L6722 datasheet(18/34 Pages) STMICROELECTRONICS

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7 Output voltage positioning

L6722

18/34

Figure 8.

Offset definition (margin mode)

●

No Offset (M1=1; M2=0)

●

Positive Margin (M1=0; M2=0)

●

Negative Margin (M1=0; M2=1)

Offset resistors may be simply defined as follow

where VTARGET-POS and VTARGET-NEG are the target voltages for positive and negative margin

mode.

Offset current is always sourced from REF_IN pin: to avoid having steps during soft-start, the

introduction of COS is required. The resulting time-constant need to be negligible with respect to

the soft-start time as well as long enough to smooth the initial step. Typical values are in the

range of few tens / hundreds of nF.

Offset function can be easily disabled by shorting REF_IN and REF_OUT together.

Warning:

Maximum offset must be limited to less than 200mV to avoid setting the OVP protection

resulting in a maximum +25% margin.

7.2

Droop function (optional)

This method "recovers" 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 9 shows the Current Sense Circuit used to implement the Droop Function. The current

flowing across the three inductors is read through the RPH - CPH filter across CS+ and CS- pins.

RD programs a transconductance gain and generates a current ICS proportional to the average

of the currents of the three phases. The current ICS is then mirrored and, multiplied by three,

sourced by the FB pin (IDROOP). RFB gives the final gain to program the desired load-line slope.

Ref

REF_IN

REF_OUT

ROS1

COS

ROS2

To Vout

(Remote Sense)

COMP

VSEN

64k

FBR

FBG

64k

RFB

REFIN

Ref

OS1

⋅

REFIN

Ref

OS1

⋅

()

OS2

OS1

OS2

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

⋅

OS1

TARGET

POS

–

Ref

–

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

OS2

OS1

TARGET

POS

–

TARGET

POS

–

TARGET

NEG

–

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

⋅

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