L6711 datasheet(15/38 Pages) STMICROELECTRONICS

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L6711

5.2

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 6 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 8):

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 con-

stant 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 equiva-

lent output impedance), it results:

where

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 current information of each

phase, information about the total current delivered (IDROOP = IINFO1 +IINFO2 + IINFO3) and the average cur-

rent for each phase (IAVG = (IINFO1 + IINFO2 + IINFO3)/3) is taken. IINFOX is then compared to IAVG to give the

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

Since Rg is designed considering the OC protection, to allow further flexibility in the system design, the

resistor in series to CSx+ can be split in two resistors as shown in Figure 8.

Figure 8. Inductor Current Sense

CSx-

µA

-------

⋅

1s R

gRC

() Cg

⋅⋅

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

⎝⎠

⎜⎟

⎛⎞

PHASEx

⋅⋅

-------

gRC

() Cg ICSx-

≥

⋅

µA

-------- I

PHASEx

⋅

µAI

INFO x

INFOx

PHASEx

--------

⋅

Rg(RC)

CSx+

CSx-

PHASEx

OUT

IPHASE

CS_SEL

Rg(a)

µA

ICSx-

Design Equations:

gRC

()

L Cg

⋅

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

gRC

()

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