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APW7098 Datasheet(PDF) 19 Page  Anpec Electronics Coropration 

APW7098 Datasheet(HTML) 19 Page  Anpec Electronics Coropration 
19 / 30 page Copyright © ANPEC Electronics Corp. Rev. A.6  Oct., 2009 APW7098 www.anpec.com.tw 19 Function Description (Cont.) Figure 3 shows the circuit of sensing inductor current. Connecting a series resistor (R S) and a capacitor (CS) network in parallel with the inductor and measuring the voltage (V C) across the capacitor can sense the in ductor current. Figure 3. Illustration of Inductor Current Sensing Circuit The equations of the sensing network are: Take for example, if the above equation is true, the voltage across the capacitor C S is equal to voltage drop across the inductor DCR, and the voltage V C is proportional to the current I L. The sensing current through the resistor R2 can be expressed as the following equation: where I CS is the sensed current I L is the inductor current DCR is the inductor resistance R2 is the sense resistor LL V (s)=I (s) (SL+DCR) × OverCurrent Protection (OCP) L DCR R s C s R2 CSP CSN PHASE I L V C V L R2 DCR I I L CS × = The APW7098 is a twophase PWM controller; therefore, the IC has two sensed current parts, I CS1 and ICS2. When I CS1 plus ICS2 is greater than 120µA, the over current occurs. In overcurrent protection, the IC shuts off the converter and then initials a new softstart process. After 3 over current events are counted, the device turns off both high side and lowside MOSFETs and the converter’s output is latched to be floating. Current Sharing The APW7098 uses inductor’s DCRs and external net works to sense the both currents flowing through the in ductors of the PWM1 and PWM2 channels. The current sharing circuit, with closedloop control, uses the sensed currents to adjust the twophase inductor currents. For example, if the sensed current of PWM1 is bigger than PWM2, the duty of PWM1 will decrease and the duty of PWM2 will increase. Then, the device will reduce I L1 current and increase I L2 current for current sharing. DROOP In some high current applications, a requirement on precisely controlled output impedance is imposed. This dependence of output voltage on load current is often termed droop regulation. As shown in figure 4, the droop control block generates a voltage through external resistor R DROOP and then set the droop voltage. The droop voltage, V DROOP, is proportional to the total current in two channels. As shown in the following equation: The V DROOP voltage is used the regulator to adjust the out put voltage, therefore, it is equal to the reference voltage minus the droop voltage. S S L S S L C C SR 1 ) DCR SL ( ) S ( I C SR 1 1 (S) V (S) V + + × = + × = DCR L C R S S = Figure 4. Illustration of Droop Setting Function Droop Control V R V REFIN/EN or 0.6V R DROOP V DROOP ] R ) I I [( 05 . 0 V DROOP 2 CS 1 CS DROOP × + × = 
