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TL4242 power dissiapation

Ashokan Arumugam1
Prodigy 50 points
Other Parts Discussed in Thread: TL4242-Q1, TL4242

 We are using TL4242-Q1 for one our application. Maximum supply voltage is 18V LED drop is 3.2V  and LED current is 80mA. its 2 layer board( no power plan). When am trying to calculate power dissipation its exceeding the rated value. Even  i have tried varying PWM duty cycle also still  am not success.

Following formula used power calculation

Power dissipation=Vin-VLED drop-Vref)*ILED*duty cycle+Vin*Iq

Power dissipation=0.944

Where Vin=18V, VLED=2.8V, Vref=168mV  ILED=80mA Iq=22mA

Allowed maximum power dissipation is  from datasheet for JEDEC 51-7 PCB is 0.63W

Please advice .

 

 

  • 0
    TI__Genius 14890 points

    Ashokan,

    A simple option would be to add a resistor in series with the LED to dissipate some of the excess power.  Keep the shunt resistance as is.  If you used a resistor to drop 15V (15V / 80mA = 188 Ohms) the TL4242 power dissipation is greatly reduced and you will not have any issues with that device.  Keep in mind that the resistor power rating must be taken into account.  Plus, variations in VLED drop, resistance and Vref need to be taken into account when selecting the resistor value.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 50 points

    Hello Dick

    Thank you very much for your answer. Need some more suggestion to select series resistor. As per datasheet I have Rref resistor 2.2ohm for 80mA current .Series resistor which you’re suggesting is different than Rref resistor used for current setting. Please clarify.

    Regards

    Ashokan

  • 0 in reply to Ashokan Arumugam1
    TI__Genius 14890 points

    Ashokan,

    I was not clear in my original answer.  The series resistor should be "above" the REF pin.  The 2.2 ohm resistor should stay exactly where it is.  Tie the power dissipation resistor between the "Q" pin and the LED.

    Let me know if that works for you.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 50 points

    Dick

    Thank you very much,Your answer helped me lot. one quick question Iq max mentioned in data sheet is 22mA. Will TLE4242 will take so much current.since it also one key factor for increasing power dissipation of driver. I assume adding series resistance will not do any change in current setting.

    How to use PWM with TLE 4244 my input will vary from 8Vt to 16V and LED current need is 80mA.

     How to decide PWM frequency and duty cycle.

    Thanks

    Ashokan

  • 0 in reply to Ashokan Arumugam1
    TI__Genius 14890 points

    Ashokan,

    The TL4242 spec shows maximum of 22mA.  I have seen much lower values in characterization.  For your conditions (8V to 16V input) I would expect below 5mA.  You should measure the current on a sample of units under your worst case operating conditions to determine how the TL4242 will perform in your setup.

    You are right that adding a series resistor will not impact the IQ of the TL4242.

    Regards,

    dick 

  • 0 in reply to Dick Stacey
    Prodigy 50 points

    Hello Dick

    Could you please tell me, How to select TL4242 PWM frequency and duty cycle. What is the maximum frequency TL4242 can support.

    Thanks

    Ashokan

  • 0 in reply to Ashokan Arumugam1
    TI__Genius 14890 points

    Ashokan,

    The maximum PWM frequency of the TL4242 depends on several factors.  The best answer is to characterize the PWM in you application.  Here are a few items to note.  The datasheet discusses the "delay".  The delay is the time from the PWM pin transitioning until the output turning on or off.  Note that the on and off delays are very similar (and maximum spec of 40us).  This means that the output will be the PWM pulse shifted by the delay factor.  The minimum period is less than 40us since the on and off delays are similar.  

    The PWM voltage and programmed current will have an impact on the timing as well.  The current slew rate while turning on and off will have an impact on the maximum PWM frequency.  

    How you define the maximum frequency will also impact your use.  When you increase the frequency high enough, the current will not reach the fully on value for the full PWM pulse, but it will reach the programmed current for some period.  As you continue to increase the frequency, the output will be on during the (shifted) PWM pulse but the the current pulse will begin to change from a rectangular pulse, to more of a triangular pulse.  

    Experimentation in your environment is the best method for determining the maximum effective PWM frequency.

    Regards,

    Dick