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mini By Tim Blythman LE river This small, low-cost module can drive relatively large 12V white LEDs from a USB or 5V DC power source. Sometimes you don’t need a floodlight; a modest amount of light is enough, and the Mini LED Driver is an economical way to deliver it. I n the June 2022 issue, we featured some 70W LED panels that are incredibly bright when run at their maximum power (around 6A <at> 12V). But those panels can still be handy when run at lower currents; they generate quite a bit of light even at 1A/12W, and there are plenty of other white LEDs out there which are designed to run at around 10W. This Mini LED Driver is perfect for them. The main motivation behind it is to safely power 12V LED panels from a 5V DC source. If you’re like us, you have many spare USB power supplies or power banks that can be pressed into service to supply 5V. This Driver can deliver enough current to drive most white LEDs to provide a handy light level. If they are large panels like the 70W types, as they are so under-driven, their lifespan will be significantly extended due to reduced heat production. The Mini LED Driver is based around the commonly-available, lowcost boost modules using the XL6009 IC, but it adds a few extra features. Those modules don’t have inbuilt current-­ l imiting except for short-­ circuit protection; our added circuitry provides an adjustable current limit. In the June LED Driver article (siliconchip.au/Article/15340), we explained why it’s preferable to run LEDs from a current-limited source. In brief, simply providing a fixed voltage to LEDs will not give consistent light output. Minor voltage variations 76 Silicon Chip can cause disproportionately large changes in current, perhaps even enough to damage the LEDs. The current limiting feature we’ve added will also protect the input supply, particularly if you’re using a small USB power supply to power LEDs that would draw too much current for it to handle at full brightness. The other feature the Driver adds is a low input voltage cut-out. This avoids the possibility that the boost module does not perform correctly with a low input voltage. Also, if the power comes from a battery, it will prevent excessive discharging of the battery, which could damage it. The XL6009 boost module Numerous DC/DC converter modules are available, both online and from stores like Jaycar and Altronics. They come in two main types, boost and buck, although some combine both capabilities. The buck types reduce the incoming voltage to a lower level. In contrast, buck/boost designs like the Altronics Z6337 (see the adjacent photo) contain two controller ICs (and duplicate many other parts) and can either reduce or increase the incoming voltage. These types of module are effectively a boost and buck module combined. But for this project, we’re specifically using dedicated boost type modules. To ensure that you can get the correct type, the Silicon Chip Shop will stock a boost module that we have tested to work, and that same module is included in our kit. That’s especially important given that there are quite a few different “XL6009” module designs floating around, and they do not all perform the same. These modules have a small PCB that includes a switchmode boost controller IC, a minimum of passive components, plus a trimpot to set the output voltage. The input and output connections are simply solder pads. We have used modules based on the MT3608 IC for some previous projects. In those cases, the module is soldered directly to another PCB and treated as though it were just another component, much like the Mini LED Driver. For example, the Water Tank Level Meter with WiFi from February 2018 (siliconchip.au/Article/10963) used such a module to provide 24V DC to a Features & Specifications ∎ ∎ ∎ ∎ Can drive 12V LEDs or LED panels from a 5V DC supply (eg, USB) Adjustable output current and voltage, up to 1A/20V Small and low in cost Input up to 4A/20V, subject to boost module capacity Australia's electronics magazine siliconchip.com.au Fig.1: the Driver circuit has two main sections. The first section provides the low-voltage cut-out function, using transistors Q2-Q4 and associated passives. The second samples the current between the boost module and the output at CON3 and injects a signal back into the boost module after diode D1 to limit the output current to a more-or-less fixed level. water depth sensor from a nominally 5V supply. Incidentally, this 5V supply was provided by another module that managed power from a solar cell and rechargeable battery. The Arduino-based Programmer for DCC Decoders (October 2018 issue; siliconchip.au/Article/11261) similarly used such a module to derive 12V power from a 5V USB supply. In that case, 12V was needed to correctly power and program the DCC decoders. For the Mini LED Driver, we have chosen a different boost module. The XL6009 IC makes it more capable than the MT3608-based module, giving headroom to operate the module comfortably within its limits. Implementing the current limiting feature with the XL6009-based module is also slightly easier. It’s somewhat larger, but the complete Mini LED