DLPC7540EVM: About LED PWM frequency for LED driver control signal

Part Number: DLPC7540EVM
Other Parts Discussed in Thread: DLPC7540, TPS92641, TIDA-080008

Tool/software:

Hello

The current DLPC7540EVM LED PWM signal (PWM RED, PWMGRN, PWMBLU) frequency is about 138.5kHz,

but can it be changed to a lower frequency?

This is because the rise time (Tr) and fall time (Tf) of the LED current drive by the driver are several usec.

Thanks & Best regards,

 Sasaoka 

  • Hi Sasaoka san

    How are you getting this result for frequency? The speed at which the LEDs need to switch depends on the bit depth of the sequence and the frame rate. Is there something with the video quality that is causing concern? 

    -Aaron

  • Hi Mr. Aaron,

    Using the Front end board for which you rewrote the EDID information, I input a 2k (1080p) 240Hz HDMI signal from a PC to operate the DMD.

    At that time, I observed the frequency of the LED PWM signals (PWM RED, PWMGRN, PWMBLU) on the EVM board with an oscilloscope.

    Since the optical system is not yet complete, I cannot tell the video quality.

    Is it possible to lower the frequency of the LED PWM signal?

    Best regards,

    Sasaoka

  • Hi Sasaoka-san,

    Aaron is out for this week. We will have another one of our engineers cover this thread while Aaron is out.

    We are sorry for any delay, but thank you very much for your patience.

    Regards,
    Michael Ly

  • Hello Sasaoka-san,

    Within the DLPC7540 GUI you will need to go to the Advanced Mode and utilize the 'Illumination' tab.

    Please try this and let us know if you run into issues. Please feel free to respond to this thread.

    Best,

    Aaron

  • Hello Aaron ,

    I don't know where to change the 'Illumination' tab in Advanced Mode.

    Please tell me more about it.

    Best regards,

    Sasaoka

  • Hello Sasaoka-san,

    Sorry, that tab will likely not do what you are trying to do, that tab is for changing the driving voltages and duty cycle but the PWM frequency on our EVM is a standard frequency that does not change.

    The software we have implemented with our solution does not have the ability to change the frequency.

    Please advise offline if there is confusion about this.

    Thank you!

    Aaron

  • Hello Mr. Aaron,

    I understood that the frequency cannot be changed on the GUI program.

    However, the current frequency of about 138.5 kHz seems to be high.

    The ON-OFF time is too short for the driver to follow.

    This is because the performance of the driver prototyped using your driver chip (TPS92641) this time has a rising time (Tr) and falling time(Tf) of about 10 us.

    Of course, since it is not designed for mass production, the impact is large due to the long cable,

    But since the EVM board is used in test-level prototypes, I would like you to take into consideration that Tr and Tf will be large.

    Please make it possible to set the frequency to be one order of magnitude smaller.

    Thanks you.

    Best regards,

    Sasaoka

  • Hello Sasaoka-san,

    I think there is some confusion about how this is operating. We are testing utilizing this chip in an LED driver and have had no issues.

    The on-off time is not for the PWM of the driver chip. The on-off is actually the enable sent from the controller - the real duty cycle to the LEDs.

    It would be extremely helpful to take a look at your schematic to resolve this. Please reach out to us offline with your schematic.

    Best,

    Aaron

  • Hello Mr. Aaron ,

    It is a little difficult to submit a circuit diagram due to confidentiality. Sorry, but could you show me the circuit diagram between your driver and EVM?

    The point is the control signal for LED light emission.

    As I understand it, the LED is turned on by taking the AND of both the RGB enable signal and the RGB PWM signal from the EVM.

    Perhaps you only use the enable signal to turn on the LED? If so, how do you adjust the white balance?

    As I understand it, the white balance is adjusted using the RGB PWM signal.

    When you change the RGB LED current with the 'Dynamicblack' LED current on the GUI, the duty of the PWM signal changes,

    so I think that this is input to the LED driver to adjust the white balance.

    Is this idea wrong?

    Best regards,

    Sasaoka

  • Sasaoka-San,

    The LED PWM signals on thr 7540 have a fixed 140khz pwm frequency; this frequency is not changeable.

    The PWM signals are not used to drive the LEDs directly; they are used to create a voltage reference for the LED driver to control the LED driver current.  

    The LEDs are enabled via separate LED enable signals.

    We have an example DLP LED driver using the same TPS part; it works with the PWM and enable signals from the 7540.  www.ti.com/.../TIDA-080008


    Regards,
    Gary

  • Hello Mr. Gary

    Thank you for your advice.
    I downloaded the TIDA-080008 schematic from the following URL:
    www.ti.com/.../TIDA-080008
    I have a basic question.
    Is the LED enable signal of the driver IC TPS902641 UDIM_IN_RED (GREEN or BLUE)?

