LED Driver with relatively high capacitance on PWM input causes PWM signal to be laggy

Question

I am using three Meanwell LDD-600L LED drivers (datasheet) to control brightness/color of a 50 Watts (total) RGB LED. This driver has a PWM input which I draw low with a PC817 opto isolator. Everything works fine in principle, but there is a little flaw:

When the internal phototransistor of the PC817 turns on, its output voltage changes to low lightning fast (500ns). When I turn the PC817 off, I have to use a voltage divider acting on the 30 V supply voltage in order to get around the 5V specified PWM voltage. This actually causes PWM input voltage to rise with a capacitor charge curve. The time constant of the exponential curve is 10us, so my conclusion is that the driver has an input capacitance of about 450 pF (given the 22k resistor from the voltage divider).

Since I want to control the driver with 16 bit PWM at ~305 Hz (i.e. a counter frequency of 20 MHz), the smallest pulse width that can be set inside the controller is 50ns. Of course I will not even get the electrical pulse width close to this (turning the phototransistor off already takes 500 ns), but being limited by the 10 us is a little too lousy for my taste.

Is there any other possible (economic) solution for a lower impedance pull-up of the PWM input capacitance than with a voltage divider?

Of course I don't want to use smaller resistors because they already get noticeably warm (+10 degrees) and I want the circuit to have as high efficiency as possible. The datasheet of the driver also specifies a maximum of 8V on the PWM input (if my interpretation is right), so pulling up directly to 30 V is probably not an option.

By the way: the whole circuit is built on a breadboard. I have not checked yet if the capacitance comes from the breadbord. I don't hope so.-

Edit: Tony's solution of connecting the digital IO's to the PWM with a resistor directly and dropping the optos actually works. Seems I have been a little to cautious with respect to EMI cause by the 5V/30V interface.

Answer 1

Design Specs

1. As your TV and computer monitor only have 8 bit intensity control per channel, I think your 16 bit control spec is a overly optimistic for optics.

2. I suggest 10 bit or 0.1% is adequate ~ 60 dB range and 12 bit is presuming too much.

3. 1% is often adequate for lighting control and 1% of 1kHz PWM max = 10 us although they do not indicate the resolution limit but this matches your present result width .

Calc. Errors

4. The Thevenin equiv. R of 22k/3.9K is not 22k but rather 3.3k for a logic "1" which suggests a much higher Cin if Cin=T/R and T is measured for a 0 to 64% asymptote.

Suggestions:

5. Consider open bin Rank (top row on spec p4) 2.5~30 mA.out / 5mA.in = equiv to hFE= 0.5 to 6 (CTR)

6. This if Vf= 1.2 and AVR=5V+50 ohm then If = (5-1.2)V/(220+50)Ω = 14mA so worst case CTR=50% = 7mA out.

7. I am assuming you got open rank(bin) : Verify

8. PWM spec = : DIM ~ -Vin >2.6 ~ 5.5VDC or open circuit
   therefore the Vin threshold is ~ 50% of 2.6V = 1.3V
   (just like TTL and 74HCT ) as expected and as I assumed. Verify

9. Change R10,11,12 to 5V/7mA = 720 Ω pullup to 5V for 7mA sink and expected rise time ~ 720 /3.3k *10us est. = 2.2us rise time.

10. "0" Power= 5V*7mA ( 35 mW out) + 1.3 V *14mA (=18mW) * 50% duty cycle = 27mW avg vs 200 mW total rated Abs max. ( << 50% of abs. max is good rule of thumb)

11. If you need less than 1us, you may consider reducing 720 Ω pullup to 5V

The PWM does have EMI filters which may be partly caused by load capacitance. If you need a faster rise time experiment with emitter follower method instead of 720 pull up on collector try 330 Ω to emitter follower to gnd for 0 to 4V non-inv. output.

update

12. Get rid of opto isolators use 50R series to match driver impedance to reduce EMI and ringing from apparent Cin load.

p.s. use a log User Experience (UX) interface for dim control with 12 bit resolution.

p.p.s. breadboard crosstalk is more likely 10 nH/cm for wires and 2 pF/cm if twisted pairs and less if not.

Answer 2

Your question is more of a red herring; it's most valuable to know how to effectively drive your LED power supplies' PWM dimming input from your microcontroller. You state that the maximum input on the PWM pin is 8 V, so I assume you're using the 300-700 mA model that has a high logic level of 3.5 - 8 V. It also looks like you're just using a 5 V regulator for your logic supply.

Some alternatives to the slow optocouplers:

• Directly drive from your microcontroller if you're using 5 V IO. Series resistors 10-50 ohm to slow down the digital signal edges slightly and provide current limiting are good practice. Ensure you have a good common ground between the microcontroller and the -VIN of the LED drivers.
• Use a level translator or digital isolator to assure signal integrity, particularly if your microcontroller is actually at a lower IO voltage or in a different power domain. Something like the SI8035 provides a fairly low cost, 3-5V compatibility on both sides, and isolation. In low quantities, level translators end up being similarly priced and with additional complexity. Either option is additional cost, but might be cheaper than 3x optocouplers.