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RT9641A Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT9641A Datasheet(HTML) 9 Page - Richtek Technology Corporation |
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9 / 11 page  RT9641A/B DS9641A/B-11 March 2007 www.richtek.com 9 Placement of the decoupling and bulk capacitors should follow a placement reflecting their purpose. As such, the high-frequency decoupling capacitors (CHF) should be placed as close as possible to the load they are decoupling; the ones decoupling the controller (C12V, C5VSB) close to the controller pins, the one decoupling the load close to the load connector or the load itself (if embedded). The bulk capacitance (aluminum electrolytic or tantalum capacitors) placement is not as critical as the high- frequency capacitor placement, but having these capacitors close to the load they serve is preferable. The only critical small signal component is the soft-start capacitor, CSS. Locate the component close to SS pin of the control IC and connect to ground though a vias placed close to the capacitor's ground pad. Minimize any leakage current paths from SS node, since the internal current source is only 5 μA. A multi-layer printed circuit board is recommended. Figure1 shows the connections of most of the components to the converter. Note that each individual capacitor could represent numerous physical capacitors. Dedicate one solid layer for a ground plane and make all critical component ground connections through vias placed as close to the component as possible. Dedicate another solid layer as a power plane and break this plane into smaller islands of common voltage levels. Ideally, the power plane should support both the input power and output power nodes. Use copper filled polygons on the top and bottom circuit layers to create power islands connecting the filtering components (output capacitors) and the loads. Use the remaining printed circuit layers for small signal wiring. Component Selection Guidelines Output Capacitors Selection The output capacitors for all outputs should be selected to allow the output voltage to meet the dynamic regulation requirements of active state operation (S0, S1). The load transient for the various microprocessor system's components may require high quality capacitors to supply the high slew rate (di/dt) current demands. Thus, it is recommended that capacitors COUT1 and COUT2 should be selected for transient load regulation. Also, during the transition between active and sleep states, there is a short interval of time during which none of the power pass elements are conducting-during this time the output capacitors have to supply all the output current. The output voltage drop during this brief period of time can be approximated with the following formula: Δ VOUT = IOUT x (ESROUT + tt / COUT), where Δ VOUT:output voltage drop ESROUT : output capacitor bank ESR IOUT : output current during transition COUT : output capacitor bank capacitance tt : active-to-sleep or sleep-to-active transition time (5 μs typical) Since the output voltage drop is heavily dependent on the ESR (equivalent series resistance) of the output capacitor bank, the capacitors should be chosen to maintain the output voltage above the lowest allowable regulation level. Input Capacitors Selection The input capacitors for an RT9641A/B application must have sufficiently low ESR so that the input voltage does not dip excessively when energy is transferred to the output capacitors. Figure 1 . Printed Circuit Board Islands SS 3V3DLSB 3V3DL 5VSB 5VDLSB 5VDL DLA VSEN2 DRV2 GND 12V RT9641A/B CIN C5VSB C12V CSS CHF1 CBULK1 CBULK2 CBULK2 CHF3 CHF2 Q1 Q5 Q4 Q2 Q3 +5VSB +12VIN VOUT1 VOUT2 VOUT3 +3.3VIN ISLAND ON POWAR PLANE LAYER ISLAND ON CIRCUIT/POWER PLANE LAYER VIA CONNECTION TO GROUND PLANE |
Similar Part No. - RT9641A_07 |
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Similar Description - RT9641A_07 |
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