RA8875 breakout board (1590) backlight does not come on

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craigethomas
 
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RA8875 breakout board (1590) backlight does not come on

Post by craigethomas »

We have purchased a 1590 board (RA8875) along with a 1596 5" TFT display from you in order to develop some prototypes for our client. Unfortunately, we cannot get the backlight to work.

Following are the steps we did to get us this far:

All register initialization per your examples were done and seem to be ok as we can read back the registers to see what we had put into them.

Set register '\x8B' to '\x80'

Set register '\x8A' to '\x90'

Attached picture shows results on pin 4 of FAN5333 PWM signal approximately 50% duty cycle, frequency 1.25 MHz. Could not attach the pdf file showing this) Note: The LITE input pin is not connected to anything

Problem is that I am only seeing 3.3 volts dc on J1 pin 2 that goes to the TFT. Should this not be 24 volts operating at the above PWM frequency? Dumb question: Where does the +24 volts as shown on the schematic of the board come from?

I have scoured the blogs, forums, etc. trying to find the answer, but to no avail. We are getting to crunch time so you early attention would be greatly appreciated.

We did notice that the backlight flickers for a short fraction of a second when power is applied.

Thank you,
Craig Thomas

VP Engineering
CNC International LLC
Loveland, Colorado

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brian49
 
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Re: RA8875 breakout board (1590) backlight does not come on

Post by brian49 »

"Where does the +24 volts as shown on the schematic of the board come from?"

24v is the output of the DC/DC regulator with a 5-pin controller. This circuit is at the upper-left of the schematic.

Brian

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Re: RA8875 breakout board (1590) backlight does not come on

Post by brian49 »

P.S. It's not a constant 24v output, but it's working as a current source, I think, depends on the PWM0 or BACKLITE signals connected to the pin4 of the said 5-pin switching
regulator.

Regards,
Brian

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brian49
 
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Re: RA8875 breakout board (1590) backlight does not come on

Post by brian49 »

The backlight booster is a switching step up regulator using the FAN5333B controller. You change the backlight brightness using PWM0 pin of the RA8875 or using the external Backlite pin. If these pins are tristate disable then it's the same as 100% pwm due to the 10k pull up resistor. Try to output a 50% pwm0 to see what happens.

I think the flash you saw when first turned on power was due to the output (24v) rampping up but for some reasons it got too high (feedback issue?) and the 5333 shut down. Might be because you didn't have any pwm0 nor Blacklite, meaning 100% pwm and having a low forward current type backlight.

Brian

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brian49
 
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Re: RA8875 breakout board (1590) backlight does not come on

Post by brian49 »

Try to probe the pwm signal to confirm its condition. Also the 5333B requires that the pwm brightness control should not exceed 1khz.

Regards,
Brian

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craigethomas
 
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Re: RA8875 breakout board (1590) backlight does not come on

Post by craigethomas »

Hey Brian,

Thank you! Problem was that I had the frequency set too high; I had used a '\x90' in the '\x8A' register, which gave a frequency of 1.25 MHz. I tried '\x88' (20MHz/256) and that gives it a soothing backlight. I knew it had to be something simple.

P.S. I realized shortly after sending this that the FAN5333 is a boost switching regulator, thus yielding the higher voltage. In my haste to get this out, I did not look at the circuit closely enough. Now obvious, sorry about that.

Cheers,
Craig Thomas

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