L9965P-FN datasheet(51/104 Pages) STMICROELECTRONICS

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Figure 25. ER cap discharge path

RDCH

ESR

CER

ERBST

ERDCHSW

RDSON_ERDCHSW

IDCH

The ER discharge switch is an integrated switch that creates a low-ohmic path between the ER capacitor

connected to ERDCHSW pin and ground. In the standard application scenario with ERBST mounted, the

ERDCHSW is externally connected to the ER capacitor with an external resistor (RDCHG). So, this resistor defines

the current used during the discharge phase of the ER capacitor diagnostic.

The ER discharge switch can also be controlled via SPI with the ERDCHSW_EN bit located in the ERCAP

The ERDCHSW_EN bit is set by the microcontroller but it is automatically cleared and ERDCHSW is disabled

when the energy stored in the ER cap is needed as energy reserve:

•

For the device itself when it enters the DEEP-SLEEP state.

•

For deployment event when the IC enters ARM state.

•

For ER cap diagnostic when the IC enters DIAG state.

Note:

There is no constraint between the ERBST enable and the ERDCHSW enable, so it is under the user's

responsibility to prevent a possible conflict and avoid an excessive thermal heating in the integrated ER

discharge switch.

The ER discharge switch has an overcurrent protection feature performed through a voltage comparator that

monitors the VDS voltage of the switch. When the voltage on ERDCHSW pin exceeds VERDCHSW_OV_H for a time

longer than tERDCHSW_FLT, the switch is disabled, the FAULTN pin goes low and the ERDCHSW_OV bit in the

ERCAP register is set to 1. The switch is reenabled if the voltage on ERDCHSW pin is lower than

VERDCHSW_OV_L for a time longer than tERDCHSW_FLT (fault is considered as disappeared) and the

ERDCHSW_OV bit is read and cleared.

Referring to a simplified schematic of the Figure 25, typical VERBST and VERDCHSW discharge profiles are shown

in the Figure 26.

L9965P

Functional description

DS14702 - Rev 4

page 51/104

L9965P-FN Datasheet(PDF) 51 Page - STMicroelectronics