RT8841 datasheet(30/32 Pages) RICHTEK

30 / 32 page

RT8841

DS8841-01 April 2011

www.richtek.com

After determining desired phase margin, according to the

following Equation :

-1

ZCO

P.M.

tan

and

F =

⎛⎞

⎛

⎞

−≥

⎜⎟

⎜

⎟

⎝⎠

⎝

⎠

FZ and FP can be determined by choosing proper FCO to

FZratio to meet Equation (25). Again, R1 can be determined

by the Equation (16).

R2 can be determined by the following Equation :

IN(MAX)

⎛⎞

=×

⎜⎟

⎝⎠

Other component values of the Type-III compensation can

then be calculated as :

2R2 F

2R1 F

2C3 F

××

−

××

Layout Considerations

For best performance of the RT8841, 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

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

Figure 14. Bode Plot of Type-II Compensation

Frequency (Hz)

Figure 15. Type-III Compensation

VREF

Figure 16. Bode Plot of the Type-III Compensation

Frequency (Hz)

FZ = FZ1 = FZ2

FP = FP1 = FP2

RT8841 Datasheet(PDF) 30 Page - Richtek Technology Corporation

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