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TPS92692: Start time is much slower in analog dimming mode

user6413162
Prodigy 250 points
Part Number: TPS92692

Hi Team,

Hope you're well.

Start time for my design is 3ms when the output is 350mA, but the start time becomes 40ms when it works in analog dimming mode with a lower output of 75mA.

Topology: buck-boost 

Input: 12V / 24V

General mode: Io=350mA, ts=3ms, V-IADJ＞2.5V

Analog dimming mode: Io=75mA, ts=40ms, V-IADJ=511mV, no any other changes with general mode. 

What I want is the start time can also be 3ms when it works in analog dimming mode. Could you please help to analyze this issue ? What might I have a try ?  Thanks a lot.

  • 0
    TI__Genius 17690 points

    Hello,

    Can I please have a schematic of your design along with the output LED voltage before I can comment more on this. 

    In general, it will be hard to get the same 'start time' for a current mode regulator since it's not a voltage regulator where we can profile the output voltage ramp with the soft-start ramp.  Is there a reason why you want the start time the same or what is your application and why this is critical for you?

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 250 points

    Hi Tuan,

    Nice to see you again. This is Han. I guess you still remember me. I update my personal email address to a company email address because I can't use the former one to reply or ask a new question.

    This question is based on my last question 'TPS92692-Q1: some issues in electronic test'. You know, I failed to pass the PWM input test via PWM dimming as we talked before. So I changed it to analog dimming...Unfortunately,  that's would be really a bad news for me if it's still impossible to manage the start time in 8ms , I mean for analog dimming with low output. The PWM input test (8ms on, 2ms off, cycle...)  is required by customer. So it's really important to manage the start time in 8ms, otherwise, it would be flicking and fail.

    I'm quiet confused why the start time becomes much slower (about 40ms) when it works in analog dimming mode, but it can start normally in 3ms as I set when it works in high output status. It seems there happens a protection (36ms) ,then it starts working. Please take the schematic below as reference. The output LED voltage is from 11V to 13V (4 LEDs in series). Hopefully you can help me. Thank you so much !!

  • 0 in reply to Tuan Tran74
    Prodigy 250 points

    Hi Tuan,

    This product is used in automotive lighting, and the PWM input test is used to simulate the output of battery from car-body for some types of cars. So I want to manage the start time in 3ms, or 8ms at least. Because we should make sure it can start working during the on-period (8ms) of the PWM input voltage.  That's the background. Please take notice. Thanks.

  • 0 in reply to user6413162
    TI__Genius 17690 points

    Hi Han,

    Getting consistent Soft-Start time will be difficult as I mentioned before because a current regulator is different than a voltage regulator where the output follows the soft-start ramp.  You can reduce the Soft-Start cap by 1/2 to get faster soft-start time but this can increase the surge current.  To get consistent soft start on the current will not be possible with different commanded current and different input voltage or load.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 250 points

    Hi Tuan,

    Thanks for your feedback. I don't need to get the start time of low output absolutely consistent with the start time of high output. For example, start time for high output is about 2ms, and for low output is about 6ms (even 7ms), that would be OK. Is this situation be possible ?

    If it's always not possible to get the start time in 8ms when it works in dimming mode ( no matter PWM or Analog), then, do you have any other solutions (IC chips)  for recommendation ? Thanks.

  • 0 in reply to user6413162
    TI__Genius 17690 points

    Hi Han,

    Please reduce the Soft-start cap lower as I mentioned in my previous post.  There is a point where too fast of Soft-start can be an issue but for now you can experiment with lowering the cap to reduce the soft-start time.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 250 points

    Hi Tuan,

    I reduced the Soft-start cap from 22nF to 15nF or 10nF, but the start time becomes longer ( about 77ms ). However, when I increase the Soft-start cap to 68nF, the start time becomes shorter ( about 9ms ), though it's still doesn't meet the requirement...

  • 0 in reply to user6413162
    TI__Genius 17690 points

    Hi Han,

    What you are saying does not make sense to us at all.  Increasing the Soft-Start Cap should increase soft-start time not decrease it.  Either you are doing something wrong or do not understand the fundamental operation of the part or not describing things correctly.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 250 points

    Hi Tuan,

    Yes, I guess there is something wrong with my design as well. As I mentioned, it seems there is a protection happening when it started under analog dimming mode. Start time for normal mode is 3ms , but it becomes 40ms when it comes to dimming mode. The problem is that I did not find any protection happened according to the datasheet.

  • 0 in reply to user6413162
    TI__Genius 17690 points

    Hi Han,

    It's hard to understand what you are describing.  Please have scope captures of what you are talking about along with actual schematic and load conditions so we can understand better in order to help.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 250 points

    Hi Tuan,

    I have already made some progress. The start time for both normal mode and analog dimming mode are in the range of my requirement. However, when I use PWM signal as input voltage, it still flicks. I think it is because the surge is too large and it causes the protection. Can you give some advice to solve this issue ?

    I have another question, is the cap necessary for the pin IADJ ? And does it influence the circuit-start ? Thanks. 

  • 0 in reply to user6413162
    TI__Genius 17690 points

    Hello,

    We will need scope captures on the issue that you are describing for us to help you with this.

    The cap on pin IADJ is there to filter out noise on the pin.  You can reduce the cap value if you find that it helps your performance,

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 250 points

    Hi Tuan，

    Below is the scope capture, you can see that the circuit started normally in the first duty, but in the following duties, it did not work. That is the problem. I need it to work normally in each duty.

    channel 1: input voltage,     channel 2: input current,     channel 3: output current

    And, I have another question, when the circuit works in analog dimming mode, I found out the output current rises very obviously as the ambient temperature rises. That happened in the oven when I do the thermal test. Also, when I heat the component TPS92692 alone, the output current rises again. So, I guess this problem is related to the internal work logic of the IC. Could you please give an explanation about this question? And is there any method to solve it ? Thanks a lot.

  • 0 in reply to user6413162
    TI__Genius 17690 points

    Hi,

    We've tried to help you with this design with many previous posts.  In general, what you are trying to do will not work as you have described here...You are asking a lot of questions without really understanding electronics and what is happening and it's hard to help you.  You need to trouble shoot your own design some and really figuring out what the issue is..

    As far as thermal is concerned, again you have to trouble shoot the circuit and understand what you are doing to solve your issue.  The TPS92692 has been in production for a while with many automotive lighting modules designed with it so I am pretty sure it's not a parts problem with thermal here.  Again, you have to trouble-shoot yourself to figure your issue. 

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 250 points

    Hi,

    If it absolutely works as datasheet described, then, I will not go to the E2E to ask for help. What you said does not give any help...

  • 0 in reply to user6413162
    TI__Genius 17690 points

    Hi,

    The data sheet of the TPS92692 does describe all the fault conditions that can happen along with under-voltage lock out ect.  We've asked for scope captures of the fault multiple times in you previous post to E2E that you did not reply back to both on August 17 and August 21.  In this new post that you started here you are asking about the same design and issue again.  You just provided a an input voltage pulsating and going to 0V the same as your previous scope capture on another titled post 4 months ago.  As we've previously mentioned to you that if input voltage is going to ground then the part will shut down ect. and that is what is happening.

    Asking multiple times about the same issue and not providing the pertinent information as to what is wrong will not help us resolve you problem.  Again, the part absolutely works a data sheet described.

    Thanks Tuan