AM335X - LCD Backlight driver

Vinay Patil2

Other Parts Discussed in Thread: TPS61081

I have one query in AM335X Starter kit .

As attached ,there is Backlight Driver TPS61081DRC circuit on page 9 of schematic .The i/p to this circuit is LCD_BACKLIGHTEN . Is this a PWM or DC signal ?

Can I vary the output current provided by this IC with help of PWM signal?

My goal is to control the brightness of LCD using PWM signal from controller.

over 12 years ago

0 Biser Gatchev-XID over 12 years ago

TI__Guru**** 393215 points

It's a PWM signal, like shown on the TPS61081 datasheet, Figure 24 (http://www.ti.com/lit/ds/symlink/tps61081.pdf). It's driven by the processor signal eCAP2_in_PWM2_out (pinmux Mode 4) on pin C12.

Best Regards

Biser

0 AFM over 12 years ago in reply to Biser Gatchev-XID

Intellectual 560 points

hello,

just i want to knwo how the 32mA and the 20V were calculated according to this figure ?

thanks

0 Biser Gatchev-XID over 12 years ago in reply to AFM

TI__Guru**** 393215 points

Check the Application Information section and Figure 24 in the TPS61081 datasheet.

Best Regards

Biser

0 AFM over 12 years ago in reply to Biser Gatchev-XID

Intellectual 560 points

hi Biser,

i checked the datasheet and the application note "TPS6108x: A boost converter with extreme versatility" and i found that the Iled=Iss*R1/Rset but when i see the figure24 i can't find the result with this equation. did i miss somethings ?!

thanks again

0 Biser Gatchev-XID over 12 years ago in reply to AFM

TI__Guru**** 393215 points

Are you taking into consideration that the AM335X drives this backlight driver with PWM?

Best Regards

Biser

0 AFM over 12 years ago in reply to Biser Gatchev-XID

Intellectual 560 points

i m working with the PWM same as the backlight driver on the AM335x starter kit but with different LCD. so i have to customize the new values of resistors but i can't find how to calculate ?

regards

0 Biser Gatchev-XID over 12 years ago in reply to AFM

TI__Guru**** 393215 points

I don't have the EVM LCD datasheet (it can't be found on the web), so I don't know what the backlight LED matrix is, and all calculations should be based on that.

Best Regards

Biser

0 AFM over 12 years ago in reply to Biser Gatchev-XID

Intellectual 560 points

http://www.newhavendisplay.com/specs/NHD-4.3-480272MF-ATXI-CTP.pdf this is the datasheet. you can find the 32ma and 20V as described on the tps61081.

in fact, how the tps61081 calculate the Iled current ? even on figure 24 on the datasheet is not clear the relation between resistors, Iss and Vfb ?

best regards

0 ddomke over 12 years ago in reply to AFM

TI__Intellectual 2150 points

Please file a post in the Power Management form asking this question. There is a simple formula for deriving the resistor values to generate a certain output current value when configuring the TPS61081 in constant current mode. They can provide a complete answer.

0 ddomke over 12 years ago in reply to ddomke

TI__Intellectual 2150 points

Short answer: The current needed to drive this LCD backlight is 180mA. On the TPS61081, Iss = 5uA and the Iout = Iss*R77/R38 for the LCD Board.

As I wrote earlier, I recommend posting to the power management forum for full details.

0 AFM over 12 years ago in reply to ddomke

Intellectual 560 points

i thought is related to the AM335x backlight driver maybe as you said i should posting on the power management forum.

just i want to know what is the circuit or the board when you talk about the 180mA, the R77 and the R38 ? do you have a link ?

thanks for your feedback.

0 DavidVescovi over 12 years ago in reply to AFM

Genius 5470 points

Correct me if I am wrong but it looks to me that you can not control the backlight level via a PWM.

It looks to me that this is just an ON or OFF from the circuit described on the starter kit. The EN pin is tied to the supply and the SS "softstart" pin is controled via the processor pin and the FET.

0 ddomke over 12 years ago in reply to DavidVescovi

TI__Intellectual 2150 points

The SS pin on the TPS61081 is used with the PWM signal from the processor or it can be an on/off signal. See the TPS61081 documentation for details.

0 DavidVescovi over 12 years ago in reply to ddomke

Genius 5470 points

I did check the data sheet and I see nothing about modulating the SS pin for backlight level control via PWM.

I understand you can shut down the regulator by grounding the SS pin, which is what the FET does (instead of using the Enable pin???).

But driving this from a PWM seems to be an unconventual use of the pin.

Please show me where it states this is acceptable.

0 Yashpal Patel49892 over 11 years ago in reply to DavidVescovi

Expert 1835 points

Hi Biser,

As you said "

It's a PWM signal, like shown on the TPS61081 datasheet, Figure 24 (http://www.ti.com/lit/ds/symlink/tps61081.pdf). It's driven by the processor signal eCAP2_in_PWM2_out (pinmux Mode 4) on pin C12. "

how could I drive that pin making change in kernel coding or how could I initialize the PWM functionality in my EVM.c file for Auto off Back-light.

Regards

Praveen

0 Biser Gatchev-XID over 11 years ago in reply to Yashpal Patel49892

TI__Guru**** 393215 points

Hi Praveen,

I'm not a software expert so I can't say.

0 Yashpal Patel49892 over 11 years ago in reply to Biser Gatchev-XID

Expert 1835 points

Hi Biser,

I will keep in Mind for hardware point of view by the way

Thanks for your Post it is always helpful.

Regard;

Praveen

0 Eliran Rols over 7 years ago

Prodigy 10 points

Hello,
According to the above circuit,
Which are the required values of C75 and R63 for Vout = 15V (where the input voltage is 3.8V)?

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

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AM335X - LCD Backlight driver