L6701TR datasheet(21/44 Pages) STMICROELECTRONICS

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L6701

10 Output Voltage Positioning

21/44

Output Voltage Positioning

Output voltage positioning is performed by selecting the reference DAC and by programming

the Droop Function and Offset to the reference (See Figure 7). The current (IDROOP) sourced

from the FB pin, directly proportional to the read current, causes the output voltage to vary

according to the external RFB resistor so implementing the desired load-line resistance. The

current (IOS) sourced from the REF_IN pin causes the reference voltage to be offset according

to the resistance RFB connected.

The output voltage is then driven by the following relationship:

Both DROOP and OFFSET function can be disabled: see Section 10.1 and Section 10.3 for

details.

Figure 7.

Voltage Positioning

10.1

Load-Line (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 8 shows the typical Current-Sense Circuit used to implement the Droop-Function in low-

cost application (saves component count). The current flowing across the three inductors is

read through the RPH - CPH filter across CS+ and CS- pins. RD programs a trans-conductance

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.

Considering the scheme reported on Figure 8, it is possible to observe that:

Time constant matching between the inductor (L / DCR) and the current reading filter

(

) is required to implement a real equivalent output impedance of the system so

avoiding over and/or under shoot of the output voltage as a consequence of a load transient. It

results:

OUT

VID

⋅

DR O OP

⋅

–

VID

COMP

VSEN

64k

FBR

FBG

64k

RFB

To Vcore

(Remote Sense)

REF_IN

REF_OUT

ROS

COS

OUT

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

1s L

⋅

DCR

⁄

1s R

⋅

⁄

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

DCR

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

⋅⋅

PH 3

⁄

⋅

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