TPS923652: TPS923652

Lingesh N

Part Number: TPS923652

Tool/software:

Dear Team,

We have used TPS923652 IC in our design for LCD back light control, Rating for LCD backlight are 270mA max current and Forward voltage 9.6V.

As per design followed all the steps, even went through forums for technical doubts, even though couldn't achieve required output.

Required output, To control the 10 to 90% brightness of the TFT display by using PWM signal(Tried PWM dimming and hybrid dimming also). tired both buck-boost and Boost as in our design as we have 24V and also we are steeping down to 5V. so tried 5V supply for boost and 24V for BUCK boost. C95 is 1nF.

Boost circuit works fine, without any issue, but problem with 5V regulator gets heated, so tried buck-boost.

Problem faced:- Output current is not constant when using in CC mode and display flickers.

Please find the attached schematic and changes are made for buck-boost by changing feedback resistor to 680K and 91K(44V(9.6V led forward voltage and 28V max input voltage).

Thanks for the help

10 months ago

0 Patrick Yao 10 months ago

TI__Mastermind 34840 points

Hi Lingesh,

Our expert will give you reply later, thanks for the patience.

0 Lingesh N 10 months ago in reply to Patrick Yao

Prodigy 20 points

Hi Patrick Yao thanks for the replay, waiting for solution.

0 Steven Li 10 months ago

TI__Mastermind 23200 points

Hi Lingesh,

So you are currently having some trouble to let the driver work correctly in Buck-boost connection with Vin = 24V, V_LED = 9.6V, Iout (max) = 0.4A. Am I correct?

Lingesh N said:
Please find the attached schematic and changes are made for buck-boost by changing feedback resistor to 680K and 91K(44V(9.6V led forward voltage and 28V max input voltage).

What do 680K and 91K resistors refer to?

Best Regards,

Steven

0 Lingesh N 10 months ago in reply to Steven Li

Prodigy 20 points

Hi Steven,

Yes your are correct , as per the datasheet circuit tried(Attached), which display was not turning ON.(Connecting Cathode to Vin)

Then tried connecting cathode of LED to ground, display is turning ON but not able to control the dimming effect and current.

0 Steven Li 10 months ago in reply to Lingesh N

TI__Mastermind 23200 points

Hi Lingesh,

Lingesh N said:
Then tried connecting cathode of LED to ground, display is turning ON but not able to control the dimming effect and current.

You cannot connect 24V supply and 9.6V LED in boost connection this there is always a current path from Vin, inductor, Schottky diode, sense resistor, LED and ground.

Lingesh N said:
Yes your are correct , as per the datasheet circuit tried(Attached), which display was not turning ON.(Connecting Cathode to Vin)

What do you mean by saying "which display was not turning ON"?

Please also kindly explain the 680K and 91K resistors. What do they refer to?

Best Regards,

Steven

0 Lingesh N 10 months ago in reply to Steven Li

Prodigy 20 points

This buck boost we are trying is for TFT Display(WF70A2SIAGDNN0) backlight and 680K(R189) and 91K(R192) are OVP resistors.

0 Steven Li 10 months ago in reply to Lingesh N

TI__Mastermind 23200 points

Hi Lingesh,

You design the OVP resistor divider wrongly. Please refer to this E2E FAQ - [FAQ] How to design the OVP resistor divider for TPS923655 / TPS923654 / TPS923653 / TPS923652? - Power management forum - Power management - TI E2E support forums (A screenshot is shown below.)

Do pay attention to the contents regarding "buck-boost connection".

Best Regards,

Steven

0 Lingesh N 10 months ago in reply to Steven Li

Prodigy 20 points

Hi Steven,

I have considered max input voltage 28V and LED forward voltage 9.6V, so total voltage is 37.6 , considering this changed resistor Rovp1 680K and Rovp2 20K, I am able to achieve required output.

While testing I found, Led current drastically increase at exactly 9.6 to 9.7V input voltage(Vin), is there any particular reason, as this circuit is Buck-boost right.?, so should work as boost also.

Any solution for this.

I have connected LED as per this attached image.

Thanks.

0 Steven Li 10 months ago in reply to Lingesh N

TI__Mastermind 23200 points

Hi Lingesh,

Lingesh N said:
While testing I found, Led current drastically increase at exactly 9.6 to 9.7V input voltage(Vin), is there any particular reason, as this circuit is Buck-boost right.?, so should work as boost also.

Please kindly specify the details of the situation. How do you increase the input voltage from 9.6V to 9.7V?

Best Regard,

Steven

0 Lingesh N 10 months ago in reply to Steven Li

Prodigy 20 points

Hi Steven,

Started decreasing from 24V by 0.1V step, till 9.8V device works fine, below 9.7V device doesn't provide required current. At 9.7 to 9.6V, current drawn by device two much and TFT LCD brightness will be max.

Is this because 9.6V is the forward voltage of the display.

0 Steven Li 10 months ago in reply to Lingesh N

TI__Mastermind 23200 points

Hi Lingesh,

Can you measure the output current when Vin is below 9.7V? Does the device have any switching behavior at this time?

Best Regards,

Steven

0 Lingesh N 10 months ago in reply to Steven Li

Prodigy 20 points

Hi steven,

Switching behaviour at 9.7V is scattered(Means not correct, it changes) and current reaches 400mA which is display will not take(max current for display is 270mA, So R sense is 0.73 Ohm(3 parallel resistor of 2.2Ohm)).

Below 9.6V, the Switching behaviour still scattered and current comes down to 131mA.

0 Patrick Yao 10 months ago in reply to Lingesh N

TI__Mastermind 34840 points

Hi Lingesh,

We will give you update later

0 Lingesh N 10 months ago in reply to Patrick Yao

Prodigy 20 points

Hi Patrick,

Sure will be waiting for your reply.

0 Steven Li 10 months ago in reply to Lingesh N

TI__Mastermind 23200 points

Hi Lingesh,

What signals do you input to EN/PWM pin and ADIM/HD pin when you are doing the test? Which components are modified on the board (different from the schematics shown above)?

Best Regards,

Steven

0 Lingesh N 10 months ago in reply to Steven Li

Prodigy 20 points

HI Steven,

20Khz(with duty cycle 10% to 90%) square wave to PWM signal, ADIM/DP pin kept High(Analog dimming).

Please find the latest updated schematic.

0 Steven Li 10 months ago in reply to Lingesh N

TI__Mastermind 23200 points

Hi Lingesh,

I find you miss the C_SENSE capacitor. Please refer to Section 9.2.2.2.5 Other External Components Selection from the datasheet.

Please add this capacitor and see if it helps.

Best Regards,

Steven