L6717 datasheet(35/56 Pages) STMICROELECTRONICS

35 / 56 page

L6717

Output voltage positioning

Doc ID 17326 Rev 1

35/56

7.1

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 GND the PWMx signal that is not required to be

used according to Table 19. For three phase operation, short PWM4 to GND while for two

phase operation, short PWM3 and PWM4 to GND.

Caution:

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

errors in current-sharing and voltage-positioning: CSxP needs to be connected to the

regulated output voltage while CSxN needs to be connected to CSxP through the same RG

resistor used for the active phases. See Figure 2 and Figure 3 for details in 3-phase and 2-

phase connections.

Table 19.

CORE section - phase number programming

7.2

CORE section - current reading and current sharing loop

L6717 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 CSxN pin toward the reading points. The current sense circuit always tracks the

current information, the pin CSxP is used as a reference keeping the CSxN pin to this volt-

age. 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 CSxN pin is then given by

the following 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

PWM3

PWM4

2GND

GND

To driver

GND

To driver

CSxN

DCR

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

s L DCR

⁄

⋅

1s R C

⋅⋅

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

⋅

PHASEx

⋅

DCR

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

CSxN

-------- I

PHASEx

⋅

⇒

⋅

INFOx

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