TPS923652: How to achieve full power operation of LED by TPS923652

Hi Lumina,

Lumina Gui said:

We drive the LED using TPS92365 and use the same signal in both PWM and ADIM modes. When using a frequency of 20KHz in PWM mode, the current control method is to generate a 10s PWM signal and then maintain a low level for 50ms. When the PWM duty cycle is 95%, the LED cannot work and remains dark after being lit once. However, when the duty cycle is 80%, the LED can continue to flash. 

We want the LED to cycle at full power for 10ms and then stop for 30ms, but found a problem during testing. When the LED running at full power, it is given a high level and stopped at a low level. In fact, it becomes a 25Hz PWM with a duty cycle of 25%. The LED cannot work properly and will turn completely dark after being lit once. Can you give us some better suggestion for this requirement?

Sorry but I do not clearly get your meaning. Please kindly clarify the target results you expect. Do you just want 25Hz, 25% duty PWM dimming?

Please provide the spec of the LED (string) you use and how you connect it to the circuit. Is the input voltage 12V?

Best Regards,

Steven

Dear Steven,

Sorry for not describing our problem clearly. The details is as below:

1) The 5th pin ADIM/HD and 6th pin EN/PWM of TPS923652 are connected to the FPGA , and the signal provided by FPGA is exactly the same, which is a 20kHz PWM signal.

2) We need the LED to turn on for 10ms and then turn off for 40ms (the time may vary) . During the LED normal working (turn on), the FPGA outputs a PWM signal to the ADIM and PWM pins. When not , the FPGA outputs a low level to the two pin.

3) In order to adjust the brightness of the fill light during operation, we need to adjust the PWM duty cycle. When the PWM duty cycle is relatively low, there is no problem with the fill light time and intensity. But when the PWM duty cycle is too high, the fill light will flash briefly and then turn off continuously.

We want to know the cause of this problem. In addition, if we want to maximize the intensity of the fill light within 10ms of working time, how should we control it?  Whether it supports 100% duty cycle?

If we want the duty cycle to work normally from 0% to 100%, what control method is recommended?

Thanks,

Kind Regards

Hi Lumina,

Sorry. I am currently out of office. I will reply to you this Friday when I am back to office. Thanks for your understanding.

Best Regards,

Steven

Hi Lumina,

Sorry for the late response.

Please provide the spec of the LED (string) you use and how you connect it to the circuit. Is the input voltage 12V? What is the package of the IC that you are using now?

Lumina Gui said:

We want to know the cause of this problem. In addition, if we want to maximize the intensity of the fill light within 10ms of working time, how should we control it?  Whether it supports 100% duty cycle?

I am wondering if the problem is caused by thermal shutdown. The device can support 100% PWM dimming duty cycle.

Lumina Gui said:

If we want the duty cycle to work normally from 0% to 100%, what control method is recommended?

PWM dimming mode or flexible dimming mode is recommended.

Best Regards,

Steven

Dear Steven,

The input voltage is 12V, and the output voltage is 36V (12 blue LEDs in series), and the package should be VSON-14. 

Steven Li said:

Is the input voltage 12V? What is the package of the IC that you are using now?

The waveform of the 5% duty cycle(5us) and normal working is as below (Blue is current signal, green is voltage signal, and yellow is PWM signal):

1) Normal working waveform:

2) 5% duty cycle of hybrid dimming waveform:

In the testing we found that when using hybrid dimming, the LED (string) will not light up when the PWM duty cycle is below 5% (5us). But when using the PWM dimming, the LED (string) will light up in 2% duty cycle. 

Can you give us some suggestion for achieving light up LED (string) in hybrid dimming with 2us (2% duty cycle)?

Thanks,

Kind Regards

Hi Lumina,

Sorry. I am currently out of office. I will reply to you next Monday when I am back to office. Thanks for your understanding.

Best Regards,

Steven

Hi Lumina,

Lumina Gui said:

1) Normal working waveform:

What dimming mode do you use when you get the first oscilloscope capture?

Lumina Gui said:

2) 5% duty cycle of hybrid dimming waveform:

In the testing we found that when using hybrid dimming, the LED (string) will not light up when the PWM duty cycle is below 5% (5us). But when using the PWM dimming, the LED (string) will light up in 2% duty cycle. 

When getting the second oscilloscope capture, is there any switching behavior on the SW pin?

Best Regards,

Steven

Hi Steven,

For the frist question, we are using the PWM dimming mode when we get the first oscilloscope capture.

For the second question, there was not load and other parameter setting do not changed when getting the second oscilloscope capture. For whether there is any switching behavior on the SW pin, we need to capture the output voltage waveform on this pin.

By the way, we also have other question about how to set it up to work in stroboscopic mode (instantaneous power), like the camera's fill light. Can you give us some suggestion?

Thanks,

Kind Regards

Hi Lumina,

Lumina Gui said:

For whether there is any switching behavior on the SW pin, we need to capture the output voltage waveform on this pin.

Please check the waveform.

Lumina Gui said:

By the way, we also have other question about how to set it up to work in stroboscopic mode (instantaneous power), like the camera's fill light. Can you give us some suggestion?

One way is to use the flexible dimming mode, where you can use ADIM/HD pin to adjust the amplitude of the output current and EN/PWM pin to strobe the light.

You can also use the PWM dimming mode if you do not need to adjust the amplitude of the output current, where you only use the EN/PWM pin to strobe the light.

Best Regards,

Steven