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TLV70012-Q1: Operating above current rating during bootup inrush current conditions - LDO TLV70012QDDCRQ1

sergio ramos
Prodigy 190 points
Part Number: TLV70012-Q1
Other Parts Discussed in Thread: TPS73433-Q1, TPS7A53-Q1

Question on operating the LDO in an over current condition at startup.   would like to use this TLV70012QDDCRQ1 LDO to feed a DC-DC boost converter which converts 3.3V to +5V.   The boos converter has an inrush current requirement of 2.6A for 500uS however the DC-DC will boot in at lower input current levels. Since the LDO can not supply the 2.6A I expect that the LDO output will fold back and reduce the output voltage/current.  The effect of this is to stretch the boot time of the DC-DC from a few mS to approx 50mS. The question is whether there is a reliability issue operating the LDO in this mode where it limits current for 50mS.  Once the +5V supply boots the constant current draw is only 30mA. 

 

  • 0
    TI__Guru 55940 points

    Hi Sergio,

    Could you please help clarify your application?  TLV70012-Q1  will output a 1.2 V so I assume you are placing it in parallel with whatever is sourcing the normal 3.3 V input to the boost converter similar to what  describes in his two part blog:

    How-to Improve Buck Converter Light Load Efficiency with an LDO - Part 1
    Designing a DC/DC converter that only consumes micro amps of current at no load can be compared to fueling a muscle car with lighter fluid – you may get it to work, but it won’t be easy! High efficiency…
    By in Technical articles > Power management

    Since TLV70012-Q1 has a "brick wall" current limit, the LDO will continue to source current at the current limit and will not be regulating the output voltage.

    Keep in mind that current limit is a protection for fault conditions and is not intended for normal operation.  It is possible that the extra current could cause the temperature of the pass element to reach the thermal shutdown threshold.  If this occurs, the LDO will cycle in and out of thermal shutdown which can degrade the lifespan and reliability of the LDO.

    Due to the short duration where you would be operating in current limit, your application may work; however, we strongly recommend prototype testing.  A more robust solution would be to select an LDO that is rated for the full current.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 190 points

    Hi Ryan, 

    Thank you for the response, it was late when I wrote it and had a couple of errors. 

    The regulator I'm using is the TPS73433TDDCRQ1 (or any other regulator that has the same pinout SOT23-5 package, existing design).  The bottom line is that the DC-DC boost that it is feeding has the large inrush current.  During this time the TPS... would current limit (I assume) and the question is whether it would be a reliability issue. The DC-DC boost will still boot fine with less current so the main issue is just with the LDO, its reliability, and expected behavior. 

  • 0 in reply to sergio ramos
    TI__Guru 55940 points
    Hi Sergio,

    The majority of my previous reply will still apply to TPS73433-Q1 as it applies to all LDOs with a brick wall current limit. Current limit is a protection feature for a fault condition outside of recommended operating conditions where the LDO is designed, tested, and characterized for operating. It is possible that the extra current could cause the temperature of the pass element to reach the thermal shutdown threshold. If this occurs, the LDO will cycle in and out of thermal shutdown which can degrade the lifespan and reliability of the LDO.

    The limitation to the SOT23-5 package does not allow for a substitution that is rated for the peak current in your application. The lack of options is actually due to thermal limitations for the SOT23-5 package.

    As you are intending to operate the TPS73433-Q1 in current limit in your application, you are correct that the combination of the high current and additional power dissipation due to the high current may impact the device reliability and lifespan. We recommend prototyping for you to determine if this is okay for your application.

    Our recommendation for a robust solution (TPS7A53-Q1) does not fit your package limitation; however, it is rated for operation under your peak application current.

    Very Respectfully,
    Ryan