L9918 datasheet(17/109 Pages) STMICROELECTRONICS

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2.2.4

UV/OV management

Overvoltage Safety function

For safety reason this function is implemented with independent circuitry respect to the Overvoltage function.

Safety Overvoltage function switches OFF the excitation driver through a dedicated path in order to guarantee

that the function is always properly operated.

Safety Overvoltage detection is referred to VBOV_safetyL,H thresholds and it is applied after TOVflt_safetyL,H filter

time. Such time is used even to calculate the elapsing of Safety Overvoltage condition.

Whenever the device exits the stand-by state, the safety overvoltage detection is disabled for 2 minutes in order

to guarantee the jump start functionality. Jump start inhibits OV safety functions but it doesn’t inhibit OV function

(related to U_FELD_AUS).

Overvoltage function:

Overvoltage beyond VBOV,x due to, i.e., switching off of electrical loads, leads to switching off the output stage, if

present for longer than VBOVft,x. To avoid complete de-energizing of the alternator, phase regulation function can

be enabled during Overvoltage function (LD_NR_DIS NVM bit). When Overvoltage condition is over, regulation

re-adjusts to the set value. The function is active in “Regulation” state only (please refer to Section 1.5 State

diagram).

The level for Overvoltage function activation and the correspondent filter time are defined by U_FELD_AUS and

U_FELD_AUS_TIME NVM bits.

Low voltage function:

In case of high electrical load switching-on, especially with defect or missing battery, the voltage of the power net

can drop to low values. If the voltage falls under the minimum operating voltage, the alternator can de-energize.

The duration of the voltage drop is minimized with the activation of the power stage as soon as, for longer than

VBLVft,x voltage drops below VLOW. While the low voltage function is active, EXC current limitation set via LIN

is disabled. When B+ rises again over VLOW, an active load response will smoothly resume its operation. The

(VLOW) threshold value is selectable by NVM bits (U_FELD_EIN).

Low voltage function can be enabled or disabled programming PRIORITY_CH_DIS bit.

Low voltage function can be implemented by two different strategies selectable by PRCH_LRC_EN NVM bit.

In case PRCH_LRC_EN = 0x0, PRIORITY_CH_DIS=0x0 is selected, 100 % EXC duty cycle is applied until B+

value returns greater than VLOW. Once this threshold is crossed, adopted EXC duty cycle is the one calculated by

the voltage regulation loop.

For PRCH_LRC_EN = 0x1, PRIORITY_CH_DIS = 0x0, Low voltage function behaviour is depicted in Figure 7:

when low voltage function is activated, 100 % EXC DC is applied until B+ crosses VLOW threshold. For B+

values greater than VLOW, a fixed DC increasing slope (0.78 % every EXC period) is applied.

In case, PRCH_LRC_EN = 0x0, PRIORITY_CH_DIS = 0x1, low voltage function is deactivated and EXC duty

cycle is the one calculated by the voltage regulation loop.

For PRCH_LRC_EN = 0x1, PRIORITY_CH_DIS = 0x1, low voltage function is deactivated and EXC duty cycle for

both B+ lower than VLOW and above VLOW , a fixed DC increasing slope (0.78 % every EXC period) is applied.

When detected, the electrical fault F_E is set to 1 if the “F-EL on Low Voltage” bit (ANZ_LV_EN) of NVM bit has

been set to 1.

L9918

B+ characteristics

DS13000 - Rev 2

page 17/109

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