    Aren't the R_EN, G_EN, and B_EN signals the enable signals?

    Best regards,

    Sasaoka

  • Sasaoka-san,

    As described in the Design guide, there are headers/connectors that allow selecting either UDIM or SDIM for the Enables.  SDIM uses the NFET shunt to turn off the LEDs.  SDIM is substantially faster than using UDIM and is the only practical option for our display applications; UDIM is too slow. 

    On the TPS92641, SDIM (and UDIM) are PWM dimming pins.  We are using them as an ON/OFF switch by selecting 100% (~3.3V) or 0% (~0V) dimming -- FULL ON or FULL OFF only.  The PWM dimming function of the TPS92641 is not compatible with our display sequences on the DLPC7540. 

    The brightness/dimming of the LEDs is controlled by using the Iadj.  The controller (DLPC7540) LED PWM signals are converted to a DC voltage through an RC filter.  Note that Iadj has a 2.54V clamp; so the useful range is only 0V - 2.54V. 

    Iadj has an absolute max rating of 6V and is tolerant to the 3.3V I/O we are using; but it would be best to limit the LED duty cycle such that Iadj max is closer to the ~2.62V max IADJ clamp voltage (perhaps 2.7V max).  


    Regards,
    Gary

  • Hello Mr. Gary

    Thank you for your answer.

    I forgot to explain our system.

    Currently we are using the DLPC7540EVM and DPL650TEEVM to prepare our own optical engine (optical system for lighting and projection) and to prototype a projector.

    Within that system we are using the TPS92641 as a driver to control the RGB LED. What I would like to understand now is how to connect the DLPC7540EVM to the LED driver (TPS92641).

    I believe that the only control signals provided from the DLPC7540EVM to the LED driver (TPS92641) are 3 lines of RGB PWM signals and 3 lines of RGB enable signals.

    Is this wrong?

    It seems that the DLPC7540EVM does not have signals with names such as UDIM or SDIM.

    Therefore, I think it is correct to connect the RGB enable signal from the DLPC7540EVM to the cathode of the diode that is connected via a resistor to pin 3 (UDIM) of the TPS92641, and that the RGB PWM signal should be connected to pin 6 (IADJ) via an RC filter, and that pin 8 (SDIM) should be left open as NC.

    Is that correct?

    Best regards,

    Sasaoka

  • I believe that the only control signals provided from the DLPC7540EVM to the LED driver (TPS92641) are 3 lines of RGB PWM signals and 3 lines of RGB enable signals.

    Is this wrong?

    The only signals you need from the 7540 are the LED_EN and LED_PWM signals, assuming you are not using WPC/CCI with an integrating sensor. 

    If you are using WPC/CCI on the 754x with an integrating sensor, then you also need the SUBFRAME and SENSE_PULSE signals.

    It seems that the DLPC7540EVM does not have signals with names such as UDIM or SDIM.

    Therefore, I think it is correct to connect the RGB enable signal from the DLPC7540EVM to the cathode of the diode that is connected via a resistor to pin 3 (UDIM) of the TPS92641, and that the RGB PWM signal should be connected to pin 6 (IADJ) via an RC filter, and that pin 8 (SDIM) should be left open as NC.

    Is that correct?

    UDIM and SDIM are signals on the TPS92641.

    We DO NOT recommend using UDIM; UDIM results in very slow rise/fall times which negatively impacts image quality.  We recommend using SDIM, which uses the external shunt FET, and is much faster.  UDIM should NOT be used for the enable.

    Per the TIDA-080008 design, you can connect the enables (R_EN, G_EN, B_EN) directly to the SDIM pin on the TPS92641.  

    Yes.  LED brightness/dimming is accomplished by connecting the LED_PWM signals form the 7540 to the Iadj on the TPS92641 (though an RC filter); this controls the LED current.

  • Thank you very much for your kind and easy-to-understand answer.

    I have a question next.

    Mr. Gary said; We recommend using SDIM, which uses the external shunt FET, and is much faster. UDIM should NOT be used for the enable.

    1. In this case, can the UDIM terminal be treated as NC (open)?

    2. Is it okay to directly connect the enable signal from the EVM to the SDIM terminal?

    Next,

    The DLPC7540 GUI software has a function called "Display-DynamicBlack-LED Current" that allows you to adjust the RGB LED current.

    Is it correct to use this function to adjust the white balance in general?

    Next,

    Is the location where the maximum current flowing through the LED is determined by the resistance value connected between pin 5 VREF and pin 6 IADJ?

    Please tell me how to determine the maximum current flowing through the LED

    Best regards,

    Sasaoka