VNF9D5F datasheet(14/106 Pages) STMICROELECTRONICS

14 / 106 page

During short circuit condition or medium/hard overload, the output voltage does not overcome current sense shut

down threshold then, the current sense is not operative, the I²t curve is not verified by the implemented algorithm

but the harness protection shall work according to the ILIM_LATCHx setting.

On top of that, the device output current is clamped to the current limitation threshold (ILIMH).

The device behavior in current limitation can be set by a dedicated OTP bit (ILIM_LATCHx) - 1x channel:

•

If ILIM_LATCHx = 1: channel is switched off (after tFILTER_OL) and latched. In order to restart the channel,

the MCU shall reset the latch-off event by refreshing the programmed value in the dedicated register

(CHLOFFTCR1 or CHLOFFTCR0). The action will clear the corresponding CHLOFFSR bit in Output Status

•

If ILIM_LATCHx = 0: channel is set in auto-restart mode, allowing power limitation.

3.5

Electronic harness protection (STi²Fuse)

The VNF9D5F embeds the STMicroelectronics proprietary I²t functionality, featuring 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. The electronic wire harness protection, I²t protection, is active in all

operating modes, except in:

•

Standby mode when the device is in the lowest quiescent current consumption mode and all analog and

digital functions are in idle mode and output stages are off. Also, when a channel is in CCM, capacitive

charging mode, the I²t protection on that specific channel is disabled.

•

In CCM mode the wire harness remains in any case fully protected, since CCM mode is the latest aborted

after tCCM_DIS with low RMS current.

•

In LED mode, when the channel is operating in high RDSON and low current limitation mode. In this case,

the wires are protected by the intrinsic thermal I-t capability of the device.

•

In reverse battery condition when outputs are automatically activated.

In all other conditions, the wire harness protection is active and works fully autonomous and in particular does not

require any MCU control or supervision. The I²t protection functionality is equipped with specific safety

mechanism such as built-in self-tests (BIST) for the current sense block responsible for acquiring the load current

and the I²t block itself, which makes this function ASILx ready. In the ITCNTSR register the current value of the

integrated I²t budget is reported and allows the application to monitor how much of the available I²t budget is

actually consumed.

The I²t protection is based on a continuous RMS output current calculation. The current sense for I²t calculation is

sampled for each channel every t_ I²t _SAMPLE (given by the internal base frequency for I²t state machine fCLK)

with linearity ensured up to ILIMH. Since the current sense block is not active in the hard short circuit condition or

whenever the output voltage drops below the current sense shutdown threshold (VOUT_FSD), the behavior of the

channel when in current limitation is programmable (see Section 3.4: Overload protection–Output current

limitation (ILIMH)).

In the case ILIM_LATCHx is programmed in latch-off mode, the current sense, and consequently the I²t protection

function are always active when output is on (for the exceptions see above).

In case ILIM_LATCHx is programmed in auto restart mode, the current sense, and consequently the I²t protection

function are active as long as the output voltage remains above the current sense shutdown threshold

(VOUT_FSD). As soon as the output voltage drops below the current sense shutdown threshold (VOUT_FSD), for

example through a hard short circuit, the current sense is inhibited. In such condition however the I²t wire harness

protection is still operative, integrating with the fastest counter speed, while RMS current is maintained at a low

value, thanks to the power limitation protection, hence still effectively protecting the wire.

The shape of the actual I²t protection curve is a staircase curve, which is determined by two configurable

parameters, I_NOM and t_NOM. Both parameters are accessible through the SPI FSITCRx register, read-and

writeable.

Nominal time tNOM

The default tNOM parameter is programmed by 3 OTP bits (tNOM0, tNOM1, tNOM2, for each channel).

Its default value is 300 s.

VNF9D5F

Protections

DS13946 - Rev 6

page 14/106

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