L6711 datasheet(20/50 Pages) STMICROELECTRONICS

20 / 50 page

Current reading and current sharing control loop

L6711

20/50

µA offset allows negative current reading, enabling the device to check for dangerous

returning current between the phases assuring the complete current equalization. From the

current information of each phase, information about the total current delivered (IDROOP =

IINFO1 + IINFO2 + IINFO3) and the average current for each phase (IAVG = (IINFO1 + IINFO2 +

IINFO3)/3 ) is taken. IINFOX is then compared to IAVG to give the correction to the PWMx

output in order to equalize the current carried by the three phases.

Figure 6.

Current reading across LS mosfet: timing (left) and circuit (right) for each

phase.

7.2

Inductor current reading

Shorting CS_SEL pin to SGND, the current flowing trough each phase is read using the

voltage drop across the output inductor or across a sense resistor (RSENSE) in its series and

internally converted into a current. The transconductance ratio is issued by the external

resistor Rg placed outside the chip between CSx- and CSx+ pins toward the reading points

(see Figure 5 right).

The current sense circuit always tracks the current sensed and still sources a constant 50

µA

current from the CSx+ pin: this pin is used as a reference keeping the CSx- 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 CSx- pin is then given by the following equation (See Figure

7):

Where IPHASEx is the current carried by the relative phase.

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. Moreover, it is also important for the load transient response and to let

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

where

IINFOx is the current information reproduced internally.

µA offset allows negative current reading, enabling the device to check for dangerous

returning current between the phases assuring the complete current equalization. From the

IPHASEx

ILSx

IINFOx

TTRACK

TSW

CSx+

CSx-

LGATEx

IPHASEx

µA

ICSx-

CSx-

µA

-------

⋅

1s R

gR C

() Cg

⋅⋅

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

⎝⎠

⎜⎟

⎛⎞

PHASEx

⋅⋅

gRC

() Cg

CSx-

≥

⋅

µA

-------- IPHASEx

⋅

µAI

INFOx

PHASEx

--------

⋅

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