L6701TR datasheet(22/44 Pages) STMICROELECTRONICS

22 / 44 page

10 Output Voltage Positioning

L6701

22/44

The device forces IDROOP = ICSx3, proportional to the read current, into the feedback resistor

RFB implementing the load regulation dependence. The output characteristic vs. load current is

then given by (Offset disabled):

Where RLL is the resulting load-line resistance implemented by the system.

The whole power supply can be then represented by a "real" voltage generator with an

equivalent output resistance RLL and a voltage value of VID.

RFB resistor can be then designed according to the RLL specifications as follow:

where RD is typically designed to have ICS = 35µA at the maximum output current (OCP).

Caution:

Droop function is optional, in case it is not desired, the Current Sense circuit can be modified so

that the device always read a null current (See Figure 8). To do this, it is enough to connect

CS+ directly to the output voltage leaving CS- unconnected. The reaction will keep CS+ and

CS- at the same voltage, always reading a null current and also assuring the FB disconnection

protection to be effective.

Figure 8.

Droop Function Current Reading Network

10.2

Fully-Differential Load-Line (Droop Function - Optional)

Fully-Differential current-reading for voltage-positioning allows the designer to save time in the

application fine-tuning since the BOM so obtained becomes layout-independent. The patent-

pending topology offered by L6701 allow implementing fully-differential current-sense still using

only two current-sense pins (CS+ and CS-). Figure 9 shows the typical Current-Sense Circuit

used to implement the Fully-Differential Droop-Function. The current flowing across the three

inductors is read through an RPH - CPH filter for each phase as well as an RD is required for

each phase to program the trans-conductance-gain. As previously mentioned, a current ICS

proportional to the average of the currents of the three phases is internally generated, mirrored

DCR

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

⋅

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

OUT

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

DCR

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

⋅

OUT

VID

DROOP

⋅

–

VID

DCR

------------- IOUT

⋅⋅

–

VID

OUT

⋅

–

DCR

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

⋅

DCR1

DCR2

DCR3

CPH

PHASE3

PHASE2

PHASE1

VOUT

ICS

CS+

CS-

IDROOP

x 3

RFB

COMP

DCR1

DCR2

DCR3

PHASE3

PHASE2

PHASE1

VOUT

ICS

CS+

CS-

IDROOP

x 3

RFB

COMP

Droop Function Enabled

Droop Function Disabled

L6701TR 数据表(PDF) 22 Page - STMicroelectronics

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