Designing 40V 200mA LED Strings on 4 PCBs Using STCS1A and 40V/3A Power Supply

Good day to all,
Please i need your help and guide on how to design this circuit.I want to drive strings of High power Leds on 4 different PCBs.Each of the PCB requires 40V and 200mA to operate.In order to power the entire 4 board,I intend to buy a power supply with constant Output voltage of 40V/3A.To drive the Leds with a constant current,I intend to use 4 pieces of STCS1A chip from ST Microelectronic and one microcontroller which will supply the PWM Signal to the
STCS1A chip.I attached a block diagram of the circuit to this message.
I will highly appreciate your ideas and suggestions on how i can modify and improve the design for maximum Efficiency-My knowledge of electronics is still very limited.
Thank you for the usual Support.Best regards.

UV_1000LED_BLOCKDIAGRAM_OVERVIEW

40 volts seems quite high for the operating voltage of an LED board, and is uncomfortably close to the maximum operating voltage of the STCS1A. The spec sheet for the ST part seems to indicate that it's up to the job, though it would make me nervous for production or overvoltage surges/spikes if used in an electrically noisy environment. You might want to check the specs of that LED board, or post it here.

Otherwise, your block diagram seems reasonable. The 5V buck regulator for the microprocessor seems like serious overkill for a part that probably won't draw 20 mA, but the 40V input probably limits your choices.

Please would you confirm the power rating of the individual LEDs, and the number of LEDs in series on each PCB ?
Is the intention to drive the LEDs at a constant brightness, or do you want this adjustable ?
Are the PCBs your own design, or are they factory built units ?

Thank you very much Sir for taking the time to answer my question.With respect to the 40V which you said is quite high for the operating voltage of an Led board,I arrived at 40V/200mA based on the way the Leds are arranged on the board(Series/parallel arrangement)and the Information supplied in the datasheet of the Led.Each of the PCB has about 120 Leds and each Led needs a Minimum of 3.2V and 20mA .
Sir,did you say my block diagram is okay except the likely Problem of overvoltage surges and the 5V buck Regulator for the microprocessor.If yes,then i will try and look for another chip with Input voltage range say up to 50V .
Once again thank you for the Support.I really appreciate your sense of understanding.
Best regards.

Thank you for the Response.
Each of the Led requires a minimum of 3.2V and 20mA to operate.Each of the PCB has 120 Leds on it with a Series/Parallel aarangement.The PCB had already been produced,my only task now is just to control the brightness of the Leds.
Once again thank you for the guide.Best regards.

OK. That's not a great LED arrangement, since the forward voltage of LEDs is not very reliable, and changes with current. With 10 strings of 12 LEDs in each module, it's possible that some strings will have a lower voltage drop than others, thus "hogging" the available current and causing uneven illumination of the module. It's possible that the modules have some equalizing resistors? But since you're stuck with that module, you'll have to do the best you can.

I suggest a 45 to 48 volt main supply, to allow for variation in the forward voltages of the modules. That will mean, unfortunately, that you'll definitely have to find a different driver, since that's well above the absolute max. of the ST part. Make sure the voltage of the PWM control is compatible with the voltage of your microprocessor (5V or 3.3V, but not both!).

Running it at 40V should be OK, because the absolute maximum is 45V and 40V is the specified upper limit.

Now, your response to Frank was that these LEDs will be run at 20ma, yet in your original description (above) you stated 200ma -- which is it. Also, you should be designing to the MAXimum forward voltage, not the minimum. For instance, if the LED spec shows a range of 3.1 to 3.6 for the forward voltage, then use the 3.6 value to add up the max voltage in a series string -- so if there are 12 LEDs in the string, that would be 3.6 * 12 = 43.2V. [sound of record scratch] -- hold the phone. I just re-read your reply to Frank and noticed that each PCB has 120 LEDs?!? In a series/parallel arrangement!?! As Mark pointed out, running LEDs in parallel is a BAD idea!! For the reasons that Mark stated [again check out my blog -- which explains in greater detail why this is a bad idea].

So, the picture is forming more clearly -- each LED is to be run at 20ma, and each group of 120 LEDs will regulated to 200ma by the STCS1A chip [I'm assuming that each series string has 12 LEDs in it based on your claim that each LED will drop 3.2 for 3.2*12=38.4V which is close to your claim of the need for a 40V regulator]. The combined current of all of the PCBs [each with one STCS1A chip and each with a set of 120 LEDs] will be 4 x 200ma = 800ma + whatever overhead current is needed by each PCB + the 2A for the microcontroller, so lets say 2.8A. Is that correct?

I suggest you remove one LED from all but one of each series string of 12 LEDs and put a CL520 in it's place. Then, for Rfb [on the STCS1A] use a 0.56 Ohm resistor with a 50 Ohm trimmer+2.2 Ohm resistor series combination across it [the 2.2 Ohm resistor in series with the 50 Ohm trimmer is a measure to prevent an adjustment that would cause excessive current in the LEDs], and adjust it to regulate the current at 200ma. The CL520 is a current regulator designed to run strings of LEDs at 20ma. They will balance the currents in the series strings to 20ma each, for a total of 180ma. Since the STCS1A will be regulating at 200ma, the left over current of 20ma will flow through the one string that doesn't have a CL520 on it.

Here's how I arrived at the Rfb resistor values:

At max Vfb of 110mV we want the current to be 200ma: 110mv/200ma = 0.55 Ohms
The nearest standard value is 0.56 Ohms so we choose that.
But, Vfb can be as low as 90mV so for 200ma at that voltage we need a resistance of: 90mv/200ma = 0.45 Ohms
So, to shift the 0.56 Ohm resistance down to 0.45 Ohms, put the following resistance in parallel:

Rp = Rt*0.56/(0.56-Rt) = 0.45*0.56/(0.56-0.45) = 2.3 Ohms

Nearest standard value is 2.2 Ohms

The parallel resistance that will produce 0.55 Ohms is:

Rp = 0.55*0.56/(0.56-0.55) = 30.8 Ohms

Nearest trimmer value is 50 Ohms [and most trimmers are 20% tolerance so that's a minimum of 50*.8 = 40 Ohms -- close enough!]



So, for each PCB, have 9 of the series strings with one CL520 in series with each and all 10 driven by the STCS1A set at 200ma. The trimmer is necessary because the control voltage across Rfb can be anything in the range of 90 to 110mv, and that will cause way too much of a difference to rely on a fixed resistor. Before powering the thing up for the first time, make sure all of the trimmers are rotated to where the least amount of current will flow, then adjust each one to a current of 200ma [or put a current meter across one of the LEDs in the string that doesn't have a CL520 in it, and adjust the trimmer [on the STCS1A] until it reads 20ma. The LED that you put the meter across will go dark, but that's OK.

What I said about determining the maximum voltage of the series LED strings is the "Engineering way". If this is just for your own use, and not something that is going to go into production, then not as critical. But, if the forward voltages on the LEDs you have turn out to be at the upper limit specified in the datasheet [for those LEDs] then you might have DIM LEDs You can get around that by removing one LED in the string(s) where the LEDs are dim and put a jumper in it's place. So, in that case, you will only have 11 [or 10 if you used the CL520] as I suggested] LEDs in series instead of 12(11).