TPS92691: TPS92691QPWPRQ1 Soft Start Issue

Tsvetomir Asenov

Part Number: TPS92691

Hello,
I am using the TPS92691QPWPRQ1 driver to control the backlight of a panel.
As there are some timing that are required in order to power up the panel i need to introduce some delay in the backlight voltage.

The way I try to do it, is to put a large capacitor on the soft start time, but it seems that at it is not properly used in my application and I am trying to find if it is normal behavior or not.

Please see the green signal from the picture below. The internal circuitry of the panel is pulling down all the signals for a short time and them bring them HIGH with a defined power up sequence.

The green signals should start about 2.5 or 3S from the yellow line (when it is high);

What I can see is that the green signals is kept at mid level and after 3s it goes high.
Is there a way to keep the signal low till 3S after the yellow signals for example?

over 1 year ago

0 Tuan Tran74 over 1 year ago

TI__Mastermind 18890 points

Hello,

I have a hard time understanding what you are trying to do. Please attach a schematic and describe with signals on the schematic what you are doing. The scope capture does not tell me what signal is what.

Thanks tuan

0 Tsvetomir Asenov over 1 year ago in reply to Tuan Tran74

Prodigy 170 points

Hello,
From the scope only the green signal is important. This is the LED drive output. (about 54V).
The idea is to provide ENABLE function to the driver. As I want to introduce delay in respect to the yellow signal.

In other words, then the yellow signal, reaches its maximum value (10V), I want to enable the LED Driver then, and to introduce delay about 3s, after the yellow signals is high!

BR,

Tsvetomir Asenov

0 Tuan Tran74 over 1 year ago in reply to Tsvetomir Asenov

TI__Mastermind 18890 points

Hello,

With 3 second delay is a long time for using soft-start capacitance since it will not be predictable. You will need to pull down Soft-Start with an open collector/drain transistor for 3 second after the input voltage goes high and this has to be done with external circuits.

Thanks Tuan

0 Tsvetomir Asenov over 1 year ago in reply to Tuan Tran74

Prodigy 170 points

Hello,
I have issue with the output voltage. The backlight of the panel requires 36V, where the input is 24V, and the output current is about 360mA.

When power the boost up, I see maximum output voltage about 26V. which is not enough to get the desired brightness.
I am using PWM with 20KHZ frequency, but I have tried also with 240Hz and the result is the same.

Would you check the schematic again, and see if there are any issues related with the schematic solution?