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TL4242-Q1 / Failure detection sequence

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: TL4242-Q1, TL4242, TPS92630-Q1
Hello,
 
I have the question about TL4242-Q1.
Please see the attached file as our application and question.
 
 
I would like to know the LED open detect functions.
I believe that this device don't latch off at the open detect.
However , please let me ask for the confirmation.
 
Are below sequence for this TL4242-Q1 schematics correct ?
-----
1.Input Voltage is decreased and Q output voltage  becomes to under the Vf  of the LED.
2.2.ST output -->  Low status
3.PWM input --> Low
4.Q output is 0 --> all LED lights out
5.All of LED lighted when Input voltage back to normal or lights out.
-----
 
Best Regards,
Ryuji Asaka
 
  • 0
    Guru 20090 points

    Hello,

    Sorry for bothering you.
    But please let me have the reply.

    I updated the information as below.

    ST pin is connected to PWM pin.Thus, TL4242 output is latched off when ST pin asserted Low.
    In this case, the led do not light again until power down the device?
    If there is power on reset threshold voltage please let me know.

    Best Regards,
    Ryiji

  • 0 in reply to Ryuji
    TI__Expert 5275 points

    Hi Ryuji,

     

    You are correct that the device does not Latch off when it detects open.  During normal operation, when the output current falls below the set point, the REF pin will fall below the reference and turn on the comparator which turns on the output (see Functional Block Diagram from Datasheet). 

    As the output continues to fall below the open detect threshold, a second comparator will drive the ST output through the status delay.  Throughout this process the first comparator is still on driving the output high.  Basically, when an open is detected, the part continues to try and drive the output.  In your case, this gets more complicated because you have connected the PWM pin to the ST pin. 

    What will happen next here, is that the bias supply turns off and effectively grounds the bandgap reference for both comparators.  And since both are comparing to REF (which is also grounded at this point), you will have a ground to ground comparison resulting in an unknown state.   The the real sequence will be:

    1.Input Voltage is decreased and Q output voltage  becomes to under the Vf  of the LED.
    2.2.ST output -->  Low status
    3.PWM input --> Low
    4.Unknown State

    If the device is powered down all the way, then the ST drive will reset, but if this doesn't happen quickly enough, the device will pass through this unknown state.  Or, if there is a brown out, the device could go into an unknown state.

    If you don't plan on using the PWM function to control the current, I would suggest you connect the PWM pin to the I pin as mentioned in the Datasheet.  This way, when the input comes back up, the bandgap reference will be available.

     

    Best Regards,

    Adam Hoover

    Drivers and Load Switch Applications Team

     

  • 0 in reply to Adam Hoover
    TI__Expert 5275 points

    Hi Ryuji,

     

    Please let me know if you have any other questions about this.

     

    Thanks,

    Adam

  • 0 in reply to Adam Hoover
    Guru 20090 points
    Hello Adam san,
     
    Sorry for my late reply.
    Our customer would like to use LED current range is 10mA to 50mA or 5mA to 300mA.
    However TL4242 LED current (min) is 18.5mA.
    Therefore, I recommended other device(TPS92630-Q1) to our customer.
     
    Best Regards,
    Ryuji Asaka