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TPS61500EVM-369 dimming

Elliott Johnson
Prodigy 100 points
Other Parts Discussed in Thread: TPS61500EVM-369, TPS61500

In slvu261 (the TPS61500EVM-369 user's guide) it indicates a circuit that can be populated to implement PWM dimming; does this provide pure PWM dimming by applying a PWM signal to the EN pin of JP1, or analog dimming via the same source?  Besides the MOSFETs I don’t see any values for the components in the bill of materials would I find these in the tps61500 user’s guide in one of the example circuits?

Regards,

Elliott Johnson

  • 0
    TI__Guru**** 167146 points

    Yes, with the simple modifications listed below, the TPS61500EVM-369 EVM can be modified to perform pure PWM dimming:

    1.   Remove the capacitor on the DIMC pin (C7)

    2.   Install 1k resistors on R5 and R6

    3.   Install FETs similar to those recommended in the Part Number column of the user's guide BOM for Q1 and Q2.  Note that Q2 needs to be a PFET with relatively low RDSon but Q1 can be a low cost NFET.

    Resistor R9 is an optional 1kohm+ pull down and would only be necessary if your EN signal goes to an undefined state thereby allowing the gate of Q1 to float high.  Capacitor C10 is an optional 100pF+ capacitor and would only be necessary to slow down the Q2's turn on if the LEDS flicker. 

  • 0 in reply to Jeff F
    Prodigy 30 points

    Hello,

    we implemented the digital-dimming in our design as you suggested and use same parts for Q1 & Q2 listed in TPS61500EVM-369 EVM. Our problem is, that on dimming the leds start to flicker. Adding a 470pF to C10 makes flicker much better but special on lower dim-levels or lower input voltages it start to flicker again very strong sometimes. We use a 150Hz / 3.3V PWM-Signal on EN (& gate Q1). In earlier revision we just used EN to dim (without C5 on TPS61500 manual) and had no problems at all, except that on higher inout voltages the leds dont turn off properly. Thats the reason why we want to implement external Q1 & Q2 PWM-dimming.

    Best regards

    TK

  • 0 in reply to TK56545
    TI__Guru**** 167146 points

    The datasheet recommends not using a PWM frequency less than 200Hz in order to ensure that the EN pin is not taken low long enough for the IC to begin shutdown.  I recommend increasing to 200Hz to see if that fixes the problem.

    If not, then the flickering may be caused by the FETs not completely turning on/off.  Can you provide a scope plot like Figure 2 of the user's guide? 

    One cause might be the 2N7002's 2.5V gate threshold voltage (Vth).  With 3.3V gate drive, the 2N7002 may not be completely turning on and therefore not completely turning on the PFET.  I would recommend replacing the NFET with a better FET, i.e. one with a lower threshold voltage.  Also, take a look at the shape of the PWM signal at the gate of the 2N7002.  If it is rounded, then your PWM signal generator doesn't have enough drive.

    Another problem at low Vin might be that either the IC is hitting current limit or the input voltage gets pulled down due to the high input current when the FETS turn on.

     

  • 0 in reply to Jeff F
    Prodigy 30 points

    Dear Jeff,

    thanks for your fast reply. Replaced the 2N7002 with BSS138 (about 1.5V VGS) also tried BSN20 (1V VGS) without success. Due Hardware limitation i cant select 200Hz Hardware-PWM and used 300Hz but without success too. I send two screenshots here. It looks like that selecting lower VGS N-FET makes it even worse. With 2N7002 it works at least sometimes :)

    We use Atmel hardware-PWM.I send two screenshots, Picture 1.jpg shows PWM + Output Led, Picture 2.jpg shows PWM + drain of Q1 (BSS138).

    Thanks

    Tobias

     

     

  • 0 in reply to TK56545
    TI__Expert 4525 points

    I modified the EVM to do PWM dimming.  Everything works as expected.

    I swept:

    PWM frequency from 150Hz to 300Hz, VIN from 5V to 10V, and Duty Cycle from 20% to 80%

    It worked properly in all scenarios.

    I have the BSN20 and SI2319DS for the transistors.

    I'll try to take some oscilloscope shots and post them here.

  • 0 in reply to Stephen Nortman
    TI__Guru**** 167146 points

    Stephen,

    You removed jumper JP2 in order to make this operate, yes?

    Jeff

  • 0 in reply to Stephen Nortman
    Prodigy 30 points

    Stephen Nortman said:

    I modified the EVM to do PWM dimming.  Everything works as expected.

    I swept:

    PWM frequency from 150Hz to 300Hz, VIN from 5V to 10V, and Duty Cycle from 20% to 80%

    It worked properly in all scenarios.

    I have the BSN20 and SI2319DS for the transistors.

    I'll try to take some oscilloscope shots and post them here.

     

    You used 3.3V PWM-Signal?

  • 0 in reply to TK56545
    TI__Expert 4525 points

    Yes.  It works for me with a 3.3V PWM.

    Two questions:

    1) Does your power supply have a current limit?  With low VIN, the circuit will draw a lot of current.  I was able to cause flickering by using a power supply with a 0.5A limit.  Perhaps, this is what you are doing.

    2) Can you verify that your PWM is offset by 1.65V (i.e. 3.3 / 2)?  From the screenshots it looks like it may go below zero when it is low.

  • 0 in reply to Stephen Nortman
    TI__Expert 4525 points

    Also, do you have JP2 open or short?

  • 0
    Prodigy 100 points

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    Thank you, everything works as expected on the dimming front, however I was wondering if there were any modifications I should make if I decided to increase the amperage of the LEDs (by replacing the 500mA LEDs included on the board with something in the range of 1 to 1.5A)

    Regards,

    Elliott Johnson

  • 0 in reply to Elliott Johnson
    TI__Guru**** 150950 points

    Elliott,

    Glad you got the dimming working.  Do you have a root cause or solution that you can post and share?

    To increase the LED current, reduce the value of R8 on the EVM, as explained on page 11 of the datasheet.  Keep in mind the 3A switch current rating of the TPS61500.  Page 12 of the datasheet explains the maximum output current that the IC can provide, which depends on how many LEDs you have and on Vin.  1.5A is pretty high for this part with a 3A switch, but you can do it with the proper Vin and LED number.