection of OCP or reduce output current when output is shorted to ground as shown in Figure 4 and 5. IMAX IMAX IMAX V I= R IMAX COMM LX(MAX) IMAX LX 3V R I= 2R R Figure 3. Over Current Comparator + - 1/3 I X 1/2 I IMAX OCP Comparator Figure 2 + - S/H CKT + T1 T2 T3 T4 Ix R COMM L X R LX I LX V CORE T1 R X C X V X +- The sensed current IX is used for current balance and droop tuning as described as followed. Since all phases share one common GM, GM offset and linearity variation effect is eliminated in practical applications. As sub-milli-ohm- grade inductors are widely used in modern mother boards, slight mismatch of GM amplifiers offset and linearity results in considerable current shift between phases. The time- sharing DCR current sensing technical is extremely important to guarantee phase current balance at mass production. Phase Current Balance The sampled and held phase current IX are summed and averaged to get the averaged current . Each phase current IX then is compared with the averaged current. The difference between IX and is injected to corresponding PWM comparator. If phase current IX is smaller than the averaged current , RT9246A increases the duty cycle of corresponding phase to increase the phase current accordingly, vice versa. X I X I |
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