The analysis process of the minimum brightness of different mobile phones in MT6753 platform project

Phenomenon:

MT6753 platform a project different phones will be the backlight height to the lowest, the minimum brightness error problem, some light, some dark. The phenomenon is more obvious.

Analysis Process:

First day:

and TCL screen days together to verify, have the following conclusions:

1.TCL and Yassi module, under the same motherboard, its minimum level=5 brightness is almost the same, the test data are as follows:

	Minimum brightness	Maximum brightness
Tcl	0.354	435
Yassi	0.35	401

2. The lowest brightness shows the case of light, follow the motherboard. The illuminated motherboard has a backlight-driven feedback end that is 50% larger than the other motherboard, causing the current to be added to the backlight to be 50% larger, causing the phone to show off. The test data is as follows:

	Feedback Terminal voltage	Sample 1 Brightness	Sample 2 Brightness
Light-biased motherboard	0.0047V	5.1	5.4
Normal motherboard	0.0031V	2.97	3

In summary, the lowest brightness of the situation following the motherboard, the motherboard output backlight current anomalies, also please continue to analyze the motherboard. Thank you ~

Day two:

Verify with the hardware colleague that the backlight IC is:SGM3720,

View MT6753 platform -dependent PWM frequency configuration: 5 KHz, Minimum brightness level setting: 5/255 =1.95%

The analysis given by the backlight IC engineer after confirming the information:

As we test and communicate today, our SGM3720 en foot PWM duty ratio supports a minimum of 2.5%, which is determined by the Vos (offset voltage) of the error amplifier inside the chip. In the case of op-amp, Vos can be described as high-precision op amp within 1mV, and general op-amp is around 5-10mv. So if below this minimum PWM duty ratio, the accuracy of the feedback voltage VfB can not be guaranteed, the screen brightness will appear very dark or even black screen phenomenon. It is recommended to use the SGM3720 when the minimum duty ratio is set at 3% and above, to ensure that the screen is not black and can see the font.

If you need a chip to support the lower brightness of the screen, can choose our large dynamic backlight driver SGM3745, offset voltage vos accuracy can be achieved 60uV, the minimum duty-free ratio can support 0.2%. Last year, several major customers have appeared similar to the problem of black screen, leading to the current ZTE,TCL, Gionee, cool and so on the mobile phone LCD flow brightness has a hard standard, the basic requirements of the darkest within 20lm. Our large dynamic backlight driver in these several clients have a large number of shipments.

If there is a demand we can provide 3% PWM duty ratio, SGM3720 the VfB voltage distribution, production line testing may take a certain amount of time. Thank!

Summarize:

The current project uses backlight IC is SGM3720, the latest version set the minimum brightness value is now: Level = 5, converted to duty ratio: 1.95%, exceeding the SGM3720 IC support minimum duty 2.5%.

A duty ratio of less than 2.5%, the accuracy of the VFB is not guaranteed, resulting in a consistent screen brightness, and even a full black screen risk.

and backlight FAE lighting over there to communicate, his side suggested that the use of SGM3720 when the minimum duty ratio is set at 3% and above, that is, the level value can not be less than 8. Because the level value is currently a top-level control modification, you need to know to apply a colleague to modify it. Thank you!

The analysis process of the minimum brightness of different mobile phones in MT6753 platform project