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diff --git a/blog/multichannel-led-driver/index.html b/blog/multichannel-led-driver/index.html index 22fa408..9f5682f 100644 --- a/blog/multichannel-led-driver/index.html +++ b/blog/multichannel-led-driver/index.html @@ -16,6 +16,9 @@ <a href="/projects/" title="Projects">Projects</a> <a href="/about/" title="About">About</a> </div> + <div class="search"> + <div id="search"></div> + </div> <div class="external"> <a href="https://git.jaseg.de/" title="cgit">cgit</a> <a href="https://github.com/jaseg" title="Github">Github</a> @@ -32,7 +35,7 @@ </ul> <strong>2018-05-02</strong> </header> - <main> + <main data-pagefind-body> <div class="document"> @@ -64,7 +67,7 @@ and it looks like it's off to your eye. If you turn it off right at the end, it' super bright to your eye. Now, if you turn it off halfway into the cycle, it's on half the time and it will look to your eye as half as bright as before. This means that you can control the LED's brightness with only a digital signal and good timing.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/pwm_schema.jpg" alt="A visualization of PWM at different duty cycles."> <figcaption>Waveforms of two PWM cycles at different duty cycles.</figcaption> </figure><p>PWM works great if you have a dedicated PWM output on your microcontroller. It's extremely simple in both hardware and @@ -104,7 +107,7 @@ channel's duty cycle into chunks the size of these bit periods. The amazingly el can guess from the name these bit periods are weighted in powers of two. Say the shortest bit period lasts 1 microsecond. Then the second-shortest bit period is 2 microseconds and the third is 4, the fifth 8, the sixth 16 and so on.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/bcm_schema.jpg" alt="A visualization of BCM at different duty cycles."> <figcaption>Waveforms of a single 4-bit BCM cycle at different duty cycles. This BCM can produce 16 different levels.</figcaption> @@ -139,7 +142,7 @@ drive a couple of amps into the LED tape from the weak outputs of the shift regi <em>reset</em> inputs. We set the timer to PWM mode so we can generate pulses with precise timing. At the beginning of each bit period, a pulse will strobe the data for this bit period that we shifted in previously. At the end of the bit period, one pulse will reset the shift register and one will strobe the freshly-reset zeros into the outputs.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/olsndot_output_schematic.jpg" alt="From left to right, we see the STM32, one of the shift registers, and the LEDs and MOSFETs. The LED tape is driven to ground by the MOSFETs, which are in turn directly driven from the shift register outputs. The shift register is wired up to the STM32 with its clock and data @@ -176,7 +179,7 @@ values. It turned out that our extremely simple LED driving circuit consisting o driving a MOSFET, which in turn directly drives the LED tape was maybe a little bit too simple. After some measurements it turned out that we were looking at about 6Vpp of ringing on the driver's output voltage. The picture below is the voltrage we saw on our oscilloscope on the LED tape.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/driver_ringing_strong.jpg" alt="Strong ringing on the LED voltage waveform edge at about 100% overshoot during about 70% of the cycle time."> <figcaption>Bad ringing on the LED output voltage caused by wiring inductance. Note that the effect on the @@ -185,7 +188,7 @@ voltrage we saw on our oscilloscope on the LED tape.</p> <h4>Dynamic switching behavior: Cause and Effect</h4> <p>A bit of <a class="reference external" href="http://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html">LTSpice</a> action later we found that the inductance of the few metres of cable leading to the LED tape is the likely culprit. The figure below is the schematic used for the simulations.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/driver_output_ltspice_schematic.jpg" alt="The LTSpice schematic of one output of the driver, taking into account the shift register's output ESR and the wiring ESL."> <figcaption>The schematic of the simulation in LTSpice</figcaption> @@ -193,7 +196,7 @@ likely culprit. The figure below is the schematic used for the simulations.</p> in full. Combined with the cable inductance, this works out to a considerable lag of the rising edge of the LED current, and bad ringing on its falling edge. Below is the voltage on the LED output from an LTSpice simulation of our driver.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/overshoot_sim_r0.svg" alt="The result of the LTSpice simulation of our driver output. The LED current shows similar ringing to what we measured using the oscilloscope. Interestingly, the gate voltage shows strong ringing, too."> @@ -206,14 +209,14 @@ capacitance. The result of this is that the LED current passing the wire's ESL r series inductance gets excited slightly less, and the overshoot decreases. Below is a picture of the waveform with the damping resistor in place and a picture of our measurement for comparison. The resistor values don't agree perfectly since the estimated ESL and stray capacitance of the wiring is probably way off.