L6759D datasheet(24/51 Pages) STMICROELECTRONICS

24 / 51 page

Output voltage positioning

L6759D

24/51

Doc ID 023240 Rev 1

Figure 9 shows the current sense circuit used to implement the load-line. The current

flowing across the inductor(s) is read through the R-C filter across the CSxP and CSxN pins.

RG programs a trans-conductance gain and generates a current ICSx proportional to the

current of the phase. The sum of the ICSx current, with proper gain eventually adjusted by

the PMBus commands, is then sourced by the FB pin (IDROOP). RFB gives the final gain to

program the desired load-line slope (Figure 8).

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

is required to implement a real equivalent output impedance of the system, so voiding over

and/or undershoot of the output voltage as a consequence of a load transient. The output

voltage characteristic vs. load current is then given by:

Equation 4

where RLL is the resulting load-line resistance implemented by the multi-phase section.

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

Equation 5

6.4

Multi-phase section - IMON information

The voltage on the IMON pin contains the analog information related to the current delivered

by the VR and it is digitized for VR12 current reporting. The pin sources a copy of the droop

current:

Equation 6

See Section 6 for details about current reading.

The Iout register contains analog-to-digital conversion of the voltage present on the IMON

pin considering the following relationships:

where RIMON is the resistor connected between IMON and

GND.

VIMON=1.24 V corresponds to IMAX. RIMON is designed according to this

relationship.

Note:

Current reporting precision may be affected by external layout. The internal ADC is

referenced to the device GND pin: in order to perform the highest accuracy in the current

monitor, RIMON must be routed to the GND pin with a dedicated net to avoid GND plane

drops affecting the precision of the measurement.

OUT

VID

DROOP

⋅

–

VID

DCR

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

OUT

⋅⋅

–

VID

OUT

⋅

–

DCR

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

⋅

IMON

DROOP

DCR

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

OUT

⋅

IMON

⋅

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