Re: Digital Current Control for LED array

On Fri, 23 Dec 2005 17:50:50 GMT, "John_H" <johnhandwork@xxxxxxxx> wrote:

>"theJackal" <f_a_s_t_g_s_f_r_i_d_e_r@xxxxxxxxxxx> wrote in message
>news:mp5oq1hshslhuq90p7qpvh93s623k0gqd1@xxxxxxxxxx
>> On Fri, 23 Dec 2005 14:34:32 GMT, John_H <johnhandwork@xxxxxxxx> wrote:
>
>>>I am suggesting 7 regulators for 7 LEDs. Of *course* connecting 7 LEDs
>>>in parallel directly with one series regulator will not provide a
>>>solution. Of *course* connecting 7 LEDs in series with one shunt
>>>regulator will not work. If one "LED stage" is defines as an LED and a
>>>regulator, 7 "LED stages" can be connected in parallel if the "LED stage
>>>is built with a series regulator and 7 "LED stages" can be connected in
>>>series if the "LED stage is built with a shunt regulator.
>>>
>>>I don't contradict a thing. EACH LED is fed with a controllable
>>>current. Separately. Not "all LEDs are fed with A controllable current."
>>
>>
>> If I get you right ... you are connecting a series resistor/regulator in
>> each parallel
>> branch of the LED array.
>> Suppose you want to brighten a LED you'd decrease the resistance in that
>> branch , but
>> that would increase the current flowing through it , which would then
>> cause a decrease in
>> the currents flowing to the other LEDs ... so those would become dimmer .
>> You'd then have
>> to work on the resistors in the other LED branches. Besides you'd have
>> to take into
>> account the limiting current and the non linear voltage current
>> characteristic of the LED.
><snip>
>
>YES! You've got it now. The only major concept that you aren't seeing is
>that a constant-current series regulator that uses feedback from the current
>running through the LED *won't* be affected by the other LEDs current
>changes as long as the supply is properly regulated. Constant-current loads
>need a supply with a voltage that can meet the LED voltage drop plus
>regulator "dropout voltage" for all the LEDs, supplying enough current to
>accommodate all 7 LEDs at full current.
>
>If you stop thinking "resistors" and start thinking "regulators" everything
>can come together.
>
>The need to ramp the intensity up and down with a single control - one
>dimmer for 7 matched intensities - means that the regulators must track each
>other as the brightness is increased and decreased. If the 7 regulators run
>from 75% to 100% rated current to match brightnesses for a particular batch
>of LEDs, half-power would servo those regulators to 37.5% to 50% of rated
>current proportionally.
>
>These high-current LEDs need a transistor (I'll use n-channel MOSFETS),
>current sense (small value resistor for my implementation), an amplifier
>with feedback to maintain a fixed current through the sense resistor through
>the gate drive voltage, and a method to calibrate the individual LEDs and
>present a "master" brightness. I may use potentiometers with tap to the
>opamp and rails of ground and "master brigtness voltage" or use an 8-channel
>multiplying DAC: change the reference voltage and the outputs scale.
>
>The main supply just needs to supply a voltage that will have all the
>regulators properly regulating (or all the op-amps out of saturation).
>
My original idea was to use only passive components.

What current regulator are you using in your setup? If you use potentiometers isn't the
setup getting too complicated? If you already have a current regulator why use a MOSFET?
I would use MOSFETs to switch to different resistances but that would limit and
complicate my brightness control.


If you are really set on connecting the LEDs in parallel I'd use the MAX1916 . I'm sure
its a cheaper option. It has a current match of 0.3%. and you can vary the brightness by
changing the duty cycle of PWM signal on the enable pin.
Let me know what you think.

theJackal
.

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