L6756D datasheet(21/36 Pages) STMICROELECTRONICS

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L6756D

Output voltage positioning

21/36

5.2

Current reading and current sharing loop

L6756D 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 ele-

ment. The fully-differential current reading rejects noise and allows placing sensing element

in different locations without affecting the measurement's accuracy. The trans-conductance

ratio is issued by the external resistor RG placed outside the chip between CSxN pin toward

the reading points. The current sense circuit always tracks the current information, the pin

CSx 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 ele-

ment. The current that flows from the CSxN pin is then given by the following equation (See

Figure 7):

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:

RG resistor is tipical designed in order to have an information current IINFOx in the range of

μA at the OC Threshold.

The current read through the CSx / 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.

Figure 7.

Current reading connections

CSx-

DCR

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

s L DCR

⁄

⋅

1s R C

⋅⋅

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

⋅

PHASEx

⋅

-------

CSxN

-------- I

PHASEx

⋅

⇒

⋅

INFOx

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