TPS61169: TPS61169 appears a different drive current at 1% PWM input

Part Number:TPS61169

Hi,

Issue description : My design is  use a TPS61169 to drive a LCD backlight circuit (LED number 1*8,Vf(typ)24.8V@30mA ). I find these is very few IC(10/10000) occurs different drive current when we use 1%  PWM input at Ctrl  Pin (24Khz,3V3 voltage level ) ,the LCD backlight show a lower light brightness,but  the same  Vfb value appears at 100% PWM input. I switch the IC  to another good PCBA,the result is whatever the PCBA we used the failure is always appear at same IC.

measure data&circuit detail :

abnormal  IC         1% PWM input                  Vfb=2mV                      Vout=20.4V       ( Rset=6.8R        L=10uH/1.4A   Diode MBR0530)

                              100% PWM input             Vfb=203mV

noraml IC              1% PWM input                  Vfb=11mV                     Vout=21.1V       (Rset=6.8R         L=10uH/1.4A   Diode MBR0530)

                              100% PWM input             Vfb=209mV

LED backlight parameter :

circuit design :

layout picture :

Questions:

1. It there any problem  in circuit design and pcb layout ? 

2. why the few IC's Vfb value shown like follow the datasheet descripted?but most of IC's Vfb at 1%PWM like NG ( base on the datasheet :Vref=204mV,Iset=204mv/6.8R=30mA,1% PWM ,Iout=0.3mA,Vfb=2.04mA)

3. why the two ICs have a same Vfb value at 100% PWM input but not happen at 1%?

4. In your opinioin, which one is normal IC in this case?  The majority of IC or the few? 

5. How can I lock the root cause in this case ?

THank you very much !

I am looking forward your feedback.

Best regards,

Jun