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?
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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
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Thank you for your support.
Best Regards
TPS92518_shunt_FET_dimming_.pptx