RT8800AGQV datasheet(11/21 Pages) RICHTEK

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RT8800A

DS8800A-04 August 2007

www.richtek.com

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Figure 2

COMP

VCORE

SSE_Internal

SS_Internal

Cross-over

VRAMP_Valley

Time-Sharing DCR Current Sensing

RT8800A adopts an innovative time-sharing DCR current

sensing technique to sense the phase currents for phase

current balance (phase thermal balance), over current

protection and load line regulation as shown in Figure 3.

The current sensing amplifier GM samples and holds

voltages VX across the current sensing capacitor CX by

turns in a switching cycle. According to the Basic Circuit

Theory, if

then

Consequently, the sensing current IX is proportional to

inductor current ILX and is expressed as

The sensed current IX is used for current balance, over

current protection, and droop tuning as described as

followed. Since all phases share one common GM, GM

offset and linearity variation effect are eliminated in

practical applications. As sub-milli-ohmgrade inductors are

widely used in modern motherboards, slight mismatch of

GM amplifiers offset and linearity results in considerable

current shift between phases. The time sharingDCR current

sensing technical is extremely important to guarantee

phase current balance at mass production.

COMM

Figure 3

Figure 4 and 5 show the linearity of GM amplifier and its

test circuit respectively. A voltage source is applied to

ISPx while other ISPx pins are short to VOUT. The voltage

VADJ across resistor RADJ is measured. It is observed from

Figure 5 shows that all ISPx share the same

transconductance linearity and voltage offset.

Figure 4. The Linearity of GMx

VOUT

S/H CKT

RLX

RCOMM

ILX

+ VX -

20406080

100

VC (mV)

GM1

GM2

GM3

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