Driver still measures just 72mm by 24mm. One caveat with these modules is that reader Jonathan Woithe wrote in to tell us that these modules do not always regulate their output voltage correctly under some input voltage conditions. This means that the module can produce up to 50V, even when set lower, which is clearly not desirable! His analysis is on page 8 of the June 2021 issue (Mailbag; siliconchip. au/Article/14875). This problem only occurs when the incoming supply voltage is below the minimum specified voltage for the XL6009 IC. So, for example, if the siliconchip.com.au module is powered by a battery that runs flat, it may be subject to these output spikes. We avoid this problem by shutting down the XL6009 module when the incoming voltage is low while also providing battery over-­ discharge protection. The Mini LED Driver is presented as a bare PCB and is intended to be used as an enhanced module as part of a larger assembly that might include a power supply and a LED panel or another device that uses power from the Driver. So the Mini LED Driver provides three main functions over a simple boost module: it’s easier to connect to, has current limiting and a low-­ voltage cut-out. We haven’t tested the Mini LED Driver in other applications. Still, it could be handy to help charge a 12V battery with the appropriate settings and a diode on the output, or anywhere a 12V DC source is needed at modest currents (up to about 1A). USB connectors will not handle more than about 2A, so the screw terminals are better for higher input currents. CON3 is another screw terminal block from which power can be drawn. If the low-voltage cut-out and current limiting are not operating, the Driver behaves just like a boost module. Circuit ground from inputs CON1 and CON2 is connected straight through to output CON3 and to the boost module’s ground terminals, IN− and OUT−. The low-voltage cut-out connects between the CON1 & CON2 inputs and the boost module, switching power to the module’s IN+ terminal. The low voltage cut-out works as follows. A divider formed by 10kW and 1.5kW resistors connects across the incoming supply. The junction of these two resistors connects to the base of NPN transistor Q3. When the voltage at this junction is above about 0.6V, Q3 Circuit details Fig.1 shows the circuit diagram of the Mini LED Driver. The input supply is wired to either CON1 or CON2 while the LEDs (or another load) connect to CON3. CON1 is a pair of screw terminals to which you can connect bare wires. This type of connector will handle up to 5A with ease. Mini-USB connector CON2 makes it convenient to power it from a USB power supply, but most Australia's electronics magazine The Altronics Z6337 buck-boost module uses two controller ICs and two inductors to provide separate buck and boost capabilities. The Mini LED Driver is intended to be used with a boost-only module. September 2022  77 The trimpot on the boost module is for changing the voltage, while the adjustment screw for the current trimpot can just be seen poking out below it. The wire just visible below the upper trimout here is critical for the Mini LED Driver’s operation. It is connected to a point on the boost module PCB that joins to the XL6009 IC’s feedback pin. is switched on, and it pulls the gate of P-channel Mosfet Q4 down, powering the boost module. The 10kW/1.5kW divider means that an input voltage of about 4.6V is needed to switch on Q3, along with Q2 and Q4. At the same time, Q3 sinks current from the base of PNP transistor Q2 via a pair of series-connected 10kW resistors, which serve both to limit the current sunk from Q2’s base and ensure it is held off when Q3 is not sinking current. These two resistors also hold Q4’s gate high when Q3 is off, so it is also switched off when appropriate. There are two resistors because Q4’s gate needs to be pulled more than 1V below the supply voltage to switch it on, while Q2’s base-emitter junction limits its base voltage to around 0.6V below the incoming supply. The 47kW resistor between Q2’s collector and Q3’s base provides some hysteresis for this voltage comparator. When Q3 switches on, Q2 supplies a small amount of extra biasing current into the junction of the 10kW/1.5kW voltage divider. This means that the input voltage needs to drop to around 3.9V before Q2, Q3 and Q4 switch off. This reduces the chance of the low-voltage cut-out oscillating when the input voltage is close to the cut-out point. The 100nF capacitor in parallel with the 1.5kW resistor also helps by further slowing down its response. The default resistors have been chosen to give correct operation with There are quite a few different modules with the XL6009 chip on them. This is the one we found worked best, and it’s pretty inexpensive. It will also be supplied as part of a complete kit for the Driver board. 