L6718TR datasheet(51/71 Pages) STMICROELECTRONICS

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L6718

Output voltage positioning

Equation 1

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 2

Figure 11. Current reading

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

proportional to the current delivered by each phase and the information regarding 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.

8.2

Multi-phase section - defining load-line

The L6718 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

dependence of the output voltage on the load current, a static error, proportional to the

output current, causes the output voltage to vary according to the sensed current.

Figure 11 shows the current sense circuit used to implement the load-line. The current

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

RG programs a trans-conductance gain and generates a current ICSx proportional to the

current of the phase. The sum of the ICSx current is then sourced by the FB pin (IDROOP).

RFB gives the final gain to program the desired load-line slope (Figure 10).

CSxN

DCR

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

DCR

⁄

⋅

⋅⋅

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

⋅

PHASEx

⋅

DCR

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

CSxN

--------

PHASEx

⋅

⇒

⋅

INFOx

CSxP

CSxN

DCRx

IPHASEx

Inductor DCR Current Sense

ICSxN=IINFOx

OUT

AM12885v1

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