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Help with LED Driver Circuit
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Luchik:
I would appriciate your help, regarding this LED driver circuit i'm working on. Please see the attached schematic file.

I am using Diodes/BCD boost IC for driving a constant 400mA trough two HP-LED components: Green and Red.
I also have two push buttons that can turn-off one of the colors, if needed (I may use toggle switch instead of push buttons). I do that by simply shorting the LED via a MOSFET transistor.
I need to use more than one transistors for switching, since i have to connect the push buttons to 3V power (for very specific reasons - it will eventually be connected to an external 3V-IC, not push buttons).

My questions:
- Will it work?
- Should I be worried about shorting the two push-buttons together? (I do not have much experience with LED drivers, will it still provide 400mA trough the short?).
- when I push one of the buttons, the voltage drop will be smaller than the input voltage (of 5V). Should I be worried about it?

Please advise.
Ian.M:
That's a disaster.   As soon as you short out one LED, the AP3032K will loose control of the current.  Short out both, and  it will certainly let the magic smoke out.   The problem is your choice to use a CC boost chip.  That forces you to put the LEDs in series so the total Vf is greater than the supply voltage.

You need two separate buck chips (one for each LED) capable of being used for constant current, with enable pins.   Alternatively, if you need *fast* on/off modulation of the LEDs, two separate buck chips + MOSFETs to short out each LED
Zero999:
The idea of using a constant current source and shorting the LEDs is not a bad one, but this implementation will not work. A boost converter can't regulate its output, once the output voltage drops below a diode drop of its input voltage. Look at L1 and D5. The inductor can be treated like a piece of wire at DC, which leaves the output connected to the input voltage, via a diode. When the output voltage is greater, than the input, the diode is reverse biased, but when the output, is less than the input voltage, the diode becomes forward biased, causing the output voltage to rise, whatever the controller does. If a single LED were connected to the output, it would smoke, since its forward voltage, is much less than 5V.

The P-MOSFETs will also never turn fully on, unless a negative gate voltage is applied and Q5's symbol is wrong and is connected incorrectly. The body diode's anode connects to the drain and cathode to the source, so it will conduct, causing the LED to be permanently short circuited.

As mentioned above: use two separate buck converters for each LED. A single constant current converter could be used, but it would need to be able to output both lower, than input, as well as higher, than input voltage, which rules out both buck and boost topologies. One could use: SEPIC, a transformer (flyback, or forward converter), buck-boost etc. but it's unlikely to be cheaper than two buck converters.
Ian.M:

--- Quote from: Hero999 on July 18, 2018, 10:39:25 am ---A single constant current converter could be used, but it would need to be able to output both lower, than input, as well as higher, than input voltage, which rules out both buck and boost topologies. One could use: SEPIC, a transformer (flyback, or forward converter), buck-boost etc. but it's unlikely to be cheaper than two buck converters.

--- End quote ---
.... and you'd also have the complexity of providing adequate drive to the transistors shorting the individual LEDs, and level shifting the control signal to the top one.   It just isn't worth it compared to using individual drivers for each LED.

mariush:
It would be easier and probably cheaper to just use two buck led drivers.

For example, Diodes Inc. AL8860 is 40 cents in small quantities (<18 cents if you buy 1000) and has a control pin which allows you to turn off or on the output, or adjust the brightness either through pwm or a voltage ( off = < 0.2v  brightness 0..100% = 0.3v .. 2.5v , and also set max current with resistor, up to around 1A) .... so just tie to ground with a mosfet or transistor or with microcontroller pin and you turn off your led.

It's high frequency, so you can use small inductors, and buck regulators are  also more efficient than boost regulators...

Here's datasheet : https://www.diodes.com/assets/Datasheets/AL8860.pdf and digikey link : https://www.digikey.com/product-detail/en/diodes-incorporated/AL8860WT-7/AL8860WT-7DICT-ND/6226981

AL8861 is almost same price, but can do more current and may be available in other packages if those are better for you.
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