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LM5022: minimizing delay during start up until PWM starts switching

Lutz Naumann
Expert 6140 points
Part Number: LM5022

Dear team,

we are facing a challenge with the LM5022 during start up. We want to minimize the time until the PWM starts switching but then have a slow soft start.. Basically a two level soft start. When selecting the SS ramp needed (470nF) the PWM starts switching with a delay of about 17ms once the VIN ramps up. See the plot attached. We want to minimize this delay but then maintain the SS ramp.

We would like to understand the following implementations to find a potential workaround:

1) how is the SS discharge circuit being implemented? Is the a FET plus a serial diode as shown in the block diagram? and what would be the remaining voltage tolerance one can expect when this charging the SS pin / cap.

2) How is the 1.4V at the comp pin implemented? Is it some sort of REF, voltage divider or diodes? What is the tolerance of the 1.4V.

3) According to the datasheet it seems the 1,4Vat the Comp pin is tracking the remaining SS voltage, can you confirm? how is this done? 

From what we can see in the measurements, the PWM starts switching once the COMP pin voltage crosses the said 1.4V. Our idea is to precharge the comp pin, so the time until we reach the 1.4V threshold is minimal, the part starts switching and then SS ramps up slowly.

Or do you have another suggestion?

many thanks

Lutz  

  • 0
    TI__Mastermind 21720 points
    Hi Lutz,

    I have contacted you directly.

    -Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 20 points
    Hi Garrett,

    the question was placed here from Lutz on my behalve. I had no account so far but now I have joined the community.
    Would you mind to give me some more delais on the startup behavior of the LM5022. Especially the three question mentioned by Lutz.

    Additionally I found during my test that precharing the SS-Cap by an external circuit to about 1V will hang up the LM5022. It wont start up until the voltage on SS-Pin is switched off. Why does a voltage on SS-Pin stop the startup procedure?

    Thanks for your help!

    Benjamin
  • 0 in reply to Benjamin Schacht
    TI__Guru* 75895 points
    Hi Benjamin,

    To reduce the SS time before the circuit is switching, the simplest way is to add a series resistor. This will lift the beginning of the SS voltage to reduce the non-switching time duration, while keep the same soft start performance once it starts switching.

    The 1.4V is implemented with two series diodes. The tolerance is not characterized.

    The COMP pin is clamped to the SS until after the SS is rising above. You will see the COMP to follow the SS during rising up, by this the soft start is achieved. Note that the PWM duty cycle is the result of peak current and COMP voltage (shifted and scaled down). SS limiting the COMP basically limits the PWM duty cycle, and controls the duty cycle to grow gradually for a SOFT START.

    Note that the SS current is only 10uA, and your precharging circuit may prevent SS pin from rising above that precharging level.

    Hope these answer your questions.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 20 points

    Hi Youhao,

    thanks for you answer. I had also the idea of inserting an series resistor before softstart cap. It improves the behavior. 

    I tried also the following circuit:

    The resistors R11 and R12 are selected to get a voltage of 1.4V at P1. By this circuit I speed up the SS charging to about 1V at P2. Then the normal softstart charging current of 10µA is doing the remaining softstart.

    The problem is the LM5022 doesnt start up when SS voltage is force to 1V by this external circuit. The PWM starts switching but the SS does not start to charge Css Cap. 

    The following sequence takes place:

    1. UVLO is exceeded by input voltage

    2. SS discharge circuit release the SS pin 

    3. SS pin is charge quickly to 1V 

    4. PWM starts switching.

    5. LM5022 remains in this state

    Why das the LM5022 doesnt perfom the softstart by turing on the 10µA current source?

    Kind regards

    Benjamin