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LM3424: loop compensation block diagram of LM3424

VINAY SAREEN
Prodigy 45 points
Part Number: LM3424

hello sir i am trying to understand the internal block diagram of LM3424 I got stuck in loop compensation part where i couldn't understand the symbol being used in block diagram which i have marked separately in the screenshot of block diagram attached with this post .

as per my understanding i assumed it as tristate buffer and considered the output from ndim comparator as control pin of the tristate buffer and input and output of tristate buffer as output from HSP/HSN comparator and  comp pin respectively 

assuming the above thing correct i setup the LM3424 as buck without dimming (i.e ndim tied to voltage above 1.24 using potential divider circuit  to act as UVLO) and tried to check voltages at pin of LM3424 what i observed was that i couldn't run my LED and the COMP pin was having voltage around 0.2volt only which i didn't expected as  CSH was around 0.1volt   forcing comparator at CSH to output +5volt and thereby forcing COMP at +5V 

please help in understanding of this symbol used in block diagram 

  • 0
    TI__Mastermind 18890 points

    Hello Vinay,

    That is basically a switch or a MUX.  When you are dimming the LED, during the OFF time we tri-stage the COMP pin to hold the charge and suspend the loop.  Basically tri-stating the output of the COMP op-amp.  With the nDIM above 1.24 V the regulator should be operating as normal.  You need to trouble shoot your circuit to see why it's not working.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 45 points

    sir i tried to debug the circuit what i observed was that gate pin is constantly set to ground so i concluded that the SR latch used internally must be reset. can you please guide the reason for that gate pin to be constantly driven to ground  

    i deliberately forced the pins(OVP ,nDIM,CSH ,IS ,(HSP-HSN)) as follows (less than 1.24V, GREATER THAN 1.24V, less than 1.24v,GROUNDED ,O volt ) respectively  so that reset gets to 0 still gate pin is at 0v only 

    all the above was done so as to see the internal clock at GATE pin 

    please guide me what to do its urgent 

  • 0 in reply to VINAY SAREEN
    TI__Intellectual 1675 points

    Hi Vinay,

    We are looking into this, and will get back to you soon.

    Best Regards,

    Caroline

  • 0 in reply to Caroline Fuschino
    TI__Mastermind 18890 points

    Hello Vinay,

    If you run the LM3424 in normal operation does it work properly and control the LED current?  If it does not then I can help you with that but I can't help you trouble shoot your circuit if you are running the part in a non-typical application.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 45 points

    I am using standard application of LM3424 only without PWM dimming i have attached my schematic with the used values. my problem is that voltage across Vcc(Cbyp) is stuck at 3.9V as a result voltage at SS ,,DDRV,GATE and COMP pins are stuck at 0v please guide me for the same 

  • 0 in reply to VINAY SAREEN
    TI__Intellectual 1675 points

    Hi Vinay,

    Tuan is our technical expert on this part, so I will continue to let him assist you. Please allow us at least 24 hours to respond to new information. Thank you for your continued support of our devices!

    Best Regards,

    Caroline

  • 0 in reply to VINAY SAREEN
    TI__Mastermind 18890 points

    Hello Vinay,

    What are the values of resistors for the Thermal foldback circuit in the block diagram which are input for the TREF and TSENSE?

    What is the input voltage and output LED voltage?  Can we have a scope captures of the following signals?

    CH1:  Input voltage (trigger on this voltage rising edge)

    CH2:  VCC

    CH3:  VGATE

    CH4:  nDIM

    Then another scope capture of the below signals:

    CH1:  Input voltage (trigger on this voltage rising edge)

    CH2:  VSS

    CH3:  TREF

    CH4:  TSENSE

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 45 points

    sir we are very small scale setup availability of oscilloscope  wont be possible all i can tell you is that voltage across Cbyp at time of powering up remains at 6.9v voltage and at Css remains at 5.3volt. nDIM is constantly at 19.4 volt and gate voltage is 0v .

    after few seconds voltage across Cbyp drops to 3.9 volt and remain there constant at 3.9volt  throughout and voltage at Css drops to 0 volt nDIM is constantly at 19.4 volt and gate voltage is 0v 

  • 0 in reply to VINAY SAREEN
    TI__Mastermind 18890 points

    Hello Vinay,

    We cannot help trouble-shoot for your designs if there is no scope data on this.  Again, we will need scope data in order to provide assistance.

    Thanks Tuan