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BQ25756E: Getting fault "DRV_SUP pin voltage is out of valid range"

Sean Solberg
Prodigy 30 points
Part Number: BQ25756E

Tool/software:

I have a circuit built around the BQ25756E that is almost identical to the EVM board from TI.   I have double checked, tripple checked, etc. etc. that I don't have a wiring mistake, but I can't seem to figure it out.  While I can get the EVM board to work as I need, I can't get my circuit working and I'm getting this fault flag: "DRV_SUP pin voltage is out of valid range".   Measuring the voltage at REGN/DRV_SUP which are tied together, I get about 214mV.  Voltage fed in to VAC is just under 36VDC.    I would expect the REGN/DRV_SUP to be around 5V, so it seems like something is dragging it down. 

My question is this:  What are the possible causes for the REGN/DRV_SUP to be too low so-as to generate the fault flag?  Where does the DRV_SUP feed go internal to the IC and how does that feed to the high-side drive?  Actually, related to this, the diodes that feed this +5V to the biasing capacitors is a bit of a mystery to me.  I am not understanding the point of these two diodes in relation to the FET driving circuit.  I realize we need a voltage higher than the incoming voltage to fire the high side FET gates, so it seems to be a part of a charge pump.  Are the diodes there mostly for protective clamping purposes?

Are there possibly I2C registers that I may be inadvertently setting that results in a disabled REGN output?

One other symptom is that the output voltage very slowly climbs up to about 2.75vdc.   Very slowly is probably do the large capacitance on the output with the (3) 56uF caps. 

Any pointers in the right direction are very much so appreciated. 

..Sean Solberg

ChargerCircuit

measurements

  • 0
    TI__Mastermind 20030 points

    Hello Sean,

    Thanks for working on this. I have a few questions to help us debug this.

    What's your VAC and VBAT voltage?

    Can you send me the other I2C registers you are reading?

    Is it possible to unconnect DRV_SUP and REGN? If so, does REGN come up to 5V when you do this?

    If you connect the battery to VBAT, then connect VAC to the board, does the battery charge then?

    You've probably already done this, but make sure to review your schematic/layout with the schematic and layout checklist: e2e.ti.com/.../faq-bq25756-schematic-and-layout-checklist

    Sean Solberg said:
    Where does the DRV_SUP feed go internal to the IC and how does that feed to the high-side drive? 
    Sean Solberg said:
    Are the diodes there mostly for protective clamping purposes?

    The IC has "bootstrap" capacitors to help drive the high side FETs. When SW1 and SW2 are low, the boot strap capacitors can charge up to DRV_SUP. When SW1 and SW2 are high, the bootstrap capacitors now have a voltage of around  VBTST = SW + DRV_SUP.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 30 points

    Answers

    VAC = 36V

     

    VBatt is connected to a 24V lithium so in reality it’s nominally around 26-27V.  The image below shows the input and output voltages with a power source and battery connected.    Currents are reported as zero since the FET switching is off.

     

    It will be difficult to separate DRV_SUP and REGN on the circuit board because they are connected together just outside the chip, but I can definitely do it.   I will make that break and report back the findings.

     

    NOTE 1:  Notice below that VFB ADC register is zero even though I have that channel turned on in the ADC Channel Control.   I did a test with a meter right at the pin on the IC and it is at 1.363V so it seems strange to me that I am getting it reporting as 0000. 

     

    NOTE 2:  I did find a connection issue from VBatt to the 249K resistor that feeds to FB

     

    Thanks for the info on the bootstrap capacitors.  That’s what I was thinking.

     

     

    ************ CHARGER ****************

    0000: Charge Voltage Limit

    0320: Charge Current Limit

    01CC: Input Current DPM Limit

    0348: Input Voltage DPM Limit

    0640: Reverse Mode Input Current Limit

    03E8: Reverse Mode Input Voltage Limit

    0140: Precharge Current Limit

    00A0: Termination Current Limit

    0F: Precharge and Term Control

    0D: Timer Control

    00: ThreeState Control

    C9: Charger Control

    C0: Pin Control

    20: Precharge and Term Control

    20: Power Path and Reverse Mode Control

    20: MPPT Control

    96: TS Charging Threshold Control

    57: TS Charging Region Control

    40: TS Reverse Mode Control

    00: Reverse Undervoltage Control

    00: VAC Max Power Point Detected

    00: Charger Status 1

    80: Charger Status 2

    00: Charger Status 3

    02: Fault Status

    00: Charger Flag 1

    00: Charger Flag 2

    00: Fault Flag

    00: Charger Mask 1

    00: Charger Mask 2

    00: Fault Mask

    90: ADC Control

    08: ADC Channel Control

    0000: IADC ADC

    0000: IBAT ADC

    45CC: VAC ADC

    32BC: VBAT ADC

    0000: TS ADC

    0000: VFB ADC

    00: Gate Driver Strength Control

    00: Gate Driver Dead Time Control

    12: Part Information

    ************ CHARGER ****************

     

     

    Thank you for the checklist, I actually did not have that document.  I will go through it in detail. 

     

     

     

  • 0 in reply to Sean Solberg
    Prodigy 30 points

    Update:  cutting the trace between the REGN and the DRV_SUP pins resulted in the same behavior.  REGN is still at about 213mV.  REGN is only left connected to the two diodes and the 10uF capacitor to ground.

  • 0 in reply to Sean Solberg
    Prodigy 30 points

    I figured I should put more information around my comment "NOTE 2:  I did find a connection issue from VBatt to the 249K resistor that feeds to FB" above.

