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ISL6431EVAL1 Datasheet(PDF) 5 Page - Intersil Corporation |
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ISL6431EVAL1 Datasheet(HTML) 5 Page - Intersil Corporation |
5 / 10 page 5 completion of the OCP sampling and hold operation, the POR function initiates the Soft Start operation. Over Current Protection The overcurrent function protects the converter from a shorted output by using the upper MOSFET’s on-resistance, rDS(ON), to monitor the current. This method enhances the converter’s efficiency and reduces cost by eliminating a current sensing resistor. 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 diagram). Immediately following POR, the ISL6431 initiates the Overcurrent Protection sampling and hold operation. First, the internal error amplifier is disabled. This allows an internal 20 µA current sink to develop a voltage across ROCSET.The ISL6431 then samples this voltage at the COMP pin. This sampled voltage, which is referenced to the VCC pin, is held internally as the Overcurrent Set Point. When the voltage across the upper MOSFET, which is also referenced to the VCC pin, exceeds the Overcurrent Set Point, the overcurrent function initiates a soft-start sequence. Figure 1 shows this operation with an overload condition. This current is repeated with a 21ms period. Note that the inductor current increases to over 14A during the Soft Start interval and causes an overcurrent trip. The converter dissipates very little power with this method. The measured input power for the conditions of Figure 1 is only 0.25W. 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’s 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’. Soft Start The POR function initiates the soft start sequence after the overcurrent set point has been sampled. Soft start clamps the error amplifier output (COMP pin) and reference input (non- inverting terminal of the error amp) to the internally generated Soft Start voltage. Figure 2 shows a typical soft start interval. Initially the clamp on the error amplifier (COMP/OCSET pin) controls the converter’s output voltage. The oscillator’s triangular waveform is compared to the ramping error amplifier voltage. This generates PHASE pulses of increasing width that charge the output capacitor(s). With sufficient output voltage, the clamp on the reference input controls the output voltage. When the internally generated Soft Start voltage exceeds the feedback (FB pin) voltage, the output voltage is in regulation. This method provides a rapid and controlled output voltage rise. Application Guidelines Layout Considerations As in any high frequency switching converter, layout is very important. Switching current from one power device to another can generate voltage transients across the impedances of the interconnecting bond wires and circuit traces. These interconnecting impedances should be minimized by using wide, short printed circuit traces. The critical components should be located as close together as possible, using ground plane construction or single point grounding. FIGURE 1. OVERCURRENT OPERATION 0A 4A 2A 6A 8A 10A 12A 14A TIME (50 µs/DIV.) I PEAK I OCSET xROCSET r DS ON () ----------------------------------------------------- = I PEAK I OUT MAX () ∆I () 2 ---------- + > 0V TIME (1ms/DIV.) FIGURE 2. SOFT START INTERVAL VOUT 500mV/DIV. ISL6431 |
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