SC2544 datasheet(17/24 Pages) SEMTECH

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SC2544

POWER MANAGEMENT

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

SC2544 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 MOSFET’s. 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

PPooooow

wer Stage

er Stage

Control Section

Circuit board layout is very important for the proper

operation of high frequency switching power con-

verters. A power ground plane is required to reduce

ground bounces. The followings are suggested for

proper layout.

Applications Information (Cont.)

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