    Basically, while investigating my problem and double checking every connection path, I found that the feed to the 249K resistor was not connected to VBatt.   I figured that was the root cause of my problem.  I fixed that situation and it is connected now.   However, I still get the same results.  Note that the output voltage ADC was reporting the correct Battery voltage even when I did not have the FB signal sourced correctly.   Now that it is sourced correctly, we get 1.363 volts at the FB pin, but it seems that I cannot read it with the FB ADC value as it reports as 0000 even though I have register ADC Channel Control set to 0x08 which should enable the VFB ADC because that bit is 0 so it is not disabled. 

    Is it possible that because at one point in the past I was running the circuit without FB getting a source from VBatt that something got smoked inside the IC and now that ADC channel is blown.  I wouldn't think that would be the case because when VBatt was originally not connected to the 249K resistor, then the voltage feeding into FB would be zero via the pulldown resistor to ground.    But this is a question that might mean me just changing the IC with new one might fix the problem. 

  • 0 in reply to Sean Solberg
    TI__Mastermind 20030 points

    Hello Sean,

    Just for clarification, the IC reads the battery voltage from the SRN pin.

    I have a few more questions to help us debug this.

    Are the I2C flags different now that you fixed the resistor that connects to FB?

    Does replacing the IC fix the issue?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 30 points

    Ethan, thanks for your help.   The I2C flags were not different after getting the correct connection and voltage to FB.  

    I then replaced the IC and the DRV_SUP low voltage fault went away, REGN is producing 5V and the FB ADC is reporting a value now.  So, replacing the IC seems to have fixed the low REGN voltage problem.  However, I still could not get charging going.  Input and output voltages were reporting good..    I then removed the battery from the circuit and removed the power source from the circuit.   Once I plugged in the power source back into the circuit (36V), I got the DRV_SUP low voltage fault back again, but this time, REGN is showing about 0.8 volts.  

    I guess at this point, I will try replacing the IC again.   Maybe I just had a bad solder connection on one of the pads of the drive circuitry which caused something to smoke internally.  

    ..Sean

  • 0 in reply to Sean Solberg
    Prodigy 30 points

    I've replaced the IC again and it is back to giving seemingly good register values.  Both voltages are good.  No faults.  REGN is at 5V, etc.

    It just won't start the charging as it is stuck in the "Not Charging" mode.   Would it be possible to scan through the register values shown below to maybe pickup something that doesn't look right in the registers?  

    Client 1 connecting to s2.mpstatus.com, port=4120, [DNS mode]

    ************ CHARGER ****************

    000A: Charge Voltage Limit

    0320: Charge Current Limit

    01CC: Input Current DPM Limit

    0348: Input Voltage DPM Limit

    0640: Reverse Mode Input Current Limit

    03E8: Reverse Mode Input Voltage Limit

    0140: Precharge Current Limit

    00A0: Termination Current Limit

    00: Precharge and Term Control

    0D: Timer Control

    00: ThreeState Control

    C9: Charger Control

    C0: Pin Control

    20: Power Path and Reverse Mode Control

    20: MPPT Control

    96: TS Charging Threshold Control

    57: TS Charging Region Control

    40: TS Reverse Mode Control

    00: Reverse Undervoltage Control

    00: VAC Max Power Point Detected

    00: Charger Status 1

    C0: Charger Status 2

    00: Charger Status 3

    00: Fault Status

    00: Charger Flag 1

    00: Charger Flag 2

    00: Fault Flag

    00: Charger Mask 1

    00: Charger Mask 2

    00: Fault Mask

    90: ADC Control

    08: ADC Channel Control

    0000: IADC ADC

    0000: IBAT ADC

    45BC: VAC ADC

    31D4: VBAT ADC

    0003: TS ADC

    0547: VFB ADC

    00: Gate Driver Strength Control

    00: Gate Driver Dead Time Control

    12: Part Information

    ************ CHARGER ****************

  • 0 in reply to Sean Solberg
    TI__Mastermind 20030 points

    Hello Sean,

    I'll get back to you next week on this question.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 20030 points

    Hello Sean,

    I've looked through your registers and I can't find anything abnormal.

    Can you measure the voltage on DRV_SUP just to make sure it's connected to REGN correctly?

    Can you also send me the FETs you are using so I can see if those will work with your circuit?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 30 points

    Ethan,  thanks for your help.  The two are tied together right at the circuit board.    I have found another situation that is affecting this.   The TS input I was leaving unconnected and I believe that instead of that I should be biasing it with a resistor divider to feed it a simulated temperature signal that is in a good range.   My belief is that the temp is preventing the charger from "turning on".  However, related to that, I can disable the TS sub-system by setting the right flags and then the charger "turns on".   But, once it turns on the switching, the chip pops and I'm completely dead again.     I'm going to build a new circuit and start over with all new parts and get the TS input biased with a resistor network to be in a good temp range. 

    I think the moral of the story is to still feed in a simulated temperature with a resistor divider into the TS to at least enable charging by default, even if you are not going to be using the TS functionality. 

    The FETS I am using are the SIR680LDP-T1-RE3 (MOSFET N-CH 80V 31.8A)  from Vishay

    ..Sean

  • 0 in reply to Sean Solberg
    TI__Mastermind 20030 points

    Hello Sean,

    I'll get back to you tomorrow about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 20030 points

    Hello Sean,

    When the IC broke did any other parts break?

    Once you get your board fixed, can you try running the board on electronic CV load at a low voltage? Will the board work charge correctly when VBAT=3V or so?

    It may also help to set ICHG low while you are working on this.

    Best Regards,
    Ethan Galloway