ADM1272 EVM board - Convergence problem in measurements according to calculations and stated in the data sheets

Hello Analog Devices' support team,

I recently started doing EVM testing for my personal project using your Hotswap component - ADM1272. During the component capability tests in FET SOA retention, results were discovered that do not converge to the data sheets and do not even converge on the theoretical calculations I have performed.

In terms of information about the set-up:
I fed the component at 30 V, limited the ISET current to 5 A (by placing the respective voltage divider in the relevant pin in the component),
And I set the Load Resistor of 2.6 Ohm. 

According to the calculation by the way of operation based on the pin "ISTART" - 
we accepted, by applying the load (by switching on using another FET for switching operation) for current damping time of about 0.8 msec

What we actually got:
We got by measured the V_gate, V_ds, and the Output current - is 416usec delay in the operation of the gate of the FET, which in turn caused for delay in the operation of the current damping!

Moreover:
We performed the same tests on different loads, including - shortcut (0 Ohm), 1.5 Ohm.
In those test - we got as expected, and convergence for what we calculated.

I would like to know if you can explain this phenomenon, and try to explain the above mismatch for this load.

I will try to upload lately the screenshot of the oscilloscope of the measurements

Thank you very much,

Evyatar

Parents
  • +1
    •  Analog Employees 
    on Sep 9, 2019 4:37 PM

    Hi Evyatar,

        The ADM1272 has three factors that limit the start-up current and timing. The first, as you have observed, is the ISTART pin, which sets the start-up current limit, which is normally NOT engaged because it is a protective limit, and not approached if the load is behaving itself. The second limit is the DVDT pin, which sets the gate voltage ramp rate, and which is usually the primary method of limiting inrush current. The third is the ESTART pin, which functions as a timer during start-up current limiting (when the ISTART limit is activated). You didn't mention ESTART, so it is unclear how you programmed the 800us time limit. The usual way is to place a series R-C on the ESTART pin so that the pin voltages charges up and reaches 1V when the desired time-out elapses.

       In your case, with 30V and 2.6 OHMS you should activate the ISTART current limit and charge up the ESTART pin. I assume from your description that this is happening within 416us. A larger ESTART capacitor will charge more slowly, of course, and may be what you need. A smaller ESTART series resistor will also help. For the 0 OHM and 1.5 OHM tests you may be activating the severe overcurrent limit, which shuts down the hotswap much more quickly, regardless of the ESTART pin.

       In order for me to help more I will need to see your circuit schematic, including load capacitance and expected operating load currents.

    Thanks!
      Nathan

Reply
  • +1
    •  Analog Employees 
    on Sep 9, 2019 4:37 PM

    Hi Evyatar,

        The ADM1272 has three factors that limit the start-up current and timing. The first, as you have observed, is the ISTART pin, which sets the start-up current limit, which is normally NOT engaged because it is a protective limit, and not approached if the load is behaving itself. The second limit is the DVDT pin, which sets the gate voltage ramp rate, and which is usually the primary method of limiting inrush current. The third is the ESTART pin, which functions as a timer during start-up current limiting (when the ISTART limit is activated). You didn't mention ESTART, so it is unclear how you programmed the 800us time limit. The usual way is to place a series R-C on the ESTART pin so that the pin voltages charges up and reaches 1V when the desired time-out elapses.

       In your case, with 30V and 2.6 OHMS you should activate the ISTART current limit and charge up the ESTART pin. I assume from your description that this is happening within 416us. A larger ESTART capacitor will charge more slowly, of course, and may be what you need. A smaller ESTART series resistor will also help. For the 0 OHM and 1.5 OHM tests you may be activating the severe overcurrent limit, which shuts down the hotswap much more quickly, regardless of the ESTART pin.

       In order for me to help more I will need to see your circuit schematic, including load capacitance and expected operating load currents.

    Thanks!
      Nathan

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