DLPC7540EVM: About LED PWM frequency for LED driver control signal

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 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

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

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

笹岡 孝至 said:

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.

笹岡 孝至 said:

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.

笹岡 孝至 said:

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

In our example design, we have a jumper for using SDIM or UDIM; in that case, UDIM is left connected and I believe sets a UVLO threshold.  I do not believe the datasheet indicates an option for UDIM to be disconnected.  I suspect you should leave the resistors connected to the UDIM pin to set the UVLO and Vhys:

I believe that sets Vturn-on to ~10.4V; If your power supply is less than ~12V, I suspect you may may need to adjust those resistors.

笹岡 孝至 said:

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

Yes, that is what we have done in the TIDA-080008 reference design.

笹岡 孝至 said:

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?

This should be a real-time adjustment for the currents.  I do not know if it is aware of our LED driver design; so I suspect it does not have valid current values for this application.  I am not certain of what the implementation is behind this.  If you need support on the GUI, please open a new thread on that specific topic so it can be addressed by the correct people.

笹岡 孝至 said:

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,

There is a current sense pin with resistors selected to meet specific requirements.  You will need to go through the TPS92641 Datasheet and the TIDA-080008 reference design Design Guide to adjust all of the components for each driver channel to match your LED requirements.  The datasheet and reference Design Guide have details on how to do all the calculations required to set the design up for your specific LEDs. 

The TIDA-080008 reference design is setup for specific LEDs that had these requirements:

If you need support determining determining the component values needed for your LEDs, please start a separate E2E thread on that topic (referencing  TIDA-080008 and TPS92641).

Please keep in mind that the LED Enable/Disable delays and Rise/Fall times are critical to the DLP Display applications; using UDIM as an enable is far to slow -- SDIM MUST be used as an enable.  PWM dimming of the LEDs (i.e. PWM Modulation of the LED current to dim the LEDs) is also not a viable option for our display applications; dimming needs to be analog and needs to be handled via the I-adj function of the TPS92641.  

The original topic was "About LED PWM frequency for LED driver control signal"; I think we have addressed that here.  If you have additional questions, please start a new thread so the topic(s) can be addressed by the appropriate people.

Regards,
Gary

Hello Mr. Gary

Thank you for all your replies.
We are building a new system using DLPC7540EVM and TPS92641.

I can't go into details, but we need to adjust the brightness of the LEDs with each RGB driver IC (TPS92641).

The configuration does not allow brightness adjustment with the RGB PWM signal controlled by the GUI.

We will start the thread again, taking into consideration matching with our system.

Thank you.

Best regards,

Sasaoka