• Resolved

TPS7A8300: TPS7A8300 Current Limit FoldBack

Part Number: TPS7A8300

I have a problem with the above LDO where thermal  limit caues the power supply to shut down. Is there a version with SAME pinout that has built in programmable foldback current limit?

  • Hi KQ,

    We do not have a pin to pin device with a programmable foldback current limit. Could you tell me more about your application? We may be able to find a way for the TPS7A8300 or the newer version TPS7A83A to work for your application.

    Could you provide a multi-channel scopeshot with Vin, Vout, and Iout so that I can see how your rails are interacting?

    Very Respectfully,
    Ryan
  • In reply to Ryan Eslinger:

    Hi Ryan

    I tested it with a controlled environment using a active DC Load and set the current to 2A constant and then time when the 2A load applied and how long it takes for the power supply to turn off due to overheat. the problem is the TPS7A8300. It is not possible to reduce the 5V VIN which was shared.   The problem with the above thermal shutdown is the VIN also dip along with the output sometimes, so it was not just the output DIP (and recover quickly in less than 100msec) It was a insufficient thermal disspiation problem we just missed it by 20% margin.  But thks about the availability of the TPS7A83A  we can switch to this on the next design or a larger package LDO with larger metal tab for conducting the heat away.  If only the VOUT dip and not affect the VIN we are OK but unfortunately the VIN went down along with it. CH1 is VIN, CH2 is VOUT, CH3 is our Load Supply which is to be the same as VOUT. It is this VIN that went down along wit the output that is our problem.



  • In reply to KQ:

    Hi KQ,

    Unfortunately your image did not properly attach to your post.  From your additional explanation it sounds like your concern is the thermal shutdown rather than the current limit.  Please note that when an LDO enters thermal shutdown, the pass element will turn off.  Due to your 2 A load, the output capacitors will quickly discharge bringing the output voltage rail to GND.  When the junction temperature of the LDO cools enough, the LDO will turn back on.  When this occurs you will see your 2 A load current and inrush current flowing through the LDO to charge the output capacitors.  

    Since your conditions are intentionally allowing the TPS7A8300 to enter thermal shutdown, the LDO will continue to cycle in and out of thermal shutdown.  Your upstream supply that is powering the TPS7A8300 will see this as a load transient.  The dip you see in your input voltage will be the load transient response of the upstream supply as it attempts to source the 2 A load and inrush current through the LDO.  You can reduce the magnitude of this dip by increasing the input capacitance to the TPS7A8300.  When the LDO turns on, the input capacitor will help source the extra current to charge the output capacitors.

    Please keep in mind that Thermal Shutdown is considered a fault condition for an LDO.  Care should be taken so that the junction temperature does not exceed 125 C during your application's normal operating conditions.  Continuously running into thermal shutdown will degrade the reliability of the LDO.

    Please refer to the following for how to attach images to the E2E forums:

    Very Respectfully,

    Ryan

  • In reply to Ryan Eslinger:

    Hi Ryan

    I had already solve the thermal problem. Now at 2A continoulsy the IC is only slightly warm and I can source it infinitely.  So it is not a problem now.  Now the issue is the occasional Overcurrent limit. I had simulated it on the board.  If the over current condition is not remove, the TPS7A8300 will divert the current internally (instead of removing the current) and the IC gets VERY HOT (much hotter than the heat generated by the overccurent condition itself where the IC is only warm). I simulated with 2.5A overcurrent load. This IC does not have foldback current limit that limit the output to 2A.   [Note that this is a worst case scenario for overcurrent continuously].  So it seem the only possibility is to change to a newer chip with same pinout and larger current out capability. Which IC is the direct pin to pin and at least 3A?  Thnks

  • In reply to Ryan Eslinger:

    I have a field problem where we experience mostly Spikes at the VIN of this TPS7A8300. we see very large spike twice the VIN in short duration of tens to 100msec. However it is difficult to duplicate back in workshop as the load is pulsating that is occasional peak pulse overcurrent around 3A for pulses where the average current is closer to make the IC goes into thermal shutdown. under what kind of condition (Overcurrent shutdown or thermal shutdown) do we gets positive spikes (our load is a pulsating load, not constant current).
  • In reply to KQ:

    Hi KQ,

    I am glad to hear that you solved your thermal issue under the normal operating conditions of your application.  You are correct that the foldback current limit for TPS7A8300 will not engage until the current is 3.4 A (typically).  Unfortunately it sounds like during your fault conditions you are still dissipating enough power to cause the LDO to enter thermal shutdown.

    Depending on which package you are using for TPS7A8300 you may be able to use TPS7A84A or TPS7A85A as a pin to pin option for a higher current; however, it is important to note that switching to one of these higher rated current devices will not solve the thermal issue during your fault condition.  The only way to reduce the risk of entering into thermal shutdown is to reduce the power being dissipated in the LDO.

    Very Respectfully,

    Ryan

  • In reply to Ryan Eslinger:

    Hi Ryan

    Thanks for these info. At least I had confirmed that the power supply had shut down thermaly in the field, although i did not expect it to be thermal since the IC only feel slightly warm only (and becomes extremely hot only when it shut down) so that was a surprise. Unlike previously before lowing the VIN, it did felt quite hot before shutting down. I will confirm by applting apply freezing gas over the IC to compare since the shut down is quite easily duplicate with active load.
  • In reply to KQ:

     Hi Ryan

    I had managed to gecapture a real case of the random spikes issue at the current output and the voltage input of the TPS7A8300. . It wan't thermal shutdown or overcurrent (I had artificially created the overcurrent using DC load as previously i created the condition artificially using DC Load). The problem was the output current of the LDO  was ringging in few nsecs when the Load was turn on but yet the output voltage of the LDO was stable. However the input voltage of the LDO rings along with the LDO current output .. This high rise/fall time high current ringging looks more like inductive but the power output to the load does not have any inductor but a wide PCB Trace with 2 low EST Tantalum. The Yellow (CH1) Line is output voltage of the  TPS7A8300. The large ringging CH4 (100mV/Amp) Current sense voltage across a 0.1ohm resistor, directly at the output of the TPA7A8300 and this shows the peak current  is about 2.6A The top 2 waveform (CH2, CH3) is the voltage at the input of the LDO (pickup at some distance away from the LDO Input). So it seem the current drawn by the LDO was in very large and short spikes and this ONLY happens very very randomly. Is this a problem with the load (a radio module) or this is a problem of the TPS7A8300??  Prior to this ringging the output current of the LDO was close to 0 as both LDO and load was turn off. The LDO has a 27nF slow start up capacitor allowing 10msec startup.

  • In reply to KQ:

    Hi KQ,

    TPS7A8300 will source whatever current is required by its load (up until current limit is reached). As such if the load is switching states, the erratic current is likely associated with the load itself.

    Very Respectfully,
    Ryan
  • In reply to Ryan Eslinger:

    Hi Ryan

    We had change out the TPS7A8300 with a extneral small power mod (TPS54231) and there was no spike observable so far. So i can easily confirmed by changing out the TPS7A8300. Can i know the list of LDO that is Pin to pin compatible with TPS7A8300?? It speed up my search and changing out TPS7A8300 was the easiest option for us given our board size restriction which was the reason we pick TPS7A8300