VBATMON Operating and Absolute Limits – L9678P

[Umesh_Kumar278]

Dear ST Support Team,

We are using the L9678P in an automotive design where the battery voltage is nominally 28 V.

From the datasheet, we understand the following for the VBATMON pin:

Absolute maximum voltage: 40 V

Operative maximum voltage: 18 V

We would like to confirm:

1. Can VBATMON be directly connected to a 28 V rail continuously, or is this outside the safe operating range?

2. Is it mandatory to use a voltage divider to reduce VBATMON below 18 V during normal operation?

3. Is VBATMON used only for battery voltage monitoring, or does it have a role in powering internal logic?

4. Does VBATMON directly influence wake-up and shutdown behavior of the IC?

We want to ensure safe operation and avoid long-term reliability issues.

Thank you in advance.

Best regards,

Umesh

[ahsrabrifat]

You must use a resistive voltage divider to scale down your 28 V rail to a level within the recommended operating range of VBATMON (typically aiming for 12–16 V as a safe design margin). A buffer (e.g. op-amp or zener clamp) may also be considered depending on the precision and protection requirements.

For example, for 28 V nominal and 18 V max input:

• Choose resistor values such that V_BATMON = 28 V × (R2 / (R1 + R2)) ≤ 18 V

• This implies a ratio where (R2 / (R1 + R2)) ≤ 0.643

[MJP · Accepted solution]

Dear Umesh,

I belong to Application Team of Passive Safety group.

Here our feedbacks:

1. You are below the AMR, but in any case you are overcoming the Operative Voltage for a permanent time.
   Usually in this condition the IC is not damaged, but parameter deviation may occur and reliability can be affected. So we don't suggest to do this.

2. Yes, it is our suggestion to use a voltage divider.

3. VBATMON is used for power-up and power-down sequences, and ERBOOST enable.

4. VBATMON is used to enable ERBOOST regulator
   
   and ISOK function
   

    It is also used to alarm the MCU in order to guarantee the correct functionality of RSU function.

   

[Umesh_Kumar278]

Dear  @ahsrabrifat and @MJP 

Thank you both for your support and previous replies. As part of finalizing our thermal and power design around the L9678P, I would appreciate your confirmation on one last point regarding the VDD5 and VDD3V3 regulators:

1. Are the following maximum current values valid for continuous operation?

   • VDD5: 200 mA max (as per Table 36)

   • VDD3V3: 125 mA max (as stated in Section 4.3.5)

2. Are these values suitable for power loss and thermal dissipation calculations, especially for dimensioning the external pass transistor (for VDD5) and ensuring reliability over full temperature range?

Please confirm if any additional derating or application-specific guidance is recommended.

This will help ensure the safe sizing of our power path. Thanks again for your assistance.

Best regards,

Umesh

[MJP]

Dear Umesh,

Yes, you are right: the values you have reported (200mA for VDD5 and 125mA for VDD3V3) are the maximum ones for continuos operation.

These values are calculated to keep the junction temperature below 150 degrees

starting from a maximum ambient temperature of 95 degrees

and considering that all the functions reported in the datasheet are enabled and are working.

Of course, if in your application you don't use some functionalities, you can have a higher starting ambient temperature.

Kind regards,

Jonathan

[Umesh_Kumar278]

Dear @ahsrabrifat  and @MJP ,

Thank you both for your support and previous replies.

We are currently finalizing our hardware design using the L9678P, with a focus only on deployment functionality and functional safety compliance according to IEC 61508.

Our goal is to clearly identify:

---

1. Which pins must be directly connected to the microcontroller (MCU)?

(For communication, monitoring, wake-up, and control logic)

We believe the following pins are required — please confirm:

Pin Function

MOSI	SPI Master Out Slave In
MISO	SPI Master In Slave Out
SCK	SPI Clock
CSN	SPI Chip Select
WAKEUP	Wake-up input
RESET	Reset output to MCU
VDDQ	I/O voltage reference (tie to MCU logic level)
ARM	Arming status output (optional for monitoring)

 2. Which pins require external hardware support?

We’ve identified the following as needing passives or control components:

Pin Description

VDD5, BVDD5
VDD3V3
VSUP, BVSUP
VER
ERBOOST
VSF	External FET drive (for safing)
SS01, SS23	Deployment outputs — any external components needed?
VBATMON
COVRACT	For external crossover switch control

---

3. Specific Clarifications Requested:

• ARM: Can this be left unconnected if not monitored by MCU?

• WDT/TM: Marked "Not for application" – should it be pulled low, left floating, or tied to MCU?

• SS01 / SS23: Are external components (e.g., resistors, capacitors, TVS) needed for these?

• COVRACT: Required only for external crossover FETs, or also with internal ER switch? Is MCU control needed?

• uC_FLEN:In the reference application diagram, I noticed that the uC_FLEN signal from the MCU appears to drive the gate of an N-channel FET in the safing path (related to the VSF-regulated supply). I would like to confirm:

  Is my understanding correct that uC_FLEN is a control signal from the MCU used to enable or disable the safing FET path (typically by pulling the gate low)?

  What is the recommended driving strategy from the MCU for this pin?

Note: We are not using ISO9141 and  the Remote Sensor Interface, so those pins are excluded.

Our use case is centered on deployment control and functional safety (IEC 61508), so we are only implementing what is essential for that scope.

