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ISL6228HRTZ Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL6228HRTZ Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 16 page 9 FN9095.2 May 7, 2008 pins. The power for the LGATE gate-driver is sourced directly from the PVCC pin. The power for the UGATE gate- driver is sourced from a “boot” capacitor connected across the BOOT and PHASE pins. The boot capacitor is charged from a 5V bias supply through a “boot diode” each time the low-side MOSFET turns on, pulling the PHASE pin low. The ISL6228 has an integrated boot diode connected from the PVCC pin to the BOOT pin. Diode Emulation The ISL6228 implements forced continuous-conduction- mode (CCM) at heavy load and diode-emulation-mode (DEM) at light load, to optimize efficiency in the entire load range. The transition is automatically achieved by detecting the output load current. Positive-going inductor current flows from either the source of the high-side MOSFET, or the drain of the low-side MOSFET. Negative-going inductor current flows into the drain of the low-side MOSFET. When the low-side MOSFET conducts positive inductor current, the phase voltage will be negative with respect to the GND and PGND pins. Conversely, when the low-side MOSFET conducts negative inductor current, the phase voltage will be positive with respect to the GND and PGND pins. The ISL6228 monitors the phase voltage, when the low-side MOSFET is conducting inductor current, to determine the direction of the inductor current. When the output load current is greater than or equal to ½ the inductor ripple current, the inductor current is always positive, and the converter is always in CCM. The ISL6228 minimizes the conduction loss in this condition by forcing the low-side MOSFET to operate as a synchronous rectifier. When the output load current is less than ½ the inductor ripple current, negative inductor current occurs. Sinking negative inductor through the low-side MOSFET lowers efficiency through unnecessary conduction losses. The ISL6228 automatically enters DEM after the PHASE pin has detected positive voltage and LGATE was allowed to go high for eight consecutive PWM switching cycles. The ISL6228 will turn off the low-side MOSFET once the phase voltage turns positive, indicating negative inductor current. The ISL6228 will return to CCM on the following cycle after the PHASE pin detects negative voltage, indicating that the body diode of the low-side MOSFET is conducting positive inductor current. Efficiency can be further improved with a reduction of unnecessary switching losses by reducing the PWM frequency. It is characteristic of the R3 architecture for the PWM frequency to decrease while in diode emulation. The extent of the frequency reduction is proportional to the reduction of load current. Upon entering DEM, the PWM frequency makes an initial step-reduction because of a 33% step-increase of the window voltage VW. Overcurrent Protection The overcurrent protection (OCP) setpoint is programmed with resistor ROCSET that is connected across the OCSET and PHASE pins. Figure 5 shows the overcurrent-set circuit. The inductor consists of inductance L and the DC resistance DCR. The inductor DC current IL creates a voltage drop across DCR, given by Equation 3: The ISL6228 sinks 10µA current into the OCSET pin, creating a DC voltage drop across the resistor ROCSET, given by Equation 4: Resistor RO is connected between the VO pin and the actual output voltage of the converter. During normal operation, the VO pin is a high impedance path, therefore there is no voltage drop across RO. The DC voltage difference between the OCSET pin and the VO pin can be established using Equation 5: FIGURE 4. LGATE AND UGATE DEAD-TIME UGATE LGATE 50% 50% tLGFUGR tUGFLGR FIGURE 5. OVERCURRENT-SET CIRCUIT PHASE CO L VO ROCSET CSEN OCSET VO RO ISL6228 DCR IL 10µA + _ VDCR + _ VROCSET V DCR I L DC R • = (EQ. 3) V ROCSET 10 μAR OCSET • = (EQ. 4) V OCSET V – VO V DCR V – ROCSET I L DC R • 10 μAR OCSET • – == (EQ. 5) ISL6228 |
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