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# LM5069QUICK-CALC: LM5069 Design Calculator, SOA margin query

Part Number: LM5069QUICK-CALC
Other Parts Discussed in Thread: LM5069

Hi,

We are using LM5069-2 IC for hot swap application, below are the specs:

- Vin = 24V +/-10% tolerance

- Current Limit : 10A,

- Max load Capacitance : 2200uF

- Ta : 60C

- Power limit : 54W, Timer capacitor : 470nF

Q1: We are using LM5069_Design_Calculator_Rev C to perform the calculations, unfortunately not able to meet the SOA margin (A margin of >1.1 is required and a margin of >1.3 is recommended to account for the variation in the power limit and timer) considering ISC012N04LM6 MOSFET. Is this because of the power limit & timer capacitor selection? Attaching the file for reference. Since, we didn’t meet the derated SOA/Power limit TI recommends to use soft start control. Even then not able to meet the SOA margin!

Q2: Power limit dependency, we understand that IC determines the Pd by monitoring its drain-source voltage (SENSE to OUT), and the drain current through the sense resistor (VIN to SENSE). The product of the current and voltage is compared to the power limit threshold programmed by the resistor at the PWR pin. If the power dissipation reaches the limiting threshold, the GATE voltage is modulated to reduce the current in  MOSFET. While the power limiting circuit is active, the fault timer is active. Right? Is the power limit dependent on the maximum power dissipation of the MOSFET? Meaning, should the power limit be less than max Pd of the MOSFET? What if it is limited close to max Pd of MOSFET

Q3: What is the maximum & minimum current limit that can be designed using LM5069-2 IC?

LM5069_Design_Calculator_Using ISC012N04LM6.xlsx

• Hi Vinod

Thanks for reaching out!

1A) The ISC012N04LM6 MOSFET won't be able to support the target specifications. We need to use stronger SOA MOSFET and dvdt soft-start method.

We have another design calculator which suggests suitable MOSFET https://www.ti.com/lit/zip/snvr506

I have entered your specs and attached the tool here

6170.LM5069 Design Calculator with FET Recommendation.xlsm

2A) Power limit setting can be higher than the power dissipation in the MOSFET. Fault timer gets activated only when the device is in power limit or current limit mode of operation.

3A) Max sense resistor should be limited to 100mOhm that means min current limit is 0.55A.

You can set current limit to higher values as along as you can manage power dissipation in the shunt resistor

Best Regards

Rakesh

• Hi Rakesh,

Thanks for the response!

Yes,  ISC012N04LM6 MOSFET doesn't meet the SOA margin as per the calculations. We will look for any other alternate part.

Q1: Is the MOSFET SOA dependent on power limit and fault timer? We have fault time > Tstart (additional margin of 50% of the fault time)

Q2: What if we use ISC012N04LM6 MOSFET that doesn't meet SOA margin, will this affect the performance of the circuit over a period?

Thanks,

Vinod

• Hi Vinod,

The SOA margin gets improved as power limit and fault timer are reduced

Design with insufficient SOA margin results in MOSFET failure for fault conditions

Best regards

Rakesh