Electronics > Projects, Designs, and Technical Stuff
9VDC + 5VDC SMPS for driving LEDS
T3sl4co1l:
I don't mind it; it is what it is. The single-failure modes aren't perfectly robust (something happens, other than the affected component), but total destruction is also unlikely. That's good design.
Personally, I'd just ballast the strings with some series resistance each, then put them all in parallel, and switch the combined bank with a MOSFET for PWM (or vary supply if possible, or I guess both for highest dynamic range). The supply needs to handle a transient load, which isn't hard to do, and at a high enough frequency (some kHz), it's feasible to ride through the pulses with a little more output filter capacitance. If you're using a COTS supply, and find it tends to sing under such load, an RC filter between it and the load would do.
Tim
TerminalJack505:
Yep. So long as the power losses are minimal then I think that is the way to go.
Blade2Raiden:
@mariush
AL8843Q would be perfect for this application, but now I started wondering on what to use for PWM, using the arduino nano would be too slow as some pointed out so if I were to use an I2C PWM generator such as the PCA9685 [https://www.nxp.com/docs/en/data-sheet/PCA9685.pdf] would I be able to have a fitting frequency or is this too fast for the AL8843Q, I see they say up to 1MHz but it on the PWM input and output? unclear to me
mariush:
--- Quote from: Blade2Raiden on February 11, 2020, 03:27:34 am ---@mariush
AL8843Q would be perfect for this application, but now I started wondering on what to use for PWM, using the arduino nano would be too slow as some pointed out so if I were to use an I2C PWM generator such as the PCA9685 [https://www.nxp.com/docs/en/data-sheet/PCA9685.pdf] would I be able to have a fitting frequency or is this too fast for the AL8843Q, I see they say up to 1MHz but it on the PWM input and output? unclear to me
--- End quote ---
Just goes to show you didn't bother to read the datasheet :
See datasheet again : https://www.diodes.com/assets/Datasheets/AL8843Q.pdf
Page 5 tells you that 1 Mhz is the maximum switching frequency of the dc-dc converter. See fsw , fsw_MAX and tON_rec at the bottom of the table.
Typically, the switching frequency will be much lower, in your case with let's say 24v in and 18v out and 1-2A, you're probably gonna see 200-250kHz switching frequency.
Page 11 gives you more details about the CTRL pin :
Analog Dimming
Applying a DC voltage from 0.4V to 2.5V on the CTRL pin can adjust output current from 10% to 100% of IOUT_NOM, as shown in Figure 1.
If the CTRL pin is brought higher than 2.5V, the LED current will be clamped to 100% of IOUT_NOM while if the CTRL voltage falls below the threshold of 0.3V, the output switch will turn off.
So :
<0.3 v - OFF
0.4v .. 2.5v = 10% .. 100%
You could literally have a 2.5v linear regulator, followed by a potentiometer (which will behave like a resistor divider), giving you the ability to adjust between 0v and 2.5v.
Even an adjustable linear regulator would work as long as its internal voltage reference would allow you to set the output voltage to as low as 0.3v..0.4v to turn off the driver completely if you want that.
Alternatively, I suppose you could complicate your life by using a DAC IC, if the microcontroller you use doesn't have one built in.
For example: https://www.digikey.com/product-detail/en/microchip-technology/MCP47FVB01A0-E-ST/MCP47FVB01A0-E-ST-ND/5033361
The above gives you 8 bits, so if you use the 5v input as voltage reference, you have an output voltage of 0v..5v as a 0..255 number, so between 0.4v and 2.4v you have around 100 steps.
The chip has a built in ~1.215v voltage reference which you can double to 2.44v through i2c commands, so you'd have 0v... ~2.42v as 0..255, almost ideal for the led driver.... but even the ~100 steps you'd get by using the 5v input as voltage reference would be enough.
One fancy thing you could do is use a dual channel DAC and two separate led drivers and interleave the leds of the drivers so if you turn off one led driver, you get 50% of brightness with every other group of leds being lit at 100% (so potentially no flickering)
This will also reduce the amount of power dissipated by the led driver which should be relatively small either way but hey, it only costs a couple dollars more or something like that.
For example, see https://www.digikey.com/product-detail/en/microchip-technology/MCP47CVB02-E-UN/MCP47CVB02-E-UN-ND/9657660
You get 2 outputs, 8 bits, just like the previous one, super easy to control through i2c
PWM Dimming
The LED current can be adjusted digitally, by applying a low frequency Pulse-Width-Modulated (PWM) logic signal to the CTRL pin to turn the device on and off. This will produce an average output current proportional to the duty cycle of the control signal.
To achieve a high resolution, the PWM frequency is recommended to be lower than 500Hz, however higher dimming frequencies can be used at the expense of dimming dynamic range and accuracy. Typically, for a PWM frequency of 500Hz, the accuracy is better than 1% for PWM ranging from 1% to 100%.
The accuracy of the low duty cycle dimming is affected by both the PWM frequency and the switching frequency of the AL8843Q. For best accuracy/resolution, the switching frequency should be increased while the PWM frequency should be reduced.
So 500 Hz PWM frequency should be doable by any micro. Also, you don't need 5v signals, since the CTRL pin considers 2.5v or higher as 100%, you could power your arduino or atmega chip or pic micro or whatever with 3.3v or 3.6v and you'd be perfectly fine.
I strongly recommend that you go with groups of 2 leds in series, or 3 leds in series. Personally, I would probably lean towards using 24v as input voltage, so using groups of 2 leds in series, but that's simply because 24v is more common and chances are you're more likely to find replacement wall wart adapters running on 24v.
Also, in that configuration, the led driver SHOULD also work well with as little as 18.5v..19v, which means if you really have to, if you have no other choice, you could get a laptop adapter charger to replace your 24v wallwart and get your lamp going until you can get an actual 24v supply. ( I really think the input voltage should be a bit higher than 2 x 8.8..9.3v = 17.6...18.6v)
The LED driver can do 3A, but using it at up to 1.5-2A would be also better, instead of using it with 12v input and nearly 3A of maximum current.
Decide on the maximum current the led driver will need to handle pick suitable resistor for the Rset value (remember you can wire 2 or 3 resistors in parallel to get 1/2 or 1/3 of resistor value if you determine you need some whacky value like 0.067 ohm) and pick a suitable inductor between 33 and 100 uH (look at the graphs and notes in datasheet... with 24v in and 18v out 1-2A 47uH would probably work well but look in datasheet)
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