RT8841 데이터 시트보기 (PDF) - Richtek Technology

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RT8841

4/3/2/1-Phase PWM Controller for High-Density Power Supply

Richtek Technology 

RT8841

After determining desired phase margin, according to the

following Equation :

tan-1



FCO

FZ





tan-1 

FZ

FCO





P.M.

2



45

and

FP

=

FCO2

FZ

FZ and FP can be determined by choosing proper FCO to

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

by the Equation (16).

R2 can be determined by the following Equation :

R2



R1

VP

VIN(MAX)

 

FCO

FLC

2



FZ

FCO

Other component values of the Type-III compensation can

then be calculated as :

C1 

2

1

 R2  FZ

C2



2

1

 R2  FP



1

C1

C3



2

1

 R1 FZ

R3



2

1

 C3 FP

FZ

FP

Frequency (Hz)

Figure 14. Bode Plot of Type-II Compensation

C2

R3 C3

R1

R2 C1

-

EA

+

+

-

VREF

FP = FP1 = FP2

FZ = FZ1 = FZ2

Frequency (Hz)

Figure 16. Bode Plot of the Type-III Compensation

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).

Figure 15. Type-III Compensation

Copyright ©2014 Richtek Technology Corporation. All rights reserved.

www.richtek.com

30

is a registered trademark of Richtek Technology Corporation.

DS8841-02 August 2014

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