RT8857 데이터 시트보기 (PDF) - Richtek Technology

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RT8857

4/3/2/1-Phase PWM Controller with Embedded Drivers for CPU Core Power Supply

Richtek Technology 

RT8857/A

C2

CFB

R1 C1

VCCP

RFB

(Positive remote

sense pin of CPU)

(Negative remote

sense pin of CPU)

VCCN

RADJ

FB

IOFSP

-

+

+-

EA

+

EAP

VDAC

-

COMP

FBRTN

IOFSN

ADJ

Figure 6. Circuit for VOUT Differential Sensing and No

Load Offset

Load Transient Quick Response

RT8857/A utilizes a new quick response feature to supply

heavy load current demand during instantaneous load

application transient. RT8857/A detects load transient and

reacts via VOUT pin. When VOUT drops during load

application transient,the quick response comparator will

send asserted signals to turn on high side MOSFETs and

turn off low side MOSFETs. The LA1 signal, which is a

weaker quick response signal, will turn on only arbitrary

two channels', high side MOSFETs while turning off low

side MOSFETs also. The LA2 signal , which is a stronger

quick response signal, will turn on all channel's high side

MOSFETs while turning off low side MOSFETs also.

Therefore, the influence of total quick response function

of RT8857/A is adjustable, and the magnitude of quick

response is flexible via fine-tuning the resistors connected

to pin QR1 and QR2.

IOUT

VOUT

QR2

QR1

www.richtek.com

20

Figure 7

Output Current Sensing

The RT8857/A provides low input offset current-sense

amplifier (CSA) to monitor the output current of every

channel. Output current of CSA (IX[n]) is used for channel

current balance and active voltage position. In this inductor

current sensing topology, RS and CS must be set according

to the equation below :

L/DCR = RS x CS

(6)

Then the output current of CSA will follow the equation

below :

IX = [IL x DCR − VOFS-CSA + 235n x (RCSP − RCSN)]

/RCSN

(7)

235nA is typical value of CSA input offset current.

VOFS-CSA is the input offset voltage of CSA. VOFS-CSA of

RT8857/A is smaller than +/- 1mV. Usually, “VOFS-CSA +

235n x (RCSP − RCSN)” is negligible except at very light

load and the equation can be simplified as the equation

below :

IX = IL x DCR/RCSN

(8)

Loadline

Output current of CSA is summed and averaged in

RT8857/A. Then 0.5Σ(IX[n]) is sent to ADJ pin. Because

ΣIX[n] is a PTC (Positive Temperature Coefficient) current,

an NTC (Negative Temperature Coefficient) resistor is

needed to connect ADJ pin to GND. If the NTC resistor is

properly selected to compensate the temperature

coefficient of IX[n], the voltage on ADJ pin will be

proportional to IOUT without temperature effect. In RT8857/

A, the positive input of ErrorAmp is “VDAC − VADJ”. VOUT

will follow “VDAC − VADJ”, too. Thus, the output voltage

decreasing linearly with IOUT is obtained. The loadline is

defined as

LL(loadline) = ΔVOUT/ΔIOUT = ΔVADJ/ΔIOUT

= 0.5 x DCR x RADJ/RCSN

(9)

Briefly, the resistance of RADJ sets the resistance of

loadline. The temperature coefficient of RADJ compensates

the temperature effect of loadline.

Current Balance

In Figure 8, IX[n] is the current signal which is proportional

to current flowing through channel n. In Figure 9, the

DS8857/A-01 April 2011

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