Thank you again for your continued support.

Best regards,

Umesh

[Umesh_Kumar278]

Dear @ahsrabrifat  and @MJP ,

As already asked in the previous post — and now clarified further as I’ve gone through the datasheet and functional descriptions in detail — I’d like to summarize the key points we’ve understood and kindly request your confirmation on the following items.

We are finalizing our hardware design using the L9678P, focused exclusively on deployment functionality in compliance with IEC 61508. Our use case does not include external sensors (i.e., no sensor data read in Safing State), and we are not using the Remote Sensor Interface or ISO9141 features.

---

Pins Required for MCU Connection

Below is the list of pins we believe must be connected to the MCU for SPI communication, control logic, arming, and optional monitoring:

Pin Function

SPI_MOSI	SPI Master Out – data from MCU to L9678P
SPI_MISO	SPI Master In – data from L9678P to MCU
SPI_SCK	SPI Clock
SPI_CS	SPI Chip Select
RESET	System Reset – issued by MCU to initialize L9678P
WAKEUP	Wake-up input – required only if low-power or sleep modes are used
FENH	External arming enable for high-side drivers (active high) – used when no sensor input is available, and safing must be handled externally (since in safing State ARM and VSF won’t be valid    figure. 9)
FENL	External arming enable for low-side drivers (active low) – also used when no sensor input is available, for the same reason as above
ACL	Additional Communication Line – required to enter Arming State, as per ACLGOOD = 3 condition in Figure 9
uC_FLEN	In the reference schematic, uC_FLEN appears to be used as a GPIO from the MCU to control a FET in the VSF-powered safing path — is this correct? Could this function alternatively be driven by one of the L9678P’s GPO drivers to reduce MCU pin usage? (typically near U1 as shown in Figure 58)
ARM	Arming status output – acts as a “ready-to-deploy” flag; can be monitored by the MCU or read via SPI
VDDQ	Logic I/O reference – should be connected to MCU's I/O voltage supply to match logic levels

---

Please confirm if the above list is sufficient and minimal for a deployment-only application using without sensors ( not reading any external sensor data) .

If needed, I can follow up with the external hardware-supported pins (power, filtering, energy reserve, etc.).

Thank you again for your continued support.

Best regards,
Umesh

[MJP]

Dear Umesh,

my feedbacks:

• RESET is an output of L9678P that is used to reset the MCU in case of fault (see paragraph 4.5 of DS11626)
• WAKEUP: if you don't need the Sleep Mode, you should connect it to main supply
• FENH and FENL: signals for external arming (SAFESEL=1 in SYS)CFG register)
• ACL: it is a signal used in automotive application to arm and deploy all the loops (it is used in vehicle end of life to deploy the squibs before its scrapping)
• uC_FLEN: in automotive application it is a signal driven by MCU to enable the switch placed between VSF regulator and the "Safing FET"; it is used for safety because in automotive application there is the need for a double confirmation by L9678P (VSF enabled when it moves in ARMING state) and by MCU (uC_FLEN signal) to enable the Safing FET (also called third FET in some applications)
• ARM: it is an output used in DIAG state for "Deployment timer diagnostic" and in ARMING state to reflect the dwell time (time in which the deploy time will be accepted)
• VDDQ: it is the logic IO reference, it can be connected to VDD5 or VDD3V3

Kind regards,

Jonathan

[MJP]

Dear Umesh,

further feedbacks.

POINT 1

Reviewed in the other answer.

POINT 2

• VDD5/BVDD5: they drive and sense the external pnp to implement a 5V regulator; it is mandatory as it supplies the VDD3V3 regulator.
  VDD3V3: output of internal 3V3 linear regulator.
• VSUP/BVSUP: they drive and sense the external pnp to implement a 5V regulator. It can be not used if, as in your case, the Remote Sensor Interface is not used.
• VER: output to charge the Energy Reserve (ER) capacitor; the ER cap is used in automotive application to sustain the deployment during multiple deployment events (multiple squibs deployed together) and in case of battery loss.
• ERBOOST: output of ER boost regulator; it gives the possibility to charge the ER cap at a voltage (24V or 33V) higher than battery (higher the voltage, lower the size of the ER cap).
• VSF: output of VSF regulator; it drives the Safing FET.
• SSxy: supply of deployment loops; SS01 supplies channels 0 and 1, SS23 upplies channels 2 and 3.
• VBATMON: battery monitor pin; it is used in power-up sequence.
• COVRACT: digital output that goes high when the IC moves in PASSIVE mode.

In Figure 2 and Figure 58 of DS11626 you can find the typical application that can help you in understanding what written above.

POINT 3

• ARM: can be left open if not used.
• WDT/TM: connect to GND if not used (it can be used during development phase to disable the watchdog, see paragraph 8.3 in DS11626).
• SSxy: a 10nF cap is needed (see Figure 58 and Table 13 in DS11626).
• COVRACT: digital output that can be used as interrupt signal for MCU to report the entering in PASSIVE mode, or to drive the enable of an external circuit in case of battery loss.
• uC_FLEN: something just explained in the other answer; in any case, your understanding is correct. 
  In automotive application, both L9678P and MCU elaborate the data from the sensors and must be in agreement to arm: L9678P will enable VSF regulator, MCU will drive uC_FLEN signal. Then, the MCU sends the commands to deploy via SPI.

Kind regards,

Jonathan