Triggering timeout of backlight led?!

Thread Starter

jzeds1491

Joined Feb 5, 2020
16
Hi guys, recently I discussed a problem here in this forum and the problem is almost solved (link:Backlight issue)(short:ic that controls mp4player backlight doesn't turn the screen off), only thing remain is that backlight is in a kind of always-on state, it dims and adjust the LED backlight perfectly but doesn't turn it off so battery drains pretty fast .
-How I can use a transistor to sense timeout (which drops voltage on pin#4 from 3.5v to 1.6v) and totally shut the ic (when 3.5v backlight should ON when 1.6v is OFF)
Lm2703 (approximate schematic) :
IMG_20200217_021708.png
 

jpanhalt

Joined Jan 18, 2008
9,783
Here's how I control the BL when the additional 40 to 60 mA is an issue:

1581894886096.png

Of course, that is a logic level mosfet. The control can be on/off or by PWM. Oops, sorry for the huge size, I didn't realize it until posting. "LCB_BL" is from the MCU. I need to change the !LCD_BL to avoid confusion. That is the cathode connection to the LDC_BL.
 
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Thread Starter

jzeds1491

Joined Feb 5, 2020
16
Here's how I control the BL when the additional 40 to 60 mA is an issue:

View attachment 199254

Of course, that is a logic level mosfet. The control can be on/off or by PWM. Oops, sorry for the huge size, I didn't realize it until posting. "LCB_BL" is from the MCU. I need to change the !LCD_BL to avoid confusion. That is the cathode connection to the LDC_BL.
Thanks, and what type of mosfet do you recommend?(i never worked with them and have no idea)
Also resistor values?
 

jpanhalt

Joined Jan 18, 2008
9,783
That is an N-channel mosfet. You can also use an NPN transistor in the same configuration with some changes. The SparkFun 4x20 display uses a PNP transistor like this:
1582028159670.png

Notice that the PNP transistor sources current rather than sinks it.

It is really quite a common circuit. In my current design, I am using a CPH3456 (ON Semiconductor). I picked a mosfet as the control gate requires less less current over time compared to the base of a transistor. My current circuit is intended simply to turn on or off (mostly off), and while capable of being PMW'd to provide dimming, it is not that way right now. It is connected to a PWM-capable pin of the MCU. There are hundreds, if not thousands of comparable devices that will work.

As for resistor values:
R2 is any low value, e.g., 10 Ω. It can probably be eliminated.
R3 is to discharge the gate when it is turned off. Any higher value will work. I am using 10 k.
R4 needs to be calculated as one would do for any other current-limiting resistor for an LED. It depends on the Vf of the LED's, supply voltage, and intended current. In my design, it will be about 50 to 60 Ω.
 
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