78 Silicon Chip a nominally 5V USB supply and protect against such things as the USB supply’s voltage dropping. Although not explicitly designed for it, the Mini LED Driver can operate from higher voltages. We will mention some of the provisos and limitations later. By the way, the 20V maximum limit of this design is due to the maximum gate-source voltage rating of Mosfet Q4, while Q4 also limits the current fed to the boost module to 4A as its drain current limit is 4.2A. Still, the XL6009 module tops out at around 4A anyway, so using a beefier Mosfet wouldn’t gain us much. We have not added any input current limiting as most USB supplies will drop their bundle before delivering 4A. Current limiting The XL6009 IC on the boost module controls the output voltage by comparing an internal voltage reference to a fraction of the output voltage, and adjusting its operation to try to keep them the same. The trimpot on the boost module is part of a resistive voltage divider used to sample an appropriate fraction of the output voltage. So the output voltage can be set by adjusting the trimpot. We provide current limiting by injecting current into this voltage divider, making it appear to the switchmode chip that the output voltage is higher than it actually is, causing it to reduce its output. A 15mW shunt resistor is connected between the boost module’s output (OUT+) and output connector CON3. The voltage across this resistor is proportional to the current drawn by the load at CON3. The ZXCT1009 shunt monitor IC (IC1) amplifies this voltage difference and converts it to a current that flows from its pin 3 output. This current is 10mA for each 1V across the shunt. Note that the 15mW shunt resistor reduces the voltage applied to the load, Australia's electronics magazine but as its value is low, the difference is only a few millivolts (15mV <at> 1A), so it is not important. Since a 1A load current will induce 15mV across the 15mW shunt resistor, that will result in 150µA flowing from pin 3 of IC1 (10mA × 15mV ÷ 1V). The upshot is that IC1 produces a current that is 1/6667 (or, if you prefer, 3/20000) that of the output current. This current is fed to the FB (feedback) pin on the attached boost module through the 4.7kW resistor, trimpot VR1 wired as a variable resistor and schottky diode D1. This current will tend to reduce the output voltage in proportion to the current, but this is not the main factor in the current-­ limiting circuitry. There is also NPN transistor Q1 to consider. Q1’s base and emitter (with a 220W emitter degeneration resistor to moderate its gain) are connected across the 4.7kW resistor and VR1. If more than 0.6V appears across those two components, Q1 will start to conduct. This action forms the bulk of the current limiting feature, with the extra current being sourced into the FB pin through Q1’s collector and emitter. The 2.2kW collector resistor limits the maximum current that can be injected, helping to keep this arrangement stable. Since the voltage between the base and emitter of Q1 depends on both the load current and the setting of potentiometer VR1, based on Ohm’s law, that means that VR1 can be used to set the load current at which Q1 will start to conduct and therefore the maximum current that the whole device can supply. Note that if you use a supply voltage different to 5V, the current limit will change due to Q1’s collector resistor connecting to the incoming supply. But most sources of 5V DC are regulated, so this generally won’t matter. It is something to keep in mind if you’re going to power this circuit directly from a battery pack. Finally, there are two capacitors siliconchip.com.au connected across the output. We have used two smaller parts here as they fit the outline of the Mini LED Driver better. They smooth out the voltage across the shunt resistor, which would otherwise be quite peaky due to the upstream capacitors on the boost module. Due to this, the Mini LED Driver is not well suited as a current-­regulated source for dynamic loads, as these capacitors can only allow a slow response. If the load resistance suddenly changed, then these capacitors would need to charge or discharge before the system could settle at a new steady state. During this time, the current through the shunt would not represent what is happening downstream of CON3. Fortunately, LEDs present a slowly changing load. The Mini LED Driver just needs to cope with changes that occur as the LED forward voltage changes with slowly changing variables such as temperature. Keep in mind that this current limiting scheme is not effective as short-circuit protection, because the boost module cannot reduce its output voltage below its input voltage (except for the small drop due to its onboard diode). Basically, the Mini LED Driver cannot limit its output voltage to anything much below its input voltage and certainly not down to levels near zero. Current adjustment VR1 is wired such that the fully clockwise position corresponds to 0W between its two connected terminals. So the clockwise position sets a 4.7kW resistance between IC1’s Iout and the diode while the fully anti-clockwise position thus sets a 9.7kW resistance. Assuming a threshold of around 0.6V for a silicon base-emitter junction, Q1 will start to conduct at 127µA from IC1 when VR1 is set fully clockwise, and 62µA from IC1 when fully anti-clockwise. This means that the usable output current setting range is nominally from 0.85A down to 0.41A (recalling the factor of 6667 from previously), although these are not hard limits. During