RT8855 데이터 시트보기 (PDF) - Richtek Technology

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RT8855

4/3/2/1-Phase PWM Controller for AMD AM2/AM2+ CPUs

Richtek Technology 

RT8855

CFB

VOUT RFB

C2

R1 C1

FB

COMP

-

+

EAP = VDAC - VADJ

-

+

EAP - VQR

IOUT

VOUT

FB

FB

QR

QR

= VEAP

= VEAP - VQR

I1

I2

IOUT, total

Constant ratio

Figure 8. Load Transient Quick Response

CORE Section- Current Balance

In Figure9, IX[n] is the current signal which is proportional

to the current flowing through channel n. The current error

signals IERR[n] ( = IX[n] − AVG(IX[n])) are used to raise or

lower the valley of internal sawtooth waveforms (EAMP[1]

to RAMP[n]) which are compared with ErrorAmp output

(COMP) to generate PWM signal. To raise the vally of

sawtooth waveform will decrease the PWM duty of the

corresponding channel while to lower the sawtooth

waveform valley will increase the PWM duty. Eventually,

current flowing through each channel will be balanced.

COMP

RAMP[1]

+-

Interleaved IERR[1] x RCB

RAMP[n]

+-

+

CMP

-

+

CMP

-

BUF

BUF

PWM[1]

PWM[n]

IERR[n] x RCB

Figure 9. Circuit Channel Current Balance

CORE Section- Phase Current Adjustment

If phase current is not balanced due to asymmetric PCB

layout of power stage, external resistors can be adjusted

to correct current imbalance. Figure10 shows two types

of current imbalance, constant ratio type and constant

difference type. If the initial current distribution is constant

ratio type, according to Equation (3), reducing RCSN[1]

can reduce IL[1] and improve current balance. If the initial

current distribution is the constant difference type,

according to Equation (2), increasing RCSP[1] can reduce

IL[1] and improve current balance.

I1

I2

IOUT, total

Constant difference

Figure 10. Category of Phase Current Imbalance

CORE Section-Over Current Protection (OCP)

OCP Comparator

PWM

Controller

VIN

ILX

HS LX DCRX

LS

RX

CX

1/4IIMAX

1/8IX

1.6V +

-

48

IIMAX

VIMAX

RIMAX

+

GM

-

RCSNX

IX

RT8855 CORE section

Figure 11. Over Current Protection for CORE section.

CORE section uses an external resistor RIMAX connected

to IMAX pin to generate a reference current IMAX for over

current protection as depicted in Figure 11.

IIMAX

=

VIMAX

RIMAX

where VIMAX is typical 1.6V. RT8855 senses each phase

current IX and OCP comparator compares sensed average

current with the reference current. Equivalently, the

maximum phase average current ILX(MAX) is calculated as

below :

www.richtek.com

14

DS8855-01 April 2011

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