SC2545TSTRT datasheet(17/24 Pages) SEMTECH

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ã 2005 Semtech Corp.

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SC2545

POWER MANAGEMENT

1) Separate the power ground from the signal ground. In

SC2545 design, use an isolated local ground plane for

the controller and tie it to power grand.

2) Minimize the size of the high pulse current loop. Keep

the top MOSFET, the bottom MOSFET and the input

capacitors within a small area with short and wide

traces. In addition to the aluminum energy storage

capacitors, add multi-layer ceramic (MLC) capacitors

from the input to the power ground to improve high

frequency bypass.

3) Reduce high frequency voltage ringing. Widen and

shorten the drain and source traces of the MOSFETs to

reduce stray inductances. Add a small RC snubber if

necessary to reduce the high frequency ringing at the

phase node. Sometimes slowing down the gate drive

signal also helps in reducing the high frequency ringing at

the phase node if the EMI is a concern for the system.

4) Shorten the gate drive trace. Integrity of the gate

drive (voltage level, leading and falling edges) is important

for circuit operation and efficiency. Short and wide gate

drive traces reduce trace inductances. Bond wire

inductance is about 2~3nH. If the length of the PCB

trace from the gate driver to the MOSFET gate is 1 inch,

the trace inductance will be about 25nH. If the gate drive

current is 2A with 10ns rise and falling times, the voltage

drops across the bond wire and the PCB trace will be

0.6V and 5V respectively. This may slow down the

switching transient of the MOSFETs. These inductances

may also ring with the gate capacitance.

5) Put the decoupling capacitor for the gate drive power

supplies (BST and PVCC) close to the IC and power

ground.

6) The frequency-setting resistor Rosc should be placed

close to Pin 23. Trace length from this resistor to the

analog ground should be minimized.

7) Place the bias decoupling capacitor right across the

VCC and analog ground AGND.

PC Board Layout Issues

Power Stage

Control Section

Circuit board layout is very important for the proper op-

eration of high frequency switching power converters. A

power ground plane is required to reduce ground bounces.

The followings are suggested for proper layout.

Applications Information (Cont.)

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