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TPS92518: shunt FET dimming

KANJI SHIBATANI
Mastermind 6205 points
Part Number: TPS92518

Dear Sir,

Thank you for your support. 

I had discussed with my customer  for the goal of 0.1% dimming with TPS92518.

I'm sorry but I have poor knowledge about shunt FET dimming.

Please allow me basic questions below.

I had attach PPT fle also.

 

Q1  What is shunt FET dimming

Shunt FET dimming means DC/DC converter output current Iout is same, but change the ratio of ILED versus Ishunt.

So, if dimming is  based on FET dimming only, large current flows through shunt FET when ILED was small.

…Is it correct?

 

Q2  How to achieve combination with PWMdimming/shunt FET dimming with lower power consumption

 

In order to have reduce power consumption, I think smaller Iout is necessary.

Can I achieve combination of PWM dimming and shunt FET dimming as following ? ( I attach the PPT file)

 * PWMx frequency  for PWM dimming set 500Hz.

 * If duty  was greater than 1% , PWM dimming is available. shunt FET is turn OFF.

 * If duty was smaller than 1% ,MCU controls shunt FET  ON/OFF (#) , but PWMx duty keep 1%  duty for DC/DC converter.

 * The shunt FET on/off should be synchronized to PWMx dimming control pulse.

 * Shunt FET on/off frequency should be  1MHz i.e. x20 higher than 50KHz ( 1% = 500Hz/100 ) for accuracy.

 

Would you please give me your opinion?

 

Q3 LEDx_MAXOFF

How do I select LEDx_MAXOFF ?  

"255" as default of EVM should be okay?

I don't know the discription  LEDx_MAXX OFF ' Optimization  ' in the data sheet well.

 

Q4  Ringing/Protection in case shunt FET dimming

In case above combination , I think shunt FET turns of/off  10mA at the most.

Even though abouve,  clamping diode is necessary , C21 should be removed and

R17 should be 10K for EVM user's guide p9~p10 discription?  

The following description ( data sheet  p16 8.3.3.1 ) is bsed on shunt FET dimming only?

 

 

=====

8.3.3.1 Output Ringing and TPS92518 Protection

During shunt dimming, ringing may occur at the channel output due to PCB and

device parasitic capacitances and inductances. This should be checked as part of the design process.

If the ringing approaches the absolute maximum of any pin, a clamping diode must be added to the design.

Connect the diode anode to the output at VLEDx and the cathode to the input voltage.

This protection must also be used if the LED load is ever to be connected or removed while the output is enabled

 

======

Thank you for your  support.

Best Regards

 

TPS92518_shunt_FET_dimming_.pptx

 

  • 0
    TI__Mastermind 28655 points
    Hello Kanji,

    Do you want to combine Shunt Fet and Enable dimming or just use shunt FET?

    1) Shunt FET dimming is bypassing the LEDs with a MOSFET. When the MOSFET is on the current through it is the same as as what was flowing in the LEDs. Toffmax needs to be set right for this to be accurate. The Shunt FET is used as a switch so the power loss is low when the MOSFET is shunting. Current stays high so it can return back to the LEDs quickly when the Shunt FET is turned off. This makes it respond much faster without having to ramp the inductor current. The output capacitance needs to be zero or a low value since it will turn into switching loss for the Shunt FET.

    2) This is not necessary. The power loss will be slightly higher because of conduction loss and some switching loss but when shunted the voltage across the MOSFET will be, most likely, less than 100 mV depending on LED current and the MOSFET chosen. You can do a combination of Shunt FET and Enable dimming but it is not necessary. If both are used and synchronized they want to be the same frequency. Running the Shunt FET at 1 MHz will make it lose accuracy. You can just switch to Shunt FET without the Enable dimming.

    3) Maxoff is calculated by using the forward voltage of the freewheel diode and the voltage drop across the MOSFET. Setting to 255 is the safest since the current cannot run away but the calculation should be done as shown in the datasheet.

    4) They probably want this clamp regardless. It protects the TPS92518 when the LED string is opened also. Yes, it shows R17 at 10K and either C17, C18 with 220 pF. 8.3.3 shows the calculations for MAXOFF, the 0.7 is the forward voltage of the freewheel diode for: "I don't know the discription LEDx_MAXX OFF ' Optimization ' in the data sheet well."

    MAXOFF is most important when the Shunt FET is on for a long time. Fine tuning can be done when testing though they have to take into account operating temperature since the Vf of the freewheel diode will get lower as it gets hot.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Mastermind 6205 points

    Dear Irwin-san

    Thank you very much for your quick answer !!!

    I had been mis-understanding...

    So,  following is my understanding now, but  is it correct ? ( please refer to attached PPT)

    I will ask customer to use shunt FET.

    1) PWMx and  shunf FET control signal is same frequency, but inverting.

    2) Frequency of inductor current during shunt FET turn-ON  depends on LEDx_MAXOFF.

    3) If LEDx_MAXOFF were NOT optimized, ripple becomes bigger.

    Best Regards

      

    TPS92518_shunt_FET_dimming2.pptx

  • 0 in reply to KANJI SHIBATANI
    TI__Mastermind 28655 points

    Hello Kanji,

    1)  Yes, high on IC PWM is enabled, high on shunt FET is shorting the output

    2)  Yes, pretty much, but it needs to be selected to try and keep the ripple the same.  This is most important if the Shunt FET is on for a long time.

    3)  If LEDx_Maxoff were NOT optimized a few things can happen.  The average current will be different than when the Shunt FET is off, can be higher or lower, and in the extreme case if Maxoff was too small the inductor current can start to climb due to the minimum on-time of the TPS92518.  This becomes a larger issue at higher switching frequencies.

    You don't need to use the Enable PWM when you are shunt FET dimming.

    Regards,

  • 0 in reply to Irwin Nederbragt
    Mastermind 6205 points
    Dear Irwins-san
    Thank you for your answer. I will investigate more.