L6751B datasheet(35/58 Pages) STMICROELECTRONICS

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L6751B

Output voltage monitoring and protection

Doc ID 024028 Rev 1

35/58

7.1

Overvoltage

When the voltage sensed by VSEN and/or SVSEN surpasses the OV threshold, the control-

ler acts in order to protect the load from excessive voltage levels avoiding any possible

undershoot. To reach this target, a special sequence is performed as per the following list:

–

The reference performs a DVID transition down to 250 mV on the section which

triggered the OV protection.

–

The PWMs of the section which triggered the protection are switched between HiZ

and zero (ENDRV is kept high) in order to follow the voltage imposed by the DVID

on-going. This limits the output voltage excursion, protects the load and assures

no undershoot is generated (if Vout < 250 mV, the section is HiZ).

–

The PWMs of the non-involved section are set permanently to HiZ (ENDRV is kept

low) in order to realize a HiZ condition.

–

OSC/ FLT pin is driven high.

–

Power supply or EN pin cycling is required to restart operation.

If the cause of the failure is removed, the converter ends the transition with all PWMs in HiZ

state and the output voltage of the section which triggered the protection lower than 250 mV.

7.2

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.

7.2.1

Multi-phase section

The L6751B 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 10

The IMON and ILIM pins are connected to GND through a resistor (RIMON and RILIM respec-

tively), implementing a load indicator with different targets.

●

IMON is used for current reporting purposes and for the DPM phase shedding. RIMON

must be designed considering that IMAX must correspond to 1.24 V (for correct IMAX

detection).

●

ILIM is used for the overcurrent protection only. RILIM must be designed considering

that the OC protection is triggered when V(ILIM)=2.5 V.

In addition, the L6751B also performs per-phase OC protection.

–

Per-phase OC.

Maximum information current per-phase (IINFOx) is internally limited to 35 μA. This

end-of-scale current (IOC_TH) is compared with the information current generated

MON

LIM

DCR

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

OUT

⋅

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