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ISL6553 Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL6553 Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 15 page FIGURE 7. SIMPLIFIED FUNCTIONAL BLOCK DIAGRAM SHOWING CURRENT AND VOLTAGE SAMPLING CURRENT SENSING COMPARATOR PWM CIRCUIT AVERAGING CURRENT FROM OTHER CHANNEL SAWTOOTH GENERATOR + DIFFERENCE RISEN + CORRECTION ERROR AMPLIFIER FB COMP REFERENCE TO OTHER CHANNELS ISEN RIN RFB Cc VCORE Q1 Q2 COMPARATOR REFERENCE TO OVER CURRENT TRIP L PHASE INDUCTOR CURRENT FROM OTHER CHANNEL PWM IL DAC ISL6553 VIN ONLY ONE OUTPUT HIP6601 - - STAGE SHOWN - + SENSING - + - + 10 Current Sensing and Balancing Overview The ISL6553 samples the on-state voltage drop across each synchronous rectifier FET, Q2, as an indication of the inductor current in that phase, see Figure 7. Neglecting AC effects (to be discussed later), the voltage drop across Q2 is simply rDS(ON)(Q2) x inductor current (IL). Note that IL, the inductor current, is 1/2 of the total current (ILT). The voltage at Q2’s drain, the PHASE node, is applied to the RISEN resistor to develop the IISEN current to the ISL6553 ISEN pin. This pin is held at virtual ground, so the current through RISEN is IL x rDS(ON)(Q2) / RISEN. The IISEN current provides information to perform the following functions: 1. Detection of an over-current condition 2. Reduce the regulator output voltage with increasing load current (droop) 3. Balance the IL currents in the two phases Over-Current, Selecting RISEN The current detected through the RISEN resistor is averaged with the current detected in the other channel. The averaged current is compared with a trimmed, internally generated current, and used to detect an over-current condition. The nominal current through the RISEN resistor should be 50 µA at full output load current, and the nominal trip point for over-current detection is 165% of that value, or 82.5 µA. Therefore, RISEN = IL x rDS(ON) (Q2) / 50µA. For a full load of 25A per phase, and an rDS(ON) (Q2) of 4m Ω, R ISEN = 2kΩ . The over-current trip point would be 165% of 25A, or ~ 41A per phase. The RISEN value can be adjusted to change the over-current trip point, but it is suggested to stay within ±25% of nominal. Droop, Selection of RIN The average of the currents detected through the RISEN resistors is also steered to the FB pin. There is no DC return path connected to the FB pin except for RIN, so the average current creates a voltage drop across RIN. This drop increases the apparent VCORE voltage with increasing load current, causing the system to decrease VCORE to maintain balance at the FB pin. This is the desired “droop” voltage used to maintain VCORE within limits under transient conditions. ISL6553 |
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