LTC4020 Strong heating of the inductor and ITH compensation

Good day.
My project uses Vin 9 ... 28 V. Lead acid battery 6 cells .. Charge current 5 A. Load current 10A.
I am faced with the problem of strong heating of the coil with a load of more than 3 A, the coil heats up> 70 degrees. The coil used is such a SRP1770C-4R7M. All mosfet Sir664.

Could it be caused by incorrect compensation work? And can I calculate the denominations of the compensation components for the LTC4020 in the LTPowerCAD program using a project for example for the LTC3780, how true will this be?

Thank you!

  • I tried to change the ratings of the parts, the voltage does not jump at the ITH terminal, but the coil still heats up with a load of 3 Amperes. Please help me identify my problem. The operating frequency is set to 100 kHz, the resistor is 261 kOhm.

  • I increased the frequency to 250 kHz, put in a PQ2614BLA-6R8K choke and changed the compensation circuit. The situation has improved a little, but the throttle still continues to warm up at 10 amperes. On the oscillogram, the choke current, I do not like ripple, what could be the problem?

    Thank you!

    •  Analog Employees 
    on Oct 13, 2020 2:16 PM 1 month ago in reply to instalator

    I can study this further.  (I do not like the inductor-current ripple either.)  Can you post a schematic of your circuit?  See the attachment which describes how best to attach your schematic.

    PDF

  • I don't use R56, only R2 is set to 0.010.

    L1 - PQ2614BLA-6R8K

    PDF

    •  Analog Employees 
    on Oct 14, 2020 4:07 PM 1 month ago in reply to instalator

    Thank you for posting the schematic.  I have some comments on the circuit.

    • The schematic shows the MODE pin is open-circuit.  For lead-acid charging the MODE pin should be tied to INT_VCC.
    • The voltage at SHDN pin will always be 0V if R24 is not installed.  The voltage at SHDN must be greater than 1.225V for the converter to operate.
    • The connection to VIN_REG pin will affect the converter operation when VIN<10V (VIN_REG<2.5V).  Is this what you want?  See VIN_REG pin description, datasheet p. 8.
    • It will be very difficult to control the voltage at RNG/SS with the circuit U3+Q15.  A colleague has suggested the circuit shown in the attached LTspice file.  Note that the LTspice simulation shows good behaviour, but I have not actually tried this circuit in the lab.

    For the compensation I suggest you try these values for the compensation at VC.

    • R42 - 97kΩ
    • C30 - 180pF
    • C32 - do not install

    If you still have problems, please send oscilloscope pictures of SW1, SW2, inductor current.  A horizontal sweep of 1µs/division is a good setting to analyze the waveforms.

    LTC4020 RNG_SS Control_gbcomp.asc