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UCC28740: Startup sequence

Maximiliano Nieto
Prodigy 20 points
Part Number: UCC28740

Hi

I saw this TI training video:

https://training.ti.com/getting-started-ucc287xx-and-ucc289xx-family

My question is, when the controller enters in a fault condition, the three initial cylces are repetead as long as the fault condition persist?

  • 0
    TI__Mastermind 40090 points
    Hello Maximiliano,

    Yes, when a controller of these families enters a fault condition, the three initial test cycles are repeated at the peak of the UVLO cycle for as long as the fault condition persists. The voltage at VCC or VDD of the controller will rise to the turn-on threshold and the 3 pulses are issued. If a fault is determined, further PWM switching is inhibited and the VCC/VDD voltage is allowed to bleed down until it reaches the UVLO threshold. This happens because the IC bias current is held higher than the available start-up current, so the VDD capacitor discharges.
    At the UVLO point, internal loading is shut off and the VCC/VDD voltage is again allowed to climb to the turn-on threshold and the 3 pulses are again issued. This UVLO cycling will persist indefinitely, until either the fault condition is cleared or input power is removed.

    In some cases a controller may be configured to latch off for a particular fault, instead cycling indefinitely (also known as "re-try"). In such a case, input power must be removed long enough (usually a few seconds) for the VDD charge to bleed further down to an un-latch threshold (usually a few volts) in order to be able to restart the controller. During latch-off mode, the IC bias current is reduced sufficiently to match the available start-up current such that the VDD capacitor can maintain a higher voltage until removal of input power also removes the source of start-up current.

    I hope this helps your understanding of the controller behavior.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 20 points
    Thanks Ulrich, i already understand. A last question, if there is no pulses on the DRV pin, the controller is damaged?
  • 0 in reply to Maximiliano Nieto
    TI__Expert 3260 points

    Hi Maximiliano,

    Just to clarify, when you say no pulses do you mean there is no pulse at all during the start-up or you are not able to observe 3 initial switching pulses?Under some conditions the controller enters the fault mode and you will only see one pulse on the DRV pin repeatedly, and these conditions are well explained in the training video (https://training.ti.com/getting-started-ucc287xx-and-ucc289xx-family) from 13:20 to 15:30.

    If you are not able to observe any pulse at all on start-up, the following conditions could also lead to no output pulse on the DRV pin but not necessarily mean the controller IC is damaged:

    1. As Ulrich discussed previously, if the controller is configured to “latch-off” protection for a particular fault event, any PWM switching is inhibited until the controller is restarted. You will need to “un-latch” protection by restarting the controller as Ulrich suggested.

    2. If there is no output pulse on DRV pin at all on the start-up, there are few possibilities:

    I. The UVLO turn-on threshold on VCC/VDD is never reached during start-up i.e. +21V for UCC28740, so the controller is never ever activated.

    Please verify the input voltage level and Vbulk is directly connected to the HV pin as UCC28740 relies on internal high voltage start-up switch to charge up the VDD capacitor to supply the bias current before output/auxiliary voltage is established. An easy way to verify this is to measure the VDD level and see if it exceeds the turn-on threshold, check the connection and input voltage level if the VDD never reaches +21V.  

    II. Insufficient bias capacitance or without the presence for the VDD capacitor.

    On startup, UCC28740 requires around 2mA of run current (bias current in addition with the gate drive current) from the VDD capacitor after it is being charged above the UVLO turn-on threshold by HV startup switch. Too small of the VDD capacitance cannot provide sufficient current for the start-up operation, the run current depletes the VDD voltage below VDD(off) threshold too quickly and the controller re-enters the shut-down state before the internal gate driver is activated and hence there is no output pulse on the DRV pin. Small capacitance could be due to the capacitor itself is broken or its legs are not soldered properly on to the pads, please check the VDD capacitor and its connection on the VDD pin.  

    You could also verify whether the controller is damaged by applying external DC source to the VDD pin above the UVLO turn-on threshold and leave the DRV pin open to measure the output pulse on DRV pin.

    • If you are able to observe 3 start-up switching pulses (only occur once as VDD will never fall below off threshold when external supply is applied), please check the connections and ensure there is nothing short on the circuit

    • If you still cannot observe any output pulse after applying the external supply, you could consider measuring the impedance for each pin to check if there is any short on pins to see if the controller is damaged.

    Or simply replace with a new IC to see if that fix the issue, if yes that means the old one is damaged.

    I hope this could answer your question and it would be helpful if you can provide the VDD, DRV, CS, VS waveforms for us to help you find out the root cause of your start-up issue.

    Thank you.

    Best Regards,

    Wei-Hao Chen