LTC3871 does not work properly

Hello all,

I have designed a LTC3871 board referenced from the demo board DC2348A3-B schematic and made some changes according to my needs. I want to use LTC3871 as DC to DC voltage converter from 48V to 24V 20A output. So far I have boards produced and I am testing them. I am facing with many interesting issues. 

I have 1 board which is regulating 24V output and drawing only 20mA while there is no load and frequency is 105KHz. I have left SETCUR, ILIM, MODE, SS, PHSMD, DRVSET and RUN pins folating. I have tied BUCK pin to V5 and 34k to the FREQ pin. In this case the board works in DCM mode and I can see the gate signals are not proper square waves. This board draws only 20mA when there is no load and I can use it to trigger a selenoid which requires 6-8A. I can tell this board is OK.

I have another board identical to the first one and I just only change the FREQ resistor to 100k and the board starts to draw 220mA and LTC3871 is heating up to 135 celcius degrees. It is still regulating the output to the 24V and I can still use it to trigger the selenoid but heating is not normal I think. These boards drawing 200-250mA current in no load and LTC3871 is heating up to 140-150 celcius degree and I can see that the LTC3871 is tripping. It is like 5-6ms running 4-5ms stopping and then running again stopping again... Maybe thermal shutdown is making it to trip but it is not cooling down to restart the IC in a short while. 

I have 2 other boards which I think they are working properly but they are not in the same configuration and this confuses me if I can make my boards identical with the same configuration. One of them has MODE pin connected to ground to make it in FCM mode, ILIM is connected to ground FREQ resistor is 100k (frequency is 435KHz) and SETCUR is connected to ground through a 200k resistor. SS, DRVSET, PHSMD and RUN pins are still floating. This board runs in forced continuous mode and draws 100mA-120mA and regulates the 24V output. LTC3871 heats up to 75-80 celcius degree and stabilizes there. I think this case is normal and that board is working properly. Second board still has ILIM, MODE, SS, DRVSET, PHSMD, and RUN pisn floating. SETCUR pin is grounded through a 217k resistor. FREQ is grounded through 100k reistor and frequency is 435KHz. This board also draws100-120mA current in no load condition and LTC3871 heats up to 75-80 celcius degree. I think this board also works properly. 

My main objective is to get a DC-DC converter with 40-55V input and 24V 20A output. Most of the time output will supply another electronic device which draws 200mA to 3A and sometimes it will draw 10-12A instantly like 1second. How can I achieve this with LTC3871? How can I adjust the frequency, SETCUR resistors, MODE, ILIM value? I need help to make all my boards identical and working properly. I need to know if it is normal the board to draw 20mA or 200mA? Is it normal that LTC3871 i,s heaating too much?

Waiting for your kind response,

With kind regards,

Ahmet

  • 0
    •  Analog Employees 
    on Nov 14, 2019 12:45 AM over 1 year ago

    Hi Ahmet, if possible please share your schematic and PCB layout to Michael.Shriver@analog.com for further support. Designing your own board from scratch can be tricky. 

    The board using LTC3871 should be able to deliver higher currents. This would be much to your power components specifically inductor and the power switches. And no, excessive heating is not normal.