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ISL6439AIRZ Datasheet(PDF) 8 Page - Intersil Corporation |
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ISL6439AIRZ Datasheet(HTML) 8 Page - Intersil Corporation |
8 / 15 page 8 The overcurrent function cycles the soft-start function in a hiccup mode to provide fault protection. A resistor (ROCSET) programs the overcurrent trip level (see Typical Application diagrams on pages 2 and 3). An internal 20 µA (typical) current sink develops a voltage across ROCSET that is referenced to VIN. When the voltage across the upper MOSFET (also referenced to VIN) exceeds the voltage across ROCSET, the overcurrent function initiates a soft-start sequence. Figure 3 illustrates the protection feature responding to an overcurrent event. At time t0, an overcurrent condition is sensed across the upper MOSFET. As a result, the regulator is quickly shutdown and the internal soft-start function begins producing soft-start ramps. The delay interval seen by the output is equivalent to three soft-start cycles. The fourth internal soft-start cycle initiates a normal soft-start ramp of the output, at time t1. The output is brought back into regulation by time t2, as long as the overcurrent event has cleared. Had the cause of the over current still been present after the delay interval, the over current condition would be sensed and the regulator would be shut down again for another delay interval of three soft-start cycles. The resulting hiccup mode style of protection would continue to repeat indefinitely. The overcurrent function will trip at a peak inductor current (IPEAK) determined by: where IOCSET is the internal OCSET current source (20µA typical). The OC trip point varies mainly due to the MOSFET rDS(ON) variations. To avoid overcurrent tripping in the normal operating load range, find the ROCSET resistor from the equation above with: 1. The maximum rDS(ON) at the highest junction temperature. 2. The minimum IOCSET from the specification table. 3. Determine IPEAK for , where ∆I is the output inductor ripple current. For an equation for the ripple current see the section under component guidelines titled ‘Output Inductor Selection’. A small ceramic capacitor should be placed in parallel with ROCSET to smooth the voltage across ROCSET in the presence of switching noise on the input voltage. Current Sinking The ISL6439 incorporates a MOSFET shoot-through protection method which allows a converter to sink current as well as source current. Care should be exercised when designing a converter with the ISL6439 when it is known that the converter may sink current. When the converter is sinking current, it is behaving as a boost converter that is regulating its input voltage. This means that the converter is boosting current into the input rail of the regulator. If there is nowhere for this current to go, such as to other distributed loads on the rail or through a voltage limiting protection device, the capacitance on this rail will absorb the current. This situation will allow the voltage level of the input rail to increase. If the voltage level of the rail is boosted to a level that exceeds the maximum voltage rating of any components attached to the input rail, then FIGURE 2. OUTPUT VOLTAGE SELECTION + R1 COUT +3.3V VOUT R4 LOUT ISL6439 C4 Q1 FB UGATE VCC BOOT COMP D1 R2 C2 C1 R3 C3 PHASE LGATE Q2 CPVOUT VIN I PEAK I OCSET x ROCSET r DS ON () ----------------------------------------------------- = FIGURE 3. OVER CURRENT PROTECTION RESPONSE 0V TIME VOUT (2.5V) T1 T0 T2 Internal Soft-Start Function Delay Interval I PEAK I OUT MAX () ∆I () 2 ---------- + > ISL6439 |
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