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