VNF1248F datasheet(18/79 Pages) STMICROELECTRONICS

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eFuse function

Protection of wire harness and PCB can be performed by defining an ideal time to fuse curve as a result of a

maximum power dissipation over the time in the wire or copper PCB traces themselves. This function can ensure

that the insulation of wires and PCB are subject to a limited temperature and time budget that is below the

reliability specified values. Not respecting such specified limits can lead to the formation of a conducting path by

carbonization across the organic insulation materials and therefore local hot spot can conduct to sparking and fire

ignition.

The VNF1248F embeds the ST proprietary eFuse functionality for the implementation of a robust and flexible

overcurrent protection mechanism. The eFuse functionality features an intelligent circuit breaking aimed at

protecting PCB traces, connectors and wire harness from overheating, with no impact on load transients like

inrush currents and capacitance charging.

This function is set by two parameters called INOM and tNOM. The value of INOM corresponds to the maximum

continuous current while tNOM will determine a current versus time-to-fuse curve when load current is higher than

INOM. The expression of current versus time-to-fuse is approximated by an optimized stepwise function, which can

be adjusted in a range between the wire I2-t limit on one side and load transient characteristics on the other side.

The value of tNOM corresponds to the first step up of the curve. The current time curve is always active in

combination with very fast overcurrent protection that will be triggered when the current reaches a defined

threshold for hard short circuit condition.

When the current in the load is pulse wide modulated the eFuse function calculates the mean square root of the

current. Mean square root of the current is also calculated when switching on/off the power switch during normal

operation or after a switch off due to short circuit/overload condition. So, if for example the circuit is broken due to

an overload and after a while the circuit is activated again, the eFuse keeps in memory the previous condition and

still avoids that maximum IRMS is higher than INOM.

VIP-Fuse is programmed via SPI as follows:

•

VOC_THRS sets INOM = VOC_THRS/Rsense

•

VHSC_THRS sets hard short circuit current = VHSC_THRS/Rsense

•

T_NOM sets tNOM from 1 to 511 s

No intervention occurs for VSENSE < VOC_THRS, whilst an immediate shut-off occurs for VSENSE >

VHSC_THRS.

The eFuse functionality operating range is defined between VOC_THRS and VHSC_THRS. In that range, the

circuit breaking profile is defined by the stepwise function reported in Figure 5. The number of steps is

consequential to the selection of VOC_THRS and VHSC_THRS, the maximum being 15, when VOC_THRS = 6

mV and VHSC_THRS = 160 mV. This corresponds to a 1:26.67 ratio between the maximum allowed continuous

current and hard short circuit.

The Figure 6 shows the I2-t curve when VOC_THRS = 26.5 mV and VHSC_THRS = 105.60 mV. The number of

steps is reduced to 9 accordingly.

VNF1248F

eFuse function

DS14109 - Rev 7

page 18/79

VNF1248F Datasheet(PDF) 18 Page - STMicroelectronics

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