RT8205CGQW datasheet(26/28 Pages) RICHTEK

26 / 28 page

RT8205A/B/C

DS8205A/B/C-06 July 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Layout Considerations

Layout is very important in high frequency switching

converter design. If the IC is designed improperly, the PCB

could radiate excessive noise and contribute to the

converter instability. Certain points must be considered

before starting a layout using the RT8205A/B/C.

Place the filter capacitor close to the IC, within 12 mm

(0.5 inch) if possible.

Keep current limit setting network as close as possible

to the IC. Routing of the network should avoid coupling

to high-voltage switching node.

Connections from the drivers to the respective gate of

the high-side or the low-side MOSFET should be as

short as possible to reduce stray inductance. Use 0.65-

mm (25 mils) or wider trace.

All sensitive analog traces and components such as

VOUTx, FBx, GND, ENTRIPx, PGOOD, and TONSEL

should be placed away from high-voltage switching

nodes such as PHASEx, LGATEx, UGATEx, or BOOTx

nodes to avoid coupling. Use internal layer(s) as ground

plane(s) and shield the feedback trace from power traces

and components.

Gather ground terminal of VIN capacitor(s), VOUTx

capacitor(s), and source of low-side MOSFETs as close

as possible. PCB trace defined as PHASEx node, which

connects to source of high-side MOSFET, drain of low-

side MOSFET and high-voltage side of the inductor,

should be as short and wide as possible.

Figure 7. Derating Curve of Maximum Power Dissipation

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

100

125

Ambient Temperature (°C)

Four Layers PCB

WQFN-24L 4x4

test board. The maximum power dissipation at TA= 25

°C

can be calculated by following formula :

PD(MAX) = (125

°C − 25°C) / (52°C/W) = 1.923W for

WQFN-24L 4x4 packages

The maximum power dissipation depends on operating

ambient temperature for fixed TJ(MAX) and thermal

resistance

θJA. The Figure 7 of derating curves allows the

designer to see the effect of rising ambient temperature

on the maximum power allowed.

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