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LM 3914

ebee
Hello,

anyone into electronics to help me (I have some suspects).


I am having trouble with LM 3914/5/6

I get the current program of LEDs by making path to ground 0V from Pin 7.

Pins 7 & 8 give use approx 1.25 volt ref

It`s the pin 6 & pin 4 Rhi and Rlo bits in order to get values of other than the supply lines.

i.e if we put in two resistors R1 & R2 from pin 7 to 0V and calculate the voltage at the junction then we could use this as a hi or low I think but what about the other (to get a hi and a lo) .


Then methinks factor in the internal comparitor divider chain into these figures to see if a significant variation occours .


I initially want to prog my LEDs to say 10 or maybe 15mA current and the hi and low volt comparisons say 1.8v to 4.0v.

Thanks
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    Ok if you want 10mA though the LEDS then you need 1.25k between the Ref out and the Ref in (1.25V accross 1.2k = 1mA, but internal current mirrors put 10 times this down each LED)


    Now to set the top end to 4V, you need the total chain to be 4k, (1mA remember) but to set low to 1.8V that needs to be 1.8k to ground, then 1k up to the ref in and 1.25k to the ref out.


    Internally the chip is about 10k  (8 to 17k according to that data sheet )between the low and high ends of the internal resistor ladder, so the error is ~ 10%.

    If you need better than please  note that the tolerance on the on chip resistors, and on the voltage of the reference (1.2-1.34 V) is not that great, so you may need trimmer resistors to fine tune for each chip if the application is that level  critical.
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  • 0
    Ok if you want 10mA though the LEDS then you need 1.25k between the Ref out and the Ref in (1.25V accross 1.2k = 1mA, but internal current mirrors put 10 times this down each LED)


    Now to set the top end to 4V, you need the total chain to be 4k, (1mA remember) but to set low to 1.8V that needs to be 1.8k to ground, then 1k up to the ref in and 1.25k to the ref out.


    Internally the chip is about 10k  (8 to 17k according to that data sheet )between the low and high ends of the internal resistor ladder, so the error is ~ 10%.

    If you need better than please  note that the tolerance on the on chip resistors, and on the voltage of the reference (1.2-1.34 V) is not that great, so you may need trimmer resistors to fine tune for each chip if the application is that level  critical.
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