LM5069: Attempting Inrush Current Limiting

Another question that came up.... If you have a single MOSFET you enter the SOA values from that device into the spreadsheet calculator. If you parallel two devices, do the SOA values need to be adjusted since now you have two devices? I tried changing the number of MOSFETS from 1 to 2 and SOA does not seem to adjust. I would think that if you parallel two devices, then the values for SOA would adjust at least somewhat or at best 2x?

Mike Nycz

Rakesh,

Thank you for the feedback. Since I have hardware for the LM5069 on my board already, I think I will work with the third option you provided above. I still have a few questions...

The MOSFET for Q1 that I chose was an Infineon IRF1405STRLPBF. The datasheet is attached. The values for SOA in the calculator are what I determined from the datasheet for the IRF1405STRLPBF.  Did you reset them for the device used on the eval board? I just want to make sure.

When the current limit is reached as set by the Current Limit setting (in my case 15.7A), does this kick in during the inrush as well? Because with the capacitors that I have on VOUT, the current could go (if not checked) very high at first. So my question is, Will this cut off momentarily and then kick back on after the current goes below the limit and then cycle again essentially clamping the output current at 15.7A?

The values that are calculated (Resulting MIN, TYPICAL, MAX Current Limits)... Are the MIN and MAX based upon the tolerances in the LM5069? Or does it make assumptions for the attached components as well?

For the UPPER UVLO / LOWER UVLO and UPPER OVLO /LOWER OVLO, Will these function as clamps of the input voltage as well? I am using a power source that meets MIL-STD 704. This says that my input range of voltage is 22V to 29V DC. I do not want the LM5069 to shut off if the voltage goes out of this range. Should I be setting the OVLO and UVLO values outside of this range to allow small excursions outside of the range?

Thank you,

Mike Nycz

Rakesh,

      I discovered another question. In the components defined in the Design Summary section of the calculator, there is a component Z1. The P/N in the calculator leaves the last 2 characters as xx. These characters define the clamp voltage. Since I am expecting the max voltage to be 29V into the power supply, I am assuming that a 30V clamp would work in this case. The options available are 28V and 30V. Let me know if you feel this choice is appropriate?

Thank you,

Mike Nycz

Hi Mike,

The MOSFET for Q1 that I chose was an Infineon IRF1405STRLPBF. The datasheet is attached. The values for SOA in the calculator are what I determined from the datasheet for the IRF1405STRLPBF.  Did you reset them for the device used on the eval board? I just want to make sure.

No, I have not updated Step-3

When the current limit is reached as set by the Current Limit setting (in my case 15.7A), does this kick in during the inrush as well? Because with the capacitors that I have on VOUT, the current could go (if not checked) very high at first. So my question is, Will this cut off momentarily and then kick back on after the current goes below the limit and then cycle again essentially clamping the output current at 15.7A?

During startup the inrush current is limited to lowest of power limited current value (Plim/Vin) or current limit setting or soft start current set by dvdt circuit.

The values that are calculated (Resulting MIN, TYPICAL, MAX Current Limits)... Are the MIN and MAX based upon the tolerances in the LM5069? Or does it make assumptions for the attached components as well?

Yes

For the UPPER UVLO / LOWER UVLO and UPPER OVLO /LOWER OVLO, Will these function as clamps of the input voltage as well? I am using a power source that meets MIL-STD 704. This says that my input range of voltage is 22V to 29V DC. I do not want the LM5069 to shut off if the voltage goes out of this range. Should I be setting the OVLO and UVLO values outside of this range to allow small excursions outside of the range?

LM5069 will cut-off the Vout if Vin violates UVLO or OVP thresholds.

Hi Mike,

TVS diode will have wide range of clamping voltage based on the current it shunts. The following link can help you https://www.powerelectronics.com/technologies/circuit-protection-ics/article/21853356/tvs-clamping-in-hotswap-circuits 

So for the UVLO and OVLO thresholds, if I do not want to the power to cut off due to voltage threshold, I should place these thresholds above and below the operating range of the input voltage? 

Can you explain what these parameters are intended to protect against?

Thank you,

Mike Nycz

You can disable them by connecting OVLO to GND and UVLO to VIN.

These are provided for protecting the load from under voltage and overvoltage's.

Best Regards, Rakesh