SP6121 datasheet(14/16 Pages) SIPEX

14 / 16 page

Date: 5/25/04

SP6121 Low Voltage, Synchronous Step Down PWM Controller

PCB layout plays a critical role in proper func-

tion of the converters and EMI control. In switch

mode power supplies, loops carrying high di/dt

give rise to EMI and ground bounces. The goal

of layout optimization is to identify these loops

and minimize them. It is also crucial on how to

connect the controller ground such that its op-

eration is not affected by noise. The following

guideline should be followed to ensure proper

operation.

1. A ground plane is recommended for minimiz-

ing noises, copper losses and maximizing heat

dissipation.

2. Begin the layout by placing the power compo-

nents first. Orient the power circuitry to achieve

a clean power flow path. If possible make all the

connections on one side of the PCB with wide,

copper filled areas.

3. Connect the ground of feedback divider and

compensation components directly to the GND

pin of the IC using a dedicated ground trace.

Then connect this pin as close as possible to the

ground of the output capacitor.

4. The VCC bypass capacitor should be right next

to the VCC and GND pins.

5. The trace connecting the feedback resistors to

the output should be short, direct and far away

from the switch node, and switching compo-

nents.

6. Minimize the trace between PDRV/NDRV

and the gates of the MOSFETs to reduce the

impedance driving the MOSFETs. This is espe-

cially important for the bottom MOSFET that

tends to turn on through its Miller capacitor

when the switch node swings high.

7. Minimize the loop composed of input capaci-

tors, top/bottom MOSFETs and Schottky diode.

This loop carries high di/dt current. Also in-

crease the trace width to reduce copper losses.

8. Maximize the trace width of the loop connect-

ing the inductor, output capacitors, Schottky

diode and bottom MOSFET.

9. ISET and ISENSE connections to Q1 for current

limiting must be make using Kelvin connec-

tions.

LAYOUT GUIDELINE

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