LM7480-Q1: 48V Automotive rail protection (in addition to 12V and 24V rails)

Hi Navin,

What is the Max voltage (in all conditions including conditions like load dump) that we can expect on 48V rail in your system ?

LM7480-Q1 can be used if the Max Operating voltage is less than 65V (Recommended Operating Voltage).

The HGATE and DGATE and are generated internally w.r.t OUT and A respectively. Their voltages shouldn't be a problem as long as the voltage n A, VS are within the Recommended Operating Conditions.

Hi Praveen

The max positive transient will be 202V (ISO16750-2:4.6.4 Load dump). If I use circuit as shown below (Fig 10-25 in LM7480 datasheet):

• D3 will clamp anything >150V. 
• D4 will clamp any negative pulse <-33V.
• D1 will protect VS pin from any voltage >60V
• What is the purpose of D2? Is it to prevent OUT and A being <Vs? 

Thanks for confirming that HGATE and DGATE wil be generated internally, with ref to OUT

Hi Praveen

Thanks for confirming the HGATE and DGATE voltages.

The max voltage on the input rail is 204V (ISO16750-2:4.6.4 Load dump) so I was thinking of using this circuit below (fig 10-25 in datasheet), Please confirm the below:

• D3 clamps all voltages >150V
• D4 clamps all negative pulses <-33V
• D1 limits voltage at Vs to 60V
• What is the purpose of D2? Is it to prevent OUT and A to be a lower voltage than VS? Or is it some protection?

Thanks in advance

Hi Navin,

Yes, The common-source topology shown in Figure 10-25 of datasheet can withstand  200-V Unsuppressed Load Dump. Please see my response below,

• D3 clamps all voltages >150V
  • D3+D4 clamps the voltage at Input to less than 200V so as to not exceed the VDS rating of the 200V FET. 
• D4 clamps all negative pulses <-33V
  • Yes, this is to make sure the -ve voltage seen by the controller is within its Abs max rating of the controller (-65V).
• D1 limits voltage at Vs to 60V
  • Yes, this is to limit the voltage on VS pin in all conditions to be within the Abs Max Rating of VS pin (70V).
• What is the purpose of D2? Is it to prevent OUT and A to be a lower voltage than VS? Or is it some protection?
  • The purpose of D2 is to supply voltage to the VS pin from A pin. The 10 kohms in series with VS pin (which is placed to limit the current into Zener) limits the current required by the charge pump during startup conditions. So, a diode from A to VS provides uninterrupted current into the VS pin. 

Hi Praveen

Thanks for the answers

You mentioned the below

Praveen GD said:

The purpose of D2 is to supply voltage to the VS pin from A pin. The 10 kohms in series with VS pin (which is placed to limit the current into Zener) limits the current required by the charge pump during startup conditions. So, a diode from A to VS provides uninterrupted current into the VS pin. 

If there is a cold crank (undervoltage) or negative transient pulse, then Vs will be interrupted, does this mean that Ca will discharge and current flows through D2 and into Vs? In what other situations will the current to Vs be interrupted?  

I am working on the schematic for this, which I will post in the next post, would you be able to review it for me? Thanks in advance

Thanks

Navin

Hi Navin,

Without D2, the charge pump may not build up to the full capacity. The diode D2 provides a low impedance path (bypassing the 10k resistor) for power supply of the chip. This is valid in  normal conditions as well. 

Yes, you can send the schematic for review once it is done.  

Hi Praveen

Thanks for the confirmation, D2 makes sense now. Please find schematic attached, could you please review for me?

Thank you very much in advance

Hi Navin,

Thanks for sharing the schematics. I will review and get back to you by coming Tuesday. 

Hi Praveen

Thanks for your reply, much appreciated. Thanks for confirming schematic.

Regarding the soft-start, please confirm if R2 needs to be 0603 and C4 needs to be 100V rated?

Thanks

Navin

Hi Navin,

R2 can be 0603 or 0402. There will be power dissipation across it only during startup and turn off. 

C4 needs to be rated for > [Vin(max) + 15V]