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TPS56121: Start up behavior - EN/SS to GND

Dierk Bronner
Intellectual 1895 points
Part Number: TPS56121
Other Parts Discussed in Thread: TPS54A24

Hi Team,

my customer whats to understand the start up behavior of TPS56121 better. For a LED application they need a as fast as possible slew rate of the output voltage.

Looking at the datasheet its mentioned that after input power is applied the device will charge the cap connected to EN/SS.

The questions are:

-how to achieve the fastest ramp up of the output voltage?

-what is the fastest ramp up time? (just EN the device without soft start)

-would connecting EN/SS to GND offer the fastest ramp up? (or what happens then)

Kind regards

Dierk Bronner 

  • 0
    TI__Mastermind 23955 points

    I do not think the TPS56121 will work for an LED application in which you want to PWM the EN/SS.   The calibration time on the EN/SS is needed to have current limit.  If there is no soft start capacitor on the EN/SS pin to get the fastest slew rate, the calibration time is still about 1.9ms.  

    Have you considered the TPS54A24?  

  • 0 in reply to David_Daniels
    Prodigy 10 points

    Hello David,

    we are just supplying the LEDs with 5V. These are  ISELEDs which have all the logic in them.

    Therefore still the question is up: what may be the fastest switch on time for the mentioned TPS?

  • 0 in reply to Christian Rost
    TI__Mastermind 23955 points

    For the fastest startup, leave the EN/SS open without a soft capacitor.    The soft start time will rely on the parasitic capacitance on EN/SS and the response time of the control loop.   It is difficult to calculate the output voltage ramp time, since it mostly depends on the output filter, control loop and load.   From past experience when checking soft start without the external capacitor, it has been ~100us. 

    Do not drive the EN/SS,  the EN/SS pin is used for a calibration sequence for current limit at startup.   To disable the TPS56121 externally use an open drain on the EN/SS.   

    Using a short soft start time will shorten the hiccup time during overcurrent faults which increases the power dissipation during faults.