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UCC28950: Device cannot restart before Vss drops below 0.5V

Brian Wang0
Expert 5310 points
Part Number: UCC28950
Other Parts Discussed in Thread: UCC28951

Hi team,

Here is a problem we met during ucc28950 test.

We noticed that if we try to perform quick restart(power off and on), the device would not power on and there would be no output.

Scope 1: There is no pwm switching after a quick restart 


Scope 2: Vss clamps if the restarts happens before bcc drops below 0.5V 



The error happens after we remove the resistor between vref and ss



After measuring CS, we are pretty sure thr device is not locked because of CBC protection. What might be the issue?

  • 0
    TI__Mastermind 25570 points

    Hello Brian

    The SS/EN pin must be pulled below 500mV (250mV worst case) to guarantee correct reset of the device. Restarting the device with SS/EN > 500mV (700mV worst case) will not guarantee that the controller has been able to configure the current sources at the SS/EN pin correctly.

    If you want to investigate the behaviour of the device during short interruptions like this then you should also make sure that the capacitor at the SS/EN pin has been discharged fully - either by a switched MOSFET or a low Vf diode.

    Your use of a 10k pull up resistor at the SS/EN pin is also not recommended. Please let me know if you want to defeat the normal Hiccup mode protection on this device and I can share details with you.

    I would also recommend that you switch to the UCC28951 device which is an enhanced version of the UCC28950. It is a fully compatible drop-in replacement for the UCC28950. Refer to application note SLUA853 at http://www.ti.com/lit/an/slua853/slua853.pdf for more details. The UCC28951 is typically the better choice, especially for applications where the system may have to operate simultaneously at a duty cycle> 90% and current limit.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points

    Hi Brian

    I forgot to mention of course that if you want to quickly turn the controller off the best way to do this is to pull the SS/EN pin low with a small signal MOSFET. The controller will stop (asynchronously) within about 1us of the voltage at SS/EN dropping below the stop threshold (nom 500mV) with a correct soft start once the MOSFET is turned off to release the SS/EN.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Expert 5310 points

    Thanks Colin,

    Actually the error only happens when customer try to restart(off and on)the device very quickly as a stress test, which is not the ordinary case.

    The problem is solved by moving the resistor. 

  • 0 in reply to Brian Wang0
    TI__Mastermind 25570 points

    Hi Brian

    That's good to hear. Please can you confirm that they have removed the resistor circled in red below - as I said, TI recommends that this resistor is NOT used.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Expert 5310 points

    Yes Colin, this is the resistor I refered to.