RT8800A

17

DS8800A-04 August 2007

www.richtek.com

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Layout Guide

Place the high-power switching components first, and separate them from sensitive nodes.

1. Most critical path:

The current sense circuit is the most sensitive part of the converter. The current sense resistors tied to ISP1,2,3 and

ICOMMON should be located not more than 0.5 inch from the IC and away from the noise switching nodes. The PCB

trace of sense nodes should be parallel and as short as possible. R&C filter of choke should place close to PWM and

the R & C connect directly to the pin of each output choke, use 10 mil differencial pair, and 20 mil gap to other phase

pair. Less via as possible.

2. Switching ripple current path:

a. Input capacitor to high side MOSFET.

b. Low side MOSFET to output capacitor.

c. The return path of input and output capacitor.

d. Separate the power and signal GND.

e. The switching nodes (the connection node of high/low side MOSFET and inductor) is the most noisy points.Keep

them away from sensitive small-signal node.

f . Reduce parasitic R, L by minimum length, enough copper thickness and avoiding of via.

3. MOSFET driver should be closed to MOSFET.

Figure 18. Power Stage Ripple Current Path

SW2

L2

SW1

L1

C

OUT

R

L

V

OUT

V

IN

R

IN

C

IN

V