one of our tests, we started by setting the Mini LED Driver voltage to 12V with no load and with VR1 set to its minimum. We then connected one of the large 70W LED panels and measured a panel current of 0.48A at 11.1V. siliconchip.com.au Setting the current limiting to maximum gave 0.84A at 11.3V but the current could be increased to 1A by increasing the voltage setpoint (at no load) to around 12.6V. We measured close to 3A at the 5V input, so we don’t expect many USB supplies will work at these levels anyway. The fact is that the current limiting comes on gradually, which is necessary to keep the Driver stable. It also means that the LED operating point can be tweaked by careful adjustment of both the current and voltage settings. Fig.2 shows the effects of changing loads on the Mini LED Driver. We made these plots with the no-load voltage set to 12V and the current-limiting trimpot set to its lowest and highest positions, plus a third point near the middle. There is a limit to how low a voltage can be achieved by the current limiting circuitry; around 8.3V in this case. That is due to the 2.2kW resistor limiting the current injected into the voltage divider. Other boost modules that use different divider resistors for their voltage setting will behave differently as the injected current will change the setpoint by a different amount. This is one of the reasons we’re specifying and supplying a specific module, as shown in the photos opposite. This is the one that worked the best in our testing. If you must try a different boost module, we recommend thoroughly testing the combination before putting it to use. We used the Arduino Programmable Load from the June 2022 issue (siliconchip.au/Article/15341) for much of our testing, including plotting Fig.2. Efficiency We also measured the module’s efficiency and found that it did not reach the 96% figure claimed by the suppliers of many of these boost modules. They usually specify the efficiency for boosting 12V to 20V; boosting 5V to 12V is both a higher ratio and starting from a lower voltage, so efficiency will not be at its peak. With a regulated 5V DC input and 12V at the output, a helpful rule of thumb is to multiply the output current by three to work out the theoretical input current. This corresponds to an approximate efficiency of 80%. Australia's electronics magazine Fig.2: these curves show the behaviour of the Mini LED Driver when set to a nominal 12V and three different current limit settings. The curves correspond to VR1 at minimum (cyan/blue), maximum (red) and roughly halfway between the two extremes (green). Options You might decide to leave off CON1 or CON2 if you know that you will definitely only use one of them, but we’ll explain the construction procedure as if fitting both. Keep in mind that the Mini LED Driver will draw a considerable current with a 5V supply. Any significant sag in its input voltage could result in the low-voltage cut-out operating. A USB connector will have a noticeably higher resistance than the screw terminals. So we recommend fitting both in case this resistance turns out to be too high, and you need to use the screw terminal instead of the USB connector. Construction This board is not difficult to assemble, but it almost exclusively uses surface-­mounting parts. So ensure you have the necessary tools and supplies, including solder, flux paste, solder wicking braid, a fine-tipped iron (or at least not a huge one), tweezers, decent lighting and a magnifier. For more tips and tricks regarding SMD soldering, see our feature on the topic (December 2021; siliconchip.au/ Article/15138). The PCB is well-marked, but you can also refer to the overlay diagram (Fig.3) to see which parts go where. The PCB is coded 16106221 and measures 72mm x 24mm. Start with CON2, the mini-USB connector. Apply flux to the pads and rest the connector in place. It has locating lugs, so it should lock into the correct position. September 2022  79 Fig.3: the trickiest part of assembling the Driver is ensuring you don’t mix up the various SOT-23 parts. Check the PCB markings before soldering these components in place. The boost module sits over the top of this PCB, as you can see from our other photos. While the feedback connects electrically to pin 5 of the XL6009 IC, it’s usually easier to solder to a trimpot lead after checking for a low resistance between it and the IC feedback pin. Clean the soldering iron tip and apply a small amount of fresh solder. Touch the iron to the two extended end pads in the row of five – only these two are needed to supply power. If you bridge them to any other pins, use the solder wick to remove any excess before proceeding. Then apply a generous amount of solder to secure the four corner leads on the shell, which will ensure that the connector is mechanically secure. Work through the transistors, diode and IC next. They are all in identical-­ looking SOT-23 packages, but there are five different types, so take care that they are not mixed up. The PCB is marked with the part numbers as well as the designators. Check the types against the overlay, working with one type at a time. The SOT-23 parts are small, but the leads are pretty