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No 3.3V on TPS40140

Larry Dunn
Prodigy 100 points
Other Parts Discussed in Thread: TPS40140, CSD16321Q5

Just got some new hardware to look at and unfortunately the 3.3V from

one port of the TPS40140 did not come up.  What we have is a circuit that seems to be trying to start.

The device is providing a duty cycle of about 10% (high 10% of the time). This is filtered and

provides an output voltage of about 0.4 volts. This voltage gets fed back to the TPS40140 but

there appears to be no attempt to change the duty cycle and increase the output voltage.

We are using 12V on the input side. We have 5V on BP5, 1.8V on VSHARE.  We are switching the device

at about 500KHz, and have three devices hooked up in a master slave configuration.  The 3.3V  is on the master device.

One thing to note is that in two case we have blown (CSD16321Q5 input FETs (this resulted in a loss of 12V on the pack

and the only way to get it back was to remove the input  FETs. Presently am checking downstream to see if something is

preventing the 3.3 volts from going higher, rather than a situation where the TPS40140 can't drive the output to 3.3V.  Any thoughts??

  • 0
    TI__Mastermind 49430 points

    Hi Larry,

    Could you provide the schematic for review?

    Thanks,

    Nancy

  • 0 in reply to Larry Dunn
    Prodigy 100 points

     

    Note that on the circuit pack, (unlike the schematic), R132 = 60K (not 127 ohms). Also RT is not connected to BP5 and PHSEL not tied to GND through 1284 (1284 and 1407 are not populated). PHSEL is connected to a second TPS40140 with an inline 39.2K resistor.

  • 0 in reply to Larry Dunn
    TI__Mastermind 49430 points

    Hi Larry,

    I review the schematic and could not find any obvious wrong there. Here are my comments.

    1. First make sure the calculation of current limit  and loop compensation are correct. Please refer to attached spreadsheet.

    2. Populate R1284 with 0ohm to connect PHSEL to GND for debugging this TPS40140.

    3. Measure the downstream outputs to GND to make sure the resistance is not too low which may cause an over current condition on start up. Also check if there is any pre-bias at  output before start up.

    4. Make sure 12Vin start up smoothly without any dip and check no high side or low side MOSFET damage, Also verify UVLO_CEx pins are above 2.1V and stay there.

    5. You could also contact the local TI FAE for help.

    Thanks,

    Nancy

     

  • 0 in reply to Nancy Zhang
    TI__Mastermind 49430 points

    Hi Larry,

    Forget to attach spreadsheet. Here you are.

    6443.TPS40140 design spreadsheet2_15_12.xls

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    Prodigy 100 points

    I'll follow up on your suggestions. I have one of your FAE people coming in today.

    Thanks very much for your help

  • 0 in reply to Larry Dunn
    Prodigy 100 points

    We are still having problems with these circuits at start up. I've  been working with TI FAE's and have resolved some issues (or at

    least I thought we had).  From the first posting, things got worse before getting better. The failures we were seeing resulted in

    shorts being created between the output rails and ground. On some circuit packs this resulted in shorts on 5 of the six power

    rails on the pack. We were unable to remove the short so the pack became unusable. To address this issue we did two

    things. We removed the 1.5uH inductors on the 12V inputs to the  TPS40140 and we removed the 5 ohm resistors in the gate

    path of each of the FETS. This appeared to resolve the blown FET/shorted power rail issue. We were able to bring up our remaing models

    and with your FAEs help ran some gain/phase plots to chracterize/improve the stability of our designs.  We were in the processs of

    updating our models with the reccommended changes when we unfortunately lost another circuit pack.  We modified the pack

    plugged it into the shelf  and determined we had no 1.8 V or 1.0V_B outputs. Measurements showed both rails  were shorted to ground.

     

    Changes were limited on these rails. On the 1.0V_B output we removed C170 and R134 to flatten gain curve and lower the crosover frequency.

    Nothing was needed to improve the stability of the 1.8V output. In both cases we changed the tracking caps from .1uF to .022uF to provide

    faster start up so we could meet power up sequencing requirements. Changing these caps was the last thing we did before the failure.

    We changed the caps on other rails as well with no problems. (it doesn't seem like this would create the prolem).

     

    To cause a short on the power rail it would seem like we demanded to much current or more likely pushed to high a voltage down the power

    rail.  Doesn't the device protect against this?

