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No 3.3V on TPS40140
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??
Could you provide the schematic for review?
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.
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.
Forget to attach spreadsheet. Here you are.
6443.TPS40140 design spreadsheet2_15_12.xls
I'll follow up on your suggestions. I have one of your FAE people coming in today.
Thanks very much for your help
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??
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).
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.)
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?
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.
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?
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
L10 IHLP-4040DZ-11, 470nh
L12 SER1590-122MLB, 1.2uH
L16 IHLP-5050CE-01, 820nH
L68 IHLP-4040DZ-11, 470nH
L15 IHLP-4040DZ-11, 560nH
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,
2335.TPS40140 design spreadsheet2_15_12.xls
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