L6751C datasheet(29/57 Pages) STMICROELECTRONICS

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L6751C

Output voltage positioning

trans-conductance ratio is issued by the external resistor RG placed outside the chip

between the CSxN pin toward the reading points. The current sense circuit always tracks

the current information; the CSxP pin is used as a reference keeping the CSxN pin to this

voltage. To correctly reproduce the inductor current, an R-C filtering network must be

introduced in parallel to the sensing element. The current that flows from the CSxN pin is

then given by the following equation (see Figure 7):

Equation 2

Considering now the matching of the time constant between the inductor and the R-C filter

applied (time constant mismatches cause the introduction of poles into the current reading

network causing instability. In addition, it is also important for the load transient response

and to let the system show resistive equivalent output impedance), it results:

Equation 3

Figure 7. Current reading

The current read through the CSxP / CSxN pairs is converted into a current IINFOx propor-

tional to the current delivered by each phase and the information about the average current

IAVG = IINFOx / N is internally built into the device (N is the number of working phases). The

error between the read current IINFOx and the reference IAVG is then converted into a voltage

that, with a proper gain, is used to adjust the duty cycle whose dominant value is set by the

voltage error amplifier in order to equalize the current carried by each phase.

6.3

Multi-phase section - defining load-line

The L6751C introduces a dependence of the output voltage on the load current recovering

part of the drop due to the output capacitor ESR in the load transient. Introducing a depen-

dence of the output voltage on the load current, a static error, proportional to the output cur-

rent, causes the output voltage to vary according to the sensed current.

ICSxN

DCR

-------------

1 s L DCR





1s R C



-------------------------------------- I



PHASEx



DCR

-------------

ICSxN

-------- IPHASEx







IINFOx

CSxP

CSxN

DCRx

PHASEx

Inductor DCR Current Sense

ICSxN=IINFOx

OUT

AM14815v1

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