    Before failure the impedance 1.8V to ground was 206 ohms, 1.0_B to ground was 14ohms. Other rails were as low as 4 ohms.  We need to

    be confident in our power designs going forward. Does anyone have any thoughts on what is causing these failures??

  • 0 in reply to Larry Dunn
    TI__Mastermind 49430 points

    Hi Larry,

    It is good to know you fixed your issue on 3.3V rail. Regarding your 1.8V and 1.0V_B rail, I don' think lower tracking capacitor will cause outputs short to GND. Yes, TPS40140 will protect against output over current and over voltage. But It seems you don't have output over current condition( 8.7mA for 1.8V and 71.4mA for 1.0V_B before start up). What do you mean outputs short to GND? Do you mean the MOSFETs damaged? If yes, which FETs are damaged? high side or low side? Which pins of MOSFETs damaged? Please continue to work with TI FAE to debug this failure( Capture some waveforms, review the schematic and PCB layout).  

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    Prodigy 100 points

    Prior to the failure the impedance 1.8V to ground was 208 ohms. After the failure it was . 3 ohms. Yes, in some cases, high side MOSFETs were damaged.

    This caused a fault in the 12V converter on the circuit pack (12 volt disappeared). Change the MOSFETs 12 V returned. In the cases where the event caused a

    short on the output we have no way to determine if the MOSFET was damaged. The short was created downstream from the TPS40140 circuit (removed the

    output inductor and the short remained.) 

  • 0 in reply to Larry Dunn
    TI__Mastermind 49430 points

    Hi Larry,

    It looks like it is most likely a voltage stress or a dV/dt turn on issue resulting in cross-conduction and thus MOSFET failure. Could you please send the schematic of 1.8V and 1.0V . What is the MOSFET part number? High side and low side?

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    Prodigy 100 points

    Nancy,

    Schematics were attached earlier in this thread.  We did remove the Inductors in the 12V path and the 4.99 ohm gate resistors. 

    All the MOSFETs high and low are CSD16321Q5 parts.

  • 0 in reply to Larry Dunn
    TI__Mastermind 49430 points

    Hi Larry,

    The schematic in earlier post is 3.3V and 1.0V_C. Not 1.8V and 1.0V_B. Are both exactly same? Is the output inductor same?

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    Prodigy 100 points

    Nancy,

    I've attached the new schematic file. There are some differences between the three TPS40140 designs. These 

    pages contain the stability modifications recommended by the FAE. The Gate resisstors are removed as are the inductors

    on the 12 volt input. The model that failed was modified to match the schematic. The output inductors are

    as follows:

    L10    IHLP-4040DZ-11, 470nh

    L12    SER1590-122MLB, 1.2uH

    L16    IHLP-5050CE-01, 820nH

    L68    IHLP-4040DZ-11, 470nH

    L14    SER2009-122MX.1.2uH

    L15    IHLP-4040DZ-11, 560nH

    Thanks,

         Larry 

    LNW910PWR.pdf
  • 0 in reply to Larry Dunn
    TI__Mastermind 49430 points

    Hi Larry,

    I review the schematic for 1.0V_B and 1.8V and have following comments.

    1. What is max output current for 1.0V_B and 1.8V?

    2. The L/DCR time constant doesn't match the RC in you schematic. Refer to datasheet equation 5-17 , 5-18, 5-19 and 5-20. Double check L12, R65, R73, C1639. If this time constant could not meet, the sense current is not correct and IC may malfunction.

    3. PHSEL pin (pin4) connected to GND and R1254(39K) at same times. Why? You only can connect either to GND or 39k resistor to determine the phase.

    4. Correct all the components value and run loop and current limit calculation again to eliminate current limit and loop issue. The spreadsheet is as attached.

    5. Capture some waveforms for highside and lowside MOSFETs, Such as Vds,  Vgs and also inductor current. CSD16321Q5 is very fast FETs. You may have some higher spike or noise on Vds or Vgs, you could increase bootstrap series resistor from 4.7ohm to 10ohm or adjust the snubber to dump them. TPS40140EVM-003 used 10ohm bootstrap resistor. This resistor will slow down the switching rising edge to prevent the shoot through.

    I will be out of office next week and please work with TI FAE for your further questions.

    Thanks and good luck,

    Nancy

    2335.TPS40140 design spreadsheet2_15_12.xls