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LCD Back light - regulator IC available to drive LED's

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jayamohan

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tps76950

Hi all,

Refer the attcahment. The backlight LED's need 5V,100ma constant voltage and current.

Can anyone tell me any regulater IC available to drive these LED's with constant current and voltage. The regulator IC with enable and disable option is preferable.

Can anyone help me out please.
 

banjo

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LCD Back light

I believe you are mis-interpreting the datasheet. The LED backlight requires a 5V power source. This will typically consume approximately 100mA. However, it does not require a power source that regulates BOTH voltage and current. Such a supply is not possible since you do not have two degrees of freedom. You can either regulate for constant voltage or regulate for constant current. When you regulate for constant current, the output voltage varies to keep the current constant. When you regulate for constant voltage, the output current varies to keep the voltage constant.

Many regulator IC will operate your LED backlight. Since the CMOS logic of the LCD panel also requires 5V at typically around 25mA, I would suggest the LM7805 linear regulator. Depending on the package, it can provide currents up to 1A. Pick a package option to allows at least 250mA of current to allow for adequate safety margin.

Note: Since the LED backlight is the majority power requirement on 5V, it is given a separate pin and specified separately in the datasheet to allow the user to place a switch in the LED circuit to conserve power when the backlight is not needed.
 

    jayamohan

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jayamohan

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Re: LCD Back light

Can i use TPS76950 ULTRALOW-POWER 100-mA LOW-DROPOUT LINEAR REGULATORS for powering on the LCD back light??

This regulator have the enable pin seperately. so that i can switch off the back light when ever it's not required.

Kindly refer the attached datasheet for the same please.
 

banjo

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LCD Back light

This part is marginal for your application. The TPS76950 has a maximum output current of 100mA. The typical requirement for the LED backlight is 100mA. Therefore, you are running the regulator right at its limit. A better choice would be a regulator with a maximum output current of at least 125mA which gives a 25% safety margin.

I would suggest something like the LP2985 from National. It can provide up to 150mA and has the on/off control you want.

**broken link removed**
 

    jayamohan

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jayamohan

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Re: LCD Back light

Thanks for your valuable feedback.

I have few more doubts.

The LCD's are having the LED's for backlight.

Can the LED's get short? if the LED's get short means then these regulators will fail na.

is there anyway to over come of this plz...
 

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LCD Back light

The regulator I mentioned, LP2985, has internal over-current and over-temperature protection. Many other regulators offer similar features. The regulator will shut itself off if it gets too hot or the current goes too high. This should protect you from a shorted LED in the LCD.
The other thing you can do is place a fuse in series with the LED +5V power supply line. The fuse would blow should the LED become shorted. Probably around a 150mA fuse would be sufficient. However, this adds cost to your design to protect against a pretty rare event. LEDs often want to fail open rather than shorted. I would personally rely on the protections within the regulator.
 

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Re: LCD Back light

Thanks for your valuable support
 

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