SSD1963 LCD controller PWM output failure

Hello,

I designed a motherboard based on a STM32F205 MCU from ST which
communicates with a 5 inches TFT LCD through a SSD1963 LCD controller from
Solomon Systech.

This LCD controller has a PWM output pin that can be used to send a PWM
signal to a backlight driver. In my design, I used this pin for driving the
control pin of a TPS61165 high brightness white LED driver from Texas
Instruments.

The problem is: the LCD controller generates a pseudo-PWM with a frequency
of ~27KHz and an amplitude of ~1V instead of a PWM signal with a frequency
of ~10KHz and an amplitude of 3.3V. I tried to configure the LCD controller
PWM with other values but the output PWM remains the same.

The communication between the MCU and the LCD controller is OK and the
values written in the registers that configures the LCD controller PWM
output are OK too.

Do you have any idea of what is happening?

Any help would be appreciated.

Regards,
Johann	   
					
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On 2013-01-17, johsey <28204@embeddedrelated> wrote:
> Hello,
>
> I designed a motherboard based on a STM32F205 MCU from ST which
> communicates with a 5 inches TFT LCD through a SSD1963 LCD controller from
> Solomon Systech.
>
> This LCD controller has a PWM output pin that can be used to send a PWM
> signal to a backlight driver. In my design, I used this pin for driving the
> control pin of a TPS61165 high brightness white LED driver from Texas
> Instruments.
>
> The problem is: the LCD controller generates a pseudo-PWM with a frequency
> of ~27KHz and an amplitude of ~1V instead of a PWM signal with a frequency
> of ~10KHz and an amplitude of 3.3V. I tried to configure the LCD controller
> PWM with other values but the output PWM remains the same.

Can you cut the trace near SSD1963 and see if it's SSD1963 that's generating
the 1 V pulse or something else (a short-circuit on the board, wrong pin of
the TPS61165 or something lik that)?

  -jm

>Can you cut the trace near SSD1963 and see if it's SSD1963 that's
generating
>the 1 V pulse or something else (a short-circuit on the board, wrong pin
of
>the TPS61165 or something lik that)?
>
>  -jm
>

It seems that the LCD controller was not very well soldered. Another
problem occurred on this board while I was trying to isolate some LCD
controller pins.

I tried another board and the PWM signal behaves as expected. So, as you
said, it was certainly a short-circuit on the board caused by the strange
LCD controller soldering. The connection to the backlight driver was OK.

Thank you for your help.

Regards,
Johann	   
					
---------------------------------------		
Posted through http://www.EmbeddedRelated.com