L6751 datasheet(31/59 Pages) STMICROELECTRONICS

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L6751

Output voltage positioning

Doc ID 023992 Rev 1

31/59

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 par-

allel 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 = ΣIINFOx / 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 L6751 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.

Figure 7 shows the current sense circuit used to implement the load-line. The current flow-

ing across the inductor(s) is read through the R-C filter across the CSxP and CSxN pins. RG

CSxN

DCR

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

DCR

⁄

⋅

⋅⋅

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

⋅

PHASEx

⋅

DCR

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

CSxN

--------

PHASEx

⋅

⇒

⋅

INFOx

CSxP

CSxN

DCRx

PHASEx

Inductor DCR Current Sense

ICSxN=IINFOx

OUT

AM14815v1

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