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ISL6524EVAL1 Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL6524EVAL1 Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 16 page 10 FN9064.1 April 8, 2005 Application Guidelines Soft-Start Interval Initially, the soft-start function clamps the error amplifier’s output of the PWM converter. This generates PHASE pulses of increasing width that charge the output capacitor(s). The resulting output voltages start-up as shown in Figure 6. The soft-start function controls the output voltage rate of rise to limit the current surge at start-up. The soft-start interval and the surge current are programmed by the soft-start capacitor, CSS. Programming a faster soft-start interval increases the peak surge current. Using the recommended 0.1 µF soft start capacitors ensure all output voltages ramp up to their set values in a quick and controlled fashion, while meeting the system timing requirements. Shutdown The PWM output does not switch until the soft-start voltage (VSS13) exceeds the oscillator’s valley voltage. Additionally, the reference on each linear’s amplifier is clamped to the soft-start voltage. Holding the SS24 pin low (with an open drain or open collector signal) turns off regulators 1, 2 and 3. Regulator 4 (MCH) will simply drop its output to the intermediate soft-start level. This output is not allowed to violate the 2V maximum potential gap to the ATX 3.3V output. Layout Considerations MOSFETs switch very fast and efficiently. The speed with which the current transitions from one device to another causes voltage spikes across the interconnecting impedances and parasitic circuit elements. The voltage spikes can degrade efficiency, radiate noise into the circuit, and lead to device over-voltage stress. Careful component layout and printed circuit design minimizes the voltage spikes in the converter. Consider, as an example, the turn-off transition of the upper MOSFET. Prior to turn-off, the upper MOSFET was carrying the full load current. During the turn- off, current stops flowing in the upper MOSFET and is picked up by the lower MOSFET or Schottky diode. Any inductance in the switched current path generates a large voltage spike during the switching interval. Careful component selection, tight layout of the critical components, and short, wide circuit traces minimize the magnitude of voltage spikes. There are two sets of critical components in a DC-DC converter using an ISL6524A controller. The switching power components are the most critical because they switch large amounts of energy, and as such, they tend to generate equally large amounts of noise. The critical small signal components are those connected to sensitive nodes or those supplying critical bypass current. The power components and the controller IC should be placed first. Locate the input capacitors, especially the high- frequency ceramic de-coupling capacitors, close to the power switches. Locate the output inductor and output capacitors between the MOSFETs and the load. Locate the PWM controller close to the MOSFETs. The critical small signal components include the bypass capacitor for VCC and the soft-start capacitor, CSS. Locate these components close to their connecting pins on the control IC. Minimize any leakage current paths from any SS node, since the internal current source is only 28 µA. A multi-layer printed circuit board is recommended. Figure 10 shows the connections of the critical components in the converter. Note that the capacitors CIN and COUT each could represent numerous physical capacitors. Dedicate one solid layer for a ground plane and make all critical TABLE 1. OUT1 OUTPUT VOLTAGE PROGRAM PIN NAME NOMINAL DACOUT VOLTAGE VID3 VID2 VID1 VID0 VID25 01000 1.050 01000 1.050 01001 1.075 00110 1.100 00111 1.125 00100 1.150 00101 1.175 00010 1.200 00011 1.225 00000 1.250 00001 1.275 11110 1.300 11111 1.325 11100 1.350 11101 1.375 11010 1.400 11011 1.425 11000 1.450 11001 1.475 10110 1.500 10111 1.525 10100 1.550 10101 1.575 10010 1.600 10011 1.625 10000 1.650 10001 1.675 01110 1.700 01111 1.725 01100 1.750 01101 1.775 01010 1.800 01011 1.825 NOTE: 0 = connected to GND, 1 = open or connected to 3.3V through pull-up resistors |
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