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TPS92692: How to set the MAX LED Current to specify value through the direct PWM Dimming mode?

Dexter
Prodigy 80 points
Part Number: TPS92692
Design: Boost LED driver with high-side current sense circuit. VIN=12V, 6 LEDs in series(Max led current can be up to 1.5A according to LED datasheet).
My TPS92692 is configured to Direct PWM Dimming mode by connecting a 249K ohm resistor from RAMP  pin to GND. And IADJ pin in conjunction with second-order low-pass filter to 3.3V. Vo(ov) is set to 19.5V(Rov2=150K ohm, Rov1=10K ohm).
But when power on my board, the Vled is steady 14.4V no matter Rcs is 0.1ohm or 0.39ohm. How to set the MAX LED Current to specify value through the direct PWM Dimming mode?
  • 0
    Prodigy 80 points

    Attached schematic

  • 0 in reply to Dexter
    TI__Mastermind 18890 points

    Hi Dexter,

    What is the votlage that you have on IADJ (pin9)?  This DC votlage controls the DC current to the LED.  With RAMP tied to 249K to ground you are direct PWM'ing and what that means is that you are PWM'ing the output current to the level that is commanded by the IADJ and 0A. 

    Are you just wanting to drive DC current on the LED at 1.5A?  Please give me your requirements for range of input voltage, output current and if PWM'ng is required or just DC current on the LED's  and I'l try to help you.  What is the forward voltage of the LED from the data sheet?

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 30 points
    Hi Tuan Tran74,

    I'm John, Dexter's colleague. We use direct PWM dimming on DIM/PWM(pin10) in our design.

    Input voltage range: 7V~18V.
    Output current: 100mA~1500mA(6 LEDs in series, LED forward voltage is 2.75V~3.5V according to LED's datasheet), and this current should be controled by direct PWM dimming on DIM/PWM(pin10) based on the dury cycle of external PWM signal.

    By the way the voltage on IADJ(pin9) is 2.47 V on our board.
  • 0 in reply to JohnFeng
    TI__Mastermind 18890 points

    Hi John and Dexter,

    Your minimum VLED voltage is 6 x 2.75V = 16.5V.  Your maximum input votlage is 18V so you would have to use the Buck Boost configuration for your design since Vin is greater than Vout.  Otherwise, you will have really high current on the LED for that case and will not be able to control current and damage the LED's. 

    Please see section 9.2.2 to help with your design.  In addition, we have an Excel spread sheet to help you with the desing at our TI website http://www.ti.com/product/TPS92692/toolssoftware.  It's under the software folder.  You can also look at the document for our Evaluation board to help.

    Fill out the excel spread sheet and line 14 you will have 31.5W (1.5A x 6 x 3.5V) for max power when VLED = 3.5V.  You would do the design for 1.5A output current and PWM control the current to get down to 100mA average current.  Let us know if you need more help.

    Thanks Tuan

     

  • 0 in reply to Tuan Tran74
    Prodigy 30 points
    Hi Tuan Tran74,

    Thank you for you advice.

    As Now we're just making demo board, we can control the input voltage range to 11V~13V.
    So What should we do to acomplish the Output current: 100mA~1500mA design in Boost configuration use direct PWM dimming on DIM/PWM(pin10)?
  • 0 in reply to JohnFeng
    TI__Mastermind 18890 points
    Hi John,

    Please go thru the same design process that I mentioned above for boost design. . From your schematic, most of the values along with type of FET's that you have chosen are wrong. The way the PWM FET is hook up is wrong. All these are in the data sheet on the application section. At a minimum, please fill out the excel spread sheet to get your values for the design then read the application section in the data sheet to help you. We'll help you thru it but there are some fundamentals that you have to understand first before we can do the design.

    You can also order our evaluation board to help you get started then modify for your application.

    Thanks Tuan
  • 0 in reply to Tuan Tran74
    Prodigy 30 points
    Hi Tuan Tran74,

    Thanks very much for your help, i will go thru the datasheet and use our TPS92692_Design_Calculator sheet to do the calculator.

    Thank you.
  • 0 in reply to JohnFeng
    TI__Mastermind 18890 points

    Hi John,

    Also look at the Evaluation user guide to help you with the schematic with the below link.  Please look a requirements for FET's and Diode current rating.  The parts you used were not rated for the current that you want out of the design.

    http://www.ti.com/product/TPS92692/technicaldocuments?keyMatch=tps92692&tisearch=Search-EN-Everything

    We will close this thread for now.  If you have additonal questions after going thru the desing material please let us know.

    Thanks Tuan