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LM5175: Buck-Boost Transition Mode is not working for Vin 6V-18V, Vout=7.3V ,Iout 5A

Debdeep Banerjee
Prodigy 25 points
Part Number: LM5175

Hi,

I'm designing a Buck-boost Automotive LED power supply with LM5175Q1, and below are the observations : 

1. Buck mode is working properly up to 5A, even at lower currents as well.

2. When Vin is lowered up to 9.5-10V, buck-boost transition mode kicks in, and then LDRV2 (boost leg low side FET) gets driven in a wider duty cycle, which overcharges the inductor up to 20-30A ! This causes huge acoustic noise, and damages Boost Leg FETs

I've used my own MathCAD calculation to determine the values and verified it against  Buck-Boost Quick start tool as well (and they match !) . We have a 4 layer PCB, and tried to follow TI guidelines for layout wherever's possible,(Input cap, FET's and inductor placement is exactly as per TI reference layout)  not to mention reviewed multiple times internally and with TI as well.

I tried a series of trials to rectify the issue, but every time I'm seeing that LDRV2 FET is getting driven by a wider pulse from the controller which is overcharging the coil ! We have secondary filter inductor at output, but tried without that as well, and till now nothing works ! Even current sense and voltage feedback signals look clean when it's operating in stable buck mode

Attached a  small report . Looking forward to some urgent help from TI !

Regards,

Deb

Board Issue .xlsx 

  • 0
    TI__Guru* 78170 points
    It does not sounds correct. When the circuit runs in the buck-boost transition mode, the boost switch (LDRV2) should have small duty cycle. Can you share your schematic, and SW1 and SW2 waveform?
  • 0 in reply to Youhao Xi
    Prodigy 25 points

    Hi Youhao,

    Thanks for helping me on this. I've shared a small report with my original post. This below capture was taken at 100KHz operating frequency trial where you can see yellow (Q13 Gate) is on for approx 10us duration, which is letting the coil charge up to 13 A ! Blue pulse is  HDRV2 (coil discharge time) 

  • 0 in reply to Debdeep Banerjee
    TI__Guru* 78170 points
    Hi Sandeep,
    I have been studying your case and I am afraid your loop is not stable. What is your output capacitor? Your schematic in your report is truncated not showing the actual output capacitors. Could you resend the schematic?

    Thanks,
    Youhao Xi