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TPS61040: TPS61040 - definition of the soft start feature

Part Number: TPS61040

Hi there,

I want to use the TPS61040 as a backlight driver (schematic as datasheet figure 21). During start-up there is a delay of 250us between setting the EN-pin to high and the switching of the power-MOSFET (SW-pin).

How are the 250us defined? What is the maximum delay I can expect the SW-pin to start switching?

As a backlight frequency I use 200Hz. This means that I always loose 5% of the brightness and has unwanted influence on the linearity of the brightness. Is there a way to shorten this start-up delay and also shorten the soft start?

Thanks for all feedback in advance.

Kind regards,


  • Hi David,

    This is a normal behavior of delay. The figure 15 shows the start up behavior of EVM. The delay time is also 250uS, same with yours.

    What do you mean the backlight frequency 200Hz. Do you disable and enable the TPS61040 frequently? What's the duty cycle of Enable time?

  • Hi Zack Liu

    Thanks for the feedback. I also saw figure 15 (typical values). But there are no maximum limits for the start-up delay.

    I switch the EN-pin with f=200Hz. DC+=10%...100% in 10%-steps. This means that with 10% backlight I actually have only 5% brightness. 20%->15%, 30% -> 25%...

    When comparing the different backlight steps this non-linearity described abouve is quite obvious. In worst case (if the delay is longer than 500us) the backlight is even switched off at 10% backlight.

    That's why I'm so keen to get to know the maximum delay time.

    Kind regards,

    David Eschler

  • Hi David,

    I also would like to find the maximum delay time for you but sadly didn't get it. This device is pretty old, RTM in 2002. 

    This delay time is pretty stable from my experience. You can increase the duty cycle 5% higher to 15%, 25%..