L9779WD datasheet(49/170 Pages) STMICROELECTRONICS

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L9779WD

Functional description

169

6.8

Main relay driver

Figure 22. Main relay driver controlled by L9779WD

6.8.1

Main relay driver functionality description

Main relay driver MRD is controlled by L9779WD 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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