RT8577 datasheet(10/12 Pages) RICHTEK

10 / 12 page

DS8577-00 November 2011

www.richtek.com

RT8577

Figure 3. Derating Curve for RT8577 Packages

Layout Considerations

Careful PCB layout is very important for designing

switching power converter circuits. The following layout

guidelines should be strictly followed for best performance

of the RT8577.

The power components L1,D1, CIN, COUT must be placed

as close as possible to the IC to reduce current loop.

The PCB trace between power components must be as

short and wide as possible.

The compensation circuit should be kept away from

the power loops and shielded with a ground trace to

prevent any noise coupling. Place the compensation

components, RC and CC, as close as possible to pin 9.

The exposed pad of the chip should be connected to

ground plane for thermal consideration.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ (MAX)

− TA) / θJA

where TJ (MAX) is the maximum junction temperature, TA is

the ambient temperature, and

θJA is the junction to ambient

thermal resistance.

For recommended operating condition specifications of

RT8577, the maximum junction temperature is 125

°C and

TA is the ambient temperature. The junction to ambient

thermal resistance,

θJA, is layout dependent. For WQFN-

20L 5x5 packages, the thermal resistance,

θJA, is 36°C/

W on a standard JEDEC 51-7 four-layer thermal test board.

The maximum power dissipation at TA = 25

°C can be

calculated by the following formula :

PD(MAX) = (125

°C − 25°C / (36°C/W) = 2.778W for

WQFN-20L 5x5 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. For RT8577 package, the derating curve

in Figure 3 allows the designer to see the effect of rising

ambient temperature on the maximum power dissipation.

0.00

0.40

0.80

1.20

1.60

2.00

2.40

2.80

3.20

025

100

125

Ambient Temperature (°C)

Four-Layer PCB

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