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LTC7810, 75 V input 15 V 30 A output. Does not work properly.


I want to use the LTC7810 to make a step-down converter with up to 75 V input and 15 V 30 A output.
I made a PCB and tested it, but it doesn't work properly.
Can you give me some advice?

The PCB is designed based on the circuit diagram in the attached image.
Many component layouts on PCBs follow the datasheet.
However, due to peripheral equipment, the current sense resistor and filter circuit are far from the IC, and the SENSE +/- signal path runs near the inductor.

The following shows a series of operations that occurred in the test.
All tests are conducted with no load.

1: Confirm that 75 V is supplied to VDD and 15 V is output. However, about 0.2 A flows in VDD and LTC7810 generates heat.
2: When checking the waveforms of SW1 and SW2 on the oscilloscope, SW1 is operating normally, but SW2 is at the GND level.
3: Consider the possibility that the filter of the current sense circuit is far from the IC, and connect 1 nF directly between the SENSE +/- pins of the IC.
4: Confirm that 75 V is supplied to VDD again and 15 V is output. However, about 0.2 A flows in VDD and LTC7810 generates heat.
5: When the waveform of SW2 is confirmed, a switching waveform with a peak of about 15 V is observed.
6: TG2 also observed a switching waveform with a peak of about 15 V, but BG2 remained at the GND level.

So far, we have tested a 15 V 30 A step-down converter, but we have also tested a 12 V 30 A circuit using the same circuit except for the value of the voltage feedback resistor.
This circuit also did not work properly, outputting about 8 V to 10 V compared to the command value of 12 V.

How can I get it to work properly?
Or has the IC already failed in step 1 above?

Best regards

  • Cannot read your schematic (low resolution). If you are saying that the VIN pin has 75V, DRVCC pin has 15V and BG pin is dead, the driver may be damaged. Gate drive layout is important with any controller capable of producing high duty cycles (short gate off times) including the LTC7810. The 7801 PC Board Layout section includes: "The TG and BG traces from the controller IC to the gates of the external MOSFETs should be kept as short as possible to minimize the parasitic inductance. This inductance can cause voltages spikes that can potentially significantly exceed the supply rails of the drivers and damage them, especially with high gate charge MOSFETs and when using gate drive settings above 8V. Small capacitors (~1nF) placed as close as possible to the TG/SW and BG/GND pins can be used to filter these spikes." 

  • Thank you so much Kurk
    I connected a 10 pF capacitor near the BG pin, and it worked!

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