TPS61193: TPS61193: Startup flickering of LCD backlight

Kim Slot

Part Number: TPS61193

I am using the TPS61193 to drive the backlight of a LCD display. The backlight is flickering during startup as can be seen on the screenshots of output voltage of the TPS61193 to the LEDs (see image)

Sometimes it takes several seconds to stabilize the LED voltage sometimes it just works without flickering.

I run with 5V input, 800khz switching, 35mA current limit. The LCD backlight diode string contains a zener diode (rating not specified in datasheet), could this cause some trouble with the TPS61193 ?

Is it normal behaviour that it takes several seconds for the output voltage to ramp up ?

over 5 years ago

0 Sean Zhou over 5 years ago

TI__Mastermind 22745 points

Hi Kim,
Could you upload your schematic and LED configuration? I need to check the configuration.
Regards
Sean

0 Kim Slot over 5 years ago in reply to Sean Zhou

Prodigy 50 points

Hi Sean, thanks for the reply. See the schematic below.

I have monitored the fault pin with 10k pull up and there is no activity, the SYNC input is held low, the PWM frequency is 100Hz.

I have done some tests and if i start with a very low pwm duty cycle (0.5%) then the backlight is turned on without blinking and i can slowly (over seconds) increase the pwm duty cycle. I will try to see if i can get a faster response but i think the behavior is very weird.

The diode configuration is ( i have tried three different LED configurations (3, 4 and 6 diodes (with increased R3 ):

0 Kim Slot over 5 years ago in reply to Kim Slot

Prodigy 50 points

Hi Sean, i connected a 1uF to the LDO pin (as suggested in the datasheet !) and it solved all the problems. Missed the part that this was for a internal LDO.

Thanks

0 Sean Zhou over 5 years ago in reply to Kim Slot

TI__Mastermind 22745 points

Hi Kim,
That's cool. If you have any question, please let me know.
Regards
Sean

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TPS61193: TPS61193: Startup flickering of LCD backlight