L6758A datasheet(22/42 Pages) STMICROELECTRONICS

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Output voltage positioning

L6758A

22/42

DocID023298 Rev 2

6.2

Multi-phase section: current reading and current sharing

loop

The L6758A embeds a flexible, fully-differential current sense circuitry that is able to read

across inductor parasitic resistance or across a sense resistor placed in series to the

inductor element. The fully-differential current reading rejects noise and allows the placing

of sensing elements in different locations without affecting the measurement accuracy. The

trans-conductance ratio is issued by the external resistor R

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 6):

Equation 1

Considering now to match 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 2

Figure 6. Current reading

The current read through the CSxP / CSxN pairs is converted into a current I

INFOx

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

current I

AVG

ΣI

INFOx

/ N is internally built into the device (N is the number of working

phases). The error between the read current I

INFOx

and the reference I

AVG

is then converted

CSxN

DC R

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

D C R

⁄

⋅

⋅⋅

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

⋅

PH AS Ex

⋅

DC R

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

CSxN

-------- I

PH ASE x

⋅



⋅

IN F O x

CSxP

CSxN

DCRx

PHASEx

Inductor DCR Current Sense

ICSxN=IINFOx

OUT

AM11139v1

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