RT8862 datasheet(28/29 Pages) RICHTEK

28 / 29 page

RT8862

DS8862-01 April 2011

www.richtek.com

Layout Considerations

For best performance of the RT8862, the following

guidelines must be strictly followed :

Input bulk capacitors and MLCCs have to be put near

high side MOSFETs. The connection plane of input

capacitors and high side MOSFETs then can be kept as

square as possible.

The shape of phase planes (the connection plane

between high side MOSFETs, low side MOSFETs and

output inductors) have to be as square as possible. Long

traces, thin bars or separated islands must be avoided

in phase planes.

Keep snubber circuits or damping elements near its

objects. Phase RC snubbers have to be close to low

side MOSFETs, UGATE damping resistors have to be

resistance

θJA is 34°C/W on the standard JEDEC 51-7

four layers thermal test board. The maximum power

dissipation at TA = 25

°C can be calculated by following

formula :

PD(MAX) = (125

°C − 25°C) / (34°C/W) = 2.941W for

WQFN-48L 7x7

The maximum power dissipation depends on operating

ambient temperature for fixed TJ(MAX) and thermal

resistance

θJA. For RT8862 package, the Figure 19 of

derating curves allows the designer to see the effect of

rising ambient temperature on the maximum power

dissipation allowed.

Figure 19. Derating Curves for RT8862 Packages

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

100

125

Ambient Temperature (°C)

WQFN-48L 7x7

Four Layers PCB

close to high side MOSFETs, and boot to phase damping

resistors have to be close to high side MOSFETs and

phase planes. Also keep the traces of these snubbers

circuits as short as possible.

The area of VIN plane (power stage 12V VIN) and VOUT

plane (output bulk capacitors and inductors connection

plane) have to be as wide as possible. Long traces or

thin bars must be avoided in these planes. The plane

trace width must be wide enough to carry large input/

output current (40mil/A).

The following traces have to be wide and short : UGATE,

LGATE, BOOT, PHASE, and VCC12. Make sure the

width of these traces are wide enough to carry large

driving current(at least 40mil).

The voltage feedback loop contains two traces, VCC

and VSS, which are Kelvin sensed from CPU socket or

output capacitors. These two traces are suggested above

10mil width and put away from high (di/dt) switching

elements such as high side MOSFETs, low side

MOSFETs, phase plane etc. The circuit elements of

voltage feedback loop, such as feedback loop short

resistors and voltage loop compensation RCs, have to

be kept near the RT8862 and also away from switching

elements.

The current sense mechanism of the RT8862 is fully

differential Kelvin sense. Therefore, the current sense

loops of the RT8862 contain two traces : the positive

traces(ISP1 to ISP4) come from the positive node of

output inductors(the node connecting phase plane) and

the negative traces (ISN1 to ISN4) come from the

negative node of output inductors(the node connecting

output plane).

DO NOT connect the current sense traces from phase

plane or output plane. Only connect these traces from

both sides of output inductors can achieve the goal of

precise Kelvin sense. The current sense feedback loops

have to be routed away from switching elements, and

the current sense RC elements have to be put near

their respective ISN or ISP pins of the RT8862 and also

away from noise switching elements. At lease 10 mil

width is suggested for current sense feedback loops.

RT8862 Datasheet(PDF) 28 Page - Richtek Technology Corporation

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