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ISL6521EVAL1 Datasheet(PDF) 6 Page - Intersil Corporation |
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ISL6521EVAL1 Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 13 page 6 the clamped error amplifier output voltage. As the internal soft-start voltage increases, the pulse-width on the PHASE pin increases to reach its steady-state duty cycle at time T2. Also at time T2, the error amplifier references of the linear controllers, ramp to their final value bringing all outputs within regulation limits. Overcurrent Protection All outputs are protected against excessive overcurrents. The PWM controller uses the upper MOSFET’s on-resistance, rDS(ON) to monitor the current for protection against a shorted output. All linear controllers monitor their respective FB pins for undervoltage events to protect against excessive currents. A sustained overload (undervoltage on linears or overcurrent on the PWM) on any output results in an independent shutdown of the respective output, followed by subsequent individual re-start attempts performed at an interval equivalent to 3 soft-start intervals. Figure 2 describes the protection feature. At time T0, an overcurrent event sensed across the switching regulator’s upper MOSFET (rDS(ON) sensing) triggers a shutdown of the VOUT1 output. As a result, its internal soft-start initiates a number of soft-start cycles. After a three-cycle wait, the fourth soft-start initiates a ramp-up attempt of the failed output, at time T2, bringing the output in regulation at time T4. To exemplify a UV event on one of the linears, at time T1, the clock regulator (VOUT2) is also subjected to an overcurrent event, resulting in a UV condition. Similarly, after three soft-start periods, the fourth cycle initiates a ramp-up of this linear output at time T3. One soft-start period after T3, the linear output is within regulation limits. UV glitches less than 1 µs (typically) in duration are ignored. Overcurrent protection is performed on the synchronous switching regulator on a cycle-by-cycle basis. OC monitoring is active as long as the regulator is operational. Since the overcurrent protection on the linear regulators is performed through undervoltage monitoring at the feedback pins (FB2, FB3, and FB4), this feature is activated approximately 25% into the soft-start interval (see Figure 2). A resistor (ROCSET) programs the overcurrent trip level for the PWM converter. As shown in Figure 3, the internal 40 µA current sink (IOCSET) develops a voltage across ROCSET (VSET) that is referenced to VIN. The DRIVE signal enables the overcurrent comparator (OCC). When the voltage across the upper MOSFET (VDS(ON)) exceeds VSET, the overcurrent comparator trips to set the overcurrent latch. Both VSET and VDS(ON) are referenced to VIN and a small capacitor across ROCSET helps VOCSET track the variations of VIN due to MOSFET switching. The overcurrent function will trip at a peak inductor current (IPEAK) determined by: FIGURE 1. SOFT-START INTERVAL 0V 0V TIME (0.5V/DIV) VOUT4 (3.3V) VOUT1 (1.5V) T1 T2 T0 (1V/DIV) VOUT3 (1.8V) VOUT2 (2.5V) +5V FIGURE 2. OVERCURRENT/UNDERVOLTAGE PROTECTION RESPONSE 0V TIME (0.5V/DIV.) VOUT3 (1.8V) VOUT2 (2.5V) T1 T2 T3 T0 VOUT4 (3.3V) VOUT1 (1.5V) UV MONITORING INACTIVE SOFT-START FUNCTION ACTIVE T4 VOUT2 VOUT1 I PEAK = I OCSET R OCSET × r DS ON () ---------------------------------------------------- ISL6521 |
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