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ISL6552 Datasheet(PDF) 15 Page - Renesas Technology Corp |
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ISL6552 Datasheet(HTML) 15 Page - Renesas Technology Corp |
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15 / 18 page  ISL6552 FN4918 Rev 2.00 Page 15 of 18 July 2004 High frequency decoupling capacitors should be placed as close to the power pins of the load as physically possible. Be careful not to add inductance in the circuit board wiring that could cancel the usefulness of these low inductance components. Consult with the manufacturer of the load on specific decoupling requirements. Use only specialized low-ESR capacitors intended for switching-regulator applications for the bulk capacitors. The bulk capacitor’s ESR determines the output ripple voltage and the initial voltage drop following a high slew-rate transient’s edge. In most cases, multiple capacitors of small case size perform better than a single large case capacitor. Bulk capacitor choices include aluminum electrolytic, OS-Con, Tantalum and even ceramic dielectrics. An aluminum electrolytic capacitor’s ESR value is related to the case size with lower ESR available in larger case sizes. However, the equivalent series inductance (ESL) of these capacitors increases with case size and can reduce the usefulness of the capacitor to high slew-rate transient loading. Unfortunately, ESL is not a specified parameter. Consult the capacitor manufacturer and measure the capacitor’s impedance with frequency to select a suitable component. Output Inductor Selection One of the parameters limiting the converter’s response to a load transient is the time required to change the inductor current. Small inductors in a multi-phase converter reduces the response time without significant increases in total ripple current. The output inductor of each power channel controls the ripple current. The control IC is stable for channel ripple current (peak-to-peak) up to twice the average current. A single channel’s ripple current is approximately: The current from multiple channels tend to cancel each other and reduce the total ripple current. Figure 12 gives the total ripple current as a function of duty cycle, normalized to the parameter at zero duty cycle. To determine the total ripple current from the number of channels and the duty cycle, multiply the y-axis value by . Small values of output inductance can cause excessive power dissipation. The ISL6552 is designed for stable operation for ripple currents up to twice the load current. However, for this condition, the RMS current is 115% above the value shown in the following MOSFET Selection and Considerations section. With all else fixed, decreasing the inductance could increase the power dissipated in the MOSFETs by 30%. I VIN VOUT – FSW L -------------------------------- VOUT VIN ---------------- = Vo LxFSW Vo LxFSW 1.0 0.8 0.6 0.4 0.2 0 0 0.1 0.2 0.3 0.4 0.5 DUTY CYCLE (VO/VIN) SINGLE CHANNEL 2 CHANNEL 3 CHANNEL 4 CHANNEL FIGURE 11. RIPPLE CURRENT vs DUTY CYCLE VCORE +12V VIA CONNECTION TO GROUND PLANE ISLAND ON POWER PLANE LAYER ISLAND ON CIRCUIT PLANE LAYER LO1 COUT CIN +5VIN KEY PHASE VCC USE INDIVIDUAL METAL RUNS COMP ISL6552 PWM RT RIN RFB CBP FB VSEN ISEN RSEN HIP6601 CBOOT CBP CT VCC FS/DIS PVCC LOCATE NEXT TO IC PIN LOCATE NEXT TO FB PIN LOCATE NEXT TO IC PIN(S) ISOLATE OUTPUT STAGES FOR EACH CHANNEL TO HELP LOCATE NEAR TRANSISTOR FIGURE 12. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS |
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