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ISL6529ACB Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6529ACB Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 15 page 11 FN9127.1 December 28, 2004 A multi-layer printed circuit board is recommended. Figure 8 shows the connections of the critical components in the converter. Note that capacitors CIN and COUT could each represent numerous physical capacitors. Dedicate one solid layer, usually a middle layer of the PC board, for a ground plane and make all critical component ground connections through vias to this layer. Dedicate another solid layer as a power plane and break this plane into smaller islands of common voltage levels. Keep the metal runs from the PHASE terminal to the output inductor short. The power plane should support the input and output power nodes. Use copper filled polygons on the top and bottom circuit layers for the phase node. Use the remaining printed circuit layers for small signal wiring. The wiring traces from the UGATE pin to the MOSFET gate should be kept short and wide enough to easily handle the 1A of drive current. The switching components should be placed close to the ISL6529A first. Minimize the length of the connections between the input capacitors, CIN, and the power switches by placing them nearby. Position both the ceramic and bulk input capacitors as close to the upper MOSFET drain as possible. Position the output inductor and output capacitors between the upper MOSFET and lower diode and the load. The critical small signal components include any bypass capacitors, feedback components, and compensation components. Position the bypass capacitors, CBP, close to the VCC pin with a via directly to the ground plane. Place the PWM converter compensation components close to the FB and COMP pins. The feedback resistors for both regulators should also be located as close as possible to the relevant FB pin with vias tied straight to the ground plane as required. Component Selection Guidelines Output Capacitor Selection Output capacitors are required to filter the output and supply the load transient current. The filtering requirements are a function of switching frequency and output current ripple. The load transient requirements are a function of the transient load current slew rate (di/dt) and magnitude. These requirements are generally met with a mix of capacitors and careful layout. PWM Regulator Output Capacitors Modern digital ICs can produce high transient load slew rates. High frequency capacitors initially supply the transient current and slow the load rate-of-change seen by the bulk capacitors. The bulk filter capacitor selection is generally determined by the effective series resistance (ESR) and voltage rating requirements rather than actual capacitance requirements. 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. Specialized low-ESR capacitors intended for switching- regulator applications are recommended 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 edge. Aluminum electrolytic, tantalum, and special polymer capacitor ESR values are 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. Work with your capacitor supplier and measure the capacitor’s impedance with frequency to select a suitable component. In most cases, multiple electrolytic capacitors of small case size perform better than a single large case capacitor. PWM Output Inductor Selection The PWM converter requires an output inductor. The output inductor is selected to meet the output voltage ripple requirements and sets the converter response time to a load transient. The inductor value determines the converter’s VOUT1 +5 VCC ISLAND ON POWER PLANE LAYER ISLAND ON CIRCUIT PLANE LAYER LOUT COUT1 CIN +3.3 VIN KEY COMP ISL6529A UGATE R4 R2 CBP FB DRIVE2 GND 5VCC FIGURE 8. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS R1 VOUT2 FB2 C2 VIA CONNECTION TO GROUND PLANE COUT2 Q1 +3.3 VIN R5 R6 PHASE R3 C3 C1 Q2 +12 VCC CBP PGND 12VCC Q3 LGATE ISL6529A |
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