L6740L datasheet(24/44 Pages) STMICROELECTRONICS

24 / 44 page

Output voltage positioning

L6740L

24/44

6.1

CORE section - Phase # programming

CORE Section implements a flexible 2 to 4 interleaved-phase converter. To program the

desired number of phase, simply short to SGND the PWMx signal that is not required to be

used according to Table 11. For three phase operation, short PWM4 to SGND while for two

phase operation, short PWM3 and PWM4 to SGND.

Caution:

For the disabled phase(s), the current reading pins need to be properly connected to avoid

errors in Current-Sharing and Voltage-Positioning: CSx+ needs to be connected to the

regulated output voltage while CSx- needs to be connected to CSx+ through the same Rg

resistor used for the active phases.

Table 11.

CORE section - Phase number programming

6.2

CORE section - Current reading and current sharing loop

L6740L embeds a flexible, fully-differential current sense circuitry for the CORE Section 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

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 CSx- pin toward the reading points. The current sense circuit always tracks the cur-

rent information, the pin CSx+ 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 par-

allel to the sensing element. The current that flows from the CSx- pin is then given by the fol-

lowing equation (See Figure 10):

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 typically designed in order to have an information current IINFOx in the range of

about 35

µA (I

OCTH) at the OC Threshold.

Phase number

PWM1

PWM2

PWM3

PWM4

1n/a

to Driver

SGND

to Driver

SGND

to Driver

CSx-

DCR

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

s L DCR

⁄

⋅

1s R C

⋅⋅

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

⋅

PHASEx

⋅

-------

CSx-

-------- I

PHASEx

⋅

⇒

⋅

INFOx

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