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VBATMON Operating and Absolute Limits – L9678P

Umesh_Kumar278
Associate III

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

1 ACCEPTED SOLUTION

Accepted Solutions
MJ_Pellegrini
ST Employee

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
    MJP_0-1748357274555.png
    and ISOK function
    MJP_1-1748358025581.png

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

    MJP_2-1748358045135.pngMJP_3-1748358051014.png

View solution in original post

12 REPLIES 12
ahsrabrifat
Senior II

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

MJ_Pellegrini
ST Employee

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
    MJP_0-1748357274555.png
    and ISOK function
    MJP_1-1748358025581.png

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

    MJP_2-1748358045135.pngMJP_3-1748358051014.png

Dear  @ahsrabrifat and @MJ_Pellegrini 

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

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

MJP_0-1748449465724.png

starting from a maximum ambient temperature of 95 degrees

MJP_1-1748449603606.png

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
Associate III

Dear @ahsrabrifat  and @MJ_Pellegrini ,

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
MOSISPI Master Out Slave In
MISOSPI Master In Slave Out
SCKSPI Clock
CSNSPI Chip Select
WAKEUPWake-up input
RESETReset output to MCU
VDDQI/O voltage reference (tie to MCU logic level)
ARMArming 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 
VSFExternal FET drive (for safing)
SS01, SS23Deployment outputs — any external components needed?
VBATMON 
COVRACTFor 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?

     
     

     

Umesh_Kumar278_2-1750058285271.png

 

 

 

 

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
Associate III

Dear @ahsrabrifat  and @MJ_Pellegrini ,

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_MOSISPI Master Out – data from MCU to L9678P
SPI_MISOSPI Master In – data from L9678P to MCU
SPI_SCKSPI Clock
SPI_CSSPI Chip Select
RESETSystem Reset – issued by MCU to initialize L9678P
WAKEUPWake-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)

Umesh_Kumar278_0-1750152605290.png

 

 

 

FENLExternal arming enable for low-side drivers (active low) – also used when no sensor input is available, for the same reason as above
ACLAdditional Communication Line – required to enter Arming State, as per ACLGOOD = 3 condition in Figure 9
uC_FLENIn 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)
ARMArming status output – acts as a “ready-to-deploy” flag; can be monitored by the MCU or read via SPI
VDDQLogic 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

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

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

Dear @MJ_Pellegrini   and @ahsrabrifat  ,

Thank you both for your support and previous replies.

One more doubts as an application that does not use sensors, remote interfaces, or PSI5 input. Our use case involves triggering deployment logic solely based on MCU-driven signals, without relying on internal safing logic driven by sensor thresholds(we are not using in vehicle application).

Our Main Question:

In order to reach the ARMING state and assert VSF_EN, as my understanding can we rely solely on:

  • ACL pin: 3 valid pulses (i.e., ACLGOOD = 3)

  • FENH pin: High

  • FENL pin: High

  • SPI SAFESEL = 1 (external safety logic)

 

Umesh_Kumar278_1-1750774237017.pngUmesh_Kumar278_2-1750774269633.pngUmesh_Kumar278_3-1750774285957.png

Umesh_Kumar278_4-1750774578153.png

 

And skipping:

  • All SPI sensor data processing

  • All safing threshold evaluations

  • SPI safing record logic

Since no sensors ( because external event has in MCU and based on that  wanna to deploy )

 Expected Transition Path (No Sensors):

  1. System starts in INIT → DIAG

  2. MCU issues SPI command SAFING_STATE

  3. L9678P enters SAFING state

  4. MCU asserts:

    • ACL pulses (3x ACLGOOD vvalid)

    • FENH = 1, FENL = 0

    • SAFESSEL = 1

  5. Device transitions to ARMING state

  6. VSF_EN is asserted

Umesh_Kumar278_0-1750774215292.png

 

Can you please confirm if this is the valid and complete minimal setup for entering the ARMING state when the internal sensor-driven safing logic is not used?  

if so yes In this setup, is it necessary to map below pins with the MCU right?

Pin Name Purpose
ACLProvide 3 pulses → ACLGOOD = 3 to enter ARMING state
FENLPulled high by MCU to indicate external safing logic
FENHPulled high by MCU to indicate external safing logic

 

Thank you!

BR 

Umesh