L6751C Datasheet, PDF (35/57 Pages) STMicroelectronics – Digitally controlled dual PWM for Intel VR12 and AMD SVI

L6751C
Output voltage monitoring and protection
7.2
7.2.1
Overcurrent and current monitor
The overcurrent threshold must be programmed to a safe value, in order to be sure that
each section does not enter OC during normal operation of the device. This value must take
into consideration also the process spread and temperature variations of the sensing ele-
ments (inductor DCR).
Furthermore, since also the internal threshold spreads, the design must consider the mini-
mum/maximum values of the threshold.
Multi-phase section
The L6751C features two independent load indicator signals, IMON and ILIM, to properly
manage OC protection, current monitoring and DPM. Both IMON and ILIM source a current
proportional to the current delivered by the regulator, as follows:
Equation 11
I_MON = I_LIM = ^D---_R--^C---_G--^R---  I_OUT
The IMON and ILIM pins are connected to GND through a resistor (R_IMON and R_ILIM respec-
tively), implementing a load indicator with different targets.
 IMON is used for current reporting purposes and for the DPM phase shedding. R_IMON
must be designed considering that I_MAX must correspond to 1.24 V (for correct IMAX
detection).
 ILIM is used for the overcurrent protection only. R_ILIM must be designed considering
that the OC protection is triggered when V(ILIM) = 2.5 V.
In addition, the L6751C also performs per-phase OC protection.
– Per-phase OC. Maximum information current per-phase (I_INFOx) is internally
limited to 35 A. This end-of-scale current (I_{OC_TH}) is compared with the
information current generated for each phase (I_INFOx). If the current information for
the single-phase exceeds the end-of-scale current (i.e. if I_INFOx > I_{OC_TH}), the
device turns on the LS MOSFET until the threshold is re-crossed (i.e. until I_INFOx <
I_{OC_TH}).
– Total current OC. the ILIM pin allows a maximum total output current for the
system (I_{OC_TOT}) to be defined. ILIM current is sourced from the ILIM pin. By
connecting a resistor R_ILIM to GND, a load indicator with 2.5 V (V_{OC_TOT}) end-of-
scale can be implemented. When the voltage present at the ILIM pin crosses
V_{OC_TOT}, the device detects an OC and immediately latches with all the MOSFETs
of all the sections OFF (Hi-Z).
Typical design considers the intervention of the total current OC before the per-phase OC,
leaving this last one as an extreme-protection in case of hardware failures in the external
components. Per-phase OC depends on the R_G design while total OC is dependent on the
ILIM design and on the application TDC and max. current supported. Typical design flow is
the following:
– Define the maximum total output current (I_{OC_TOT}) according to system
requirements (I_MAX, I_TDC). Considering I_MON design, I_MAX must correspond to 1.24
V (for correct IMAX detection) while considering ILIM design I_{OC_TOT} must
correspond to 2.5 V.
– Design per-phase OC and R_G resistor in order to have I_INFOx = I_{OC_TH} (35 A)
when I_OUT is about 10% higher than the I_{OC_TOT} current. It results:
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