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/driver_ringing_weak.jpg" alt="Weak ringing on the LED voltage waveform edge at about 30% overshoot during about 20% of the cycle time."> <figcaption>Adding a resistor in front of the MOSFET gate to slow the transition damped the ringing somewhat, but ultimately it cannot be eliminated entirely. Note how you can actually see the miller plateau on the trailing edge of this signal. </figcaption> -</figure><figure> +</figure><figure data-pagefind-ignore> <img src="images/overshoot_sim_r100.svg" alt="The result of the LTSpice simulation of our driver output with an extra 100 Ohms between shift register output and MOSFET gate. Similar to the oscilloscope measurement the ringing is much reduced in its amplitude."> @@ -224,13 +227,13 @@ shift register's output at very small duty cycles (1µs or less). This is caused plateau</a>. For illustration, below is a graph of both the excitation waveform (the boxy line) and the resulting LED current (the other ones) both without damping (top) and with 220Ω damping (bottom). As you can see the effective duty cycle of the LED current is not at all equal to the 50% duty cycle of the excitation square wave.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/asymmetric_iled.svg" alt="The result of an LTSpice simulation of the LED duty cycle without and with damping. Dampening widens the LED current waveform from 50% duty cycle with sharp edges to about 80% duty cycle with soft edges."> <figcaption>Simulated LED duty cycle with and without damping. The damping resistance used in this simulation was 220Ω.</figcaption> -</figure><figure> +</figure><figure data-pagefind-ignore> <img src="images/asymmetric_vgate.svg" alt="The gate voltages in the spice simulation above. The undamped response shows sharp edges with the miller plateau being a barely noticeable step, but with strong ringing on the trailing edge. The damped response shows RC-like slow-edges, but has wide miller plateaus on both edges @@ -261,7 +264,7 @@ calibration, the LED driver is set to enable each single BCM period in turn, i.e shielding against both stray light and electromagnetic interference and a photodiode looking at the LED tape. We used the venerable <a class="reference external" href="http://www.vishay.com/docs/81521/bpw34.pdf">BPW34</a> photodiode in our setup as I had a bunch leftover from another project and because they are quite sensitive owing to their physically large die area.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/linearization_setup.jpg" alt="The led measurement setup consists of several PCBs and a breadboard linked with a bunch of wires and a big tin can to shield the LEDs and the photodiode. A large sub-D connector is put into the top of the tin can as a feed-through for the LED tape's control signals and the @@ -283,7 +286,7 @@ ADC chips I had in my parts bin.</p> measurement at each step. Later on, these measurements can be plotted to visualize the resulting slope's linearity, and we can even do a simulation of the resulting brightness for all possible control values by just adding the measured photocurrents for a certain BCM setpoint just as our retinas would do.</p> -<figure> +<figure data-pagefind-ignore> <img src="images/driver_linearity_raw.svg" alt=""> <figcaption> A plot of the measured brightness of our LED tape for each BCM period. The brightness values are normalized @@ -298,7 +301,7 @@ photocurrents for a certain BCM setpoint just as our retinas would do.</p> </figure><p>While it would be possible to fully automate the optimization of BCM driver lookup tables, we needed only one and in the end I just sat down and manually tweaked the ideal values we initially calculated until I liked the result. You can see the resulting brightness curve below.</p> -<div class="subfigure"> +<div class="subfigure" data-pagefind-ignore> <figure> <img src="images/uncorrected_brightness_sim.svg" alt=""> <figcaption> @@ -344,7 +347,7 @@ and control their outputs.</p> </div> <div class="section" id="conclusion"> <h3>Conclusion</h3> -<div class="subfigure"> +<div class="subfigure" data-pagefind-ignore> <figure> <a href="images/olsndot_schematic.png"> <img src="images/olsndot_schematic.png" alt="A picture of the LED driver schematic"> @@ -372,13 +375,18 @@ analyze the brightness measurement data <a class="reference external" href="http / <a href="/about/">About</a> / <a href="/imprint/">Imprint</a> </footer> -<script> +<script src="/pagefind/pagefind-ui.js"></script> + <script> if(navigator.getEnvironmentIntegrity!==undefined)document.querySelector('body').innerHTML=`<h1>Your browser contains Google DRM</h1>"Web Environment Integrity" is a Google euphemism for a DRM that is designed to prevent ad-blocking, and which Google has forced into their browsers against widespread public opposition. 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