L9779WD-SPI datasheet(44/141 Pages) STMICROELECTRONICS

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Functional description

L9779WD-SPI

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DocID027721 Rev 2

6.7

Main relay driver

Figure 22. Main relay driver controlled by L9779WD-SPI

6.7.1

Main relay driver functionality description

Main relay driver MRD is controlled by L9779WD-SPI depending on the voltage levels at

pins KEY_ON, VB and the power latch mode set by the µC as described in the previous

sections.

The output stage MRD for main-relay-control is realized with a low-side-switch with

integrated clamping at VCL voltage realized with a zener diode.

When VB is present (VB>VB_LV) the MRD driver is protected, in ON condition, against the

over temperature fault. When the temperature is above junction the MRD is switched off.

After

HYSTERESIS the MRD returns to normal operation automatically.

In case of MRD short to battery without VB present i.e. during start-up sequence, when the

current exceeds the IOVC value, this pin will be switched off after a certain filter time

TFILTEROVC; to turn on MRD again it is necessary a high to low transition on KEY_ON pin.

Refer to scenario 5 (Figure 29).

In case of MRD short to battery with VB present i.e. during normal mode, when the current

exceeds the IOVC value, this pin will be switched off after a certain filter time TFILTEROVC;

the uC can try to turn on the MRD using the command MRD_REACT until the VB voltage is

above VB_UV. Below this threshold the MRD retries to switch on, then if the fault is still

present the MRD switches off and to turn it on again it is necessary a high to low transition

on KEY_ON pin. Refer to scenario 6-7-8 (Figure 30, 31 and 32).

In every condition the bit MRD_OVC reports that the MRD is currently off due to a previous

over current event.

Diagnosis of MRD short to ground may be done as described in the power up/down

management unit, switching off the MRD keeping alive all other regulators.

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