spread out, so they are quite easy to work with. Apply flux to the pads for these parts, then use tweezers to roughly place each part in turn. Tack one lead and check that the remaining leads are all within their pads. If not, adjust as necessary using the iron and tweezers. Then solder the remaining leads. Do the same with the eight small (3.2 × 1.6mm) resistors, checking their values against the silkscreen as you go. Much the same technique is used for these parts as for the semiconductors. Fit the larger (6.3 × 3.2mm) current shunt resistor next. It is harder to mix up with the other parts due to its unique size for this project. The solitary SMD capacitor goes next to the 1.5kW resistor and can be soldered similarly. That completes the fitment of the surface mounting parts. Clean the PCB of any flux residue before proceeding 80 Silicon Chip further and allow the board to dry thoroughly. You can test the low-voltage cut-out feature if you can connect a variable power supply to the CON1 or CON2 inputs. It’s best to do so now, before connecting the boost module, as it’s easier to fix any problems you find. Do not exceed 20V, and mind the polarity of the connections to CON1. Ramp the input voltage up and down. Check that the voltage between IN+ and IN− points is present when CON1 or CON2 is above the upper threshold (around 4.6V). When the input is below the lower threshold (near 3.7V), it should drop out. Completion The remaining parts to mount are CON1, CON3, trimpot VR1, the two electrolytic capacitors and finally, the boost module. Solder CON1 and CON3 first. They should sit far enough apart to allow the boost module to sit between the connectors on the ends. Fit VR1 next. While you could solder it in the standard vertical position, the boost module will sit over the Driver PCB, blocking adjustment. So instead, install it on its side, as shown in our photos. Ensure that the adjustment screw is positioned correctly. You should also adjust the trimpot to its minimum (fully anti-clockwise) in preparation for testing. The two electrolytic capacitors sit near CON3. The longer positive leads go into the pads marked with small + symbols. Push them down firmly against the PCB before soldering and trimming the leads. A warning before fitting the boost module; we have seen some boost modules that (confusingly) increase their voltage when the trimpot is Australia's electronics magazine adjusted counter-clockwise. If you are using a different module from the type we supply, check its voltage by powering it up and measuring its output with a multimeter before soldering it to the Driver board. Otherwise, you could cook those two capacitors the first time you power it up. Having checked that, solder the short length of wire to the feedback pin at the reverse of the voltage adjust trimpot on the underside of the boost module. It is intended to be connected to the middle pin, to align with the other PCB, but you might see that two of the boost module’s trimpot’s pins are connected together anyway. You can see where this connects in our photos. On the XL6009 modules that we are using, this should line up directly with the FBPIN pad on the Driver PCB, but it might be in a different location if you are using a different module. Since it lines up directly, a component lead off-cut might be adequate, but if you can’t run the wire directly, use a short length of fine insulated wire instead (eg, Kynar or wire wrap wire). Now solder component lead offcuts or short lengths of stiff wire to the four corners of the boost module at the IN+, IN−, OUT+ and OUT− pads. These should all face down in the same fashion as the wire going to FBPIN. We found a pair of tweezers or pliers handy to grip the wire while soldering it (to avoid burned fingers). Now you can join the two boards together with the boost module above the Driver PCB, ensuring that the pad labels match. Allow some clearance between the two PCBs if possible, and tack one lead in place. Adjust the boards to ensure that nothing is making contact where it siliconchip.com.au shouldn’t and check that they are square and parallel, then solder the four corner pads followed by the wire for FBPIN. Trim any wires that are longer than necessary. Testing During testing, remember that the Mini LED Driver is not short-circuit proof. So take care with the attached loads to ensure that there is no chance of a short circuit or very low resistance that might overload Mosfet Q4. As we mentioned earlier, the Arduino Programmable Load works well for testing, but you could use a fixed resistor (eg, 22W 10W or two 10W 5W resistors in series) or a high-power white LED. The following assumes a 5V supply and might not work if you have a much higher supply. Apply power without a load and adjust the output at CON2 to 12V using the trimpot on the XL6009 module. If you can’t smoothly adjust the voltage at the output, check the Driver assembly before proceeding further. Remember to not set the output above 20V! With VR1 on the Driver set to its minimum position, a 20W or lower resistance load should draw near 0.6A and cause the output to enter current limiting. Referring to Fig.2, check that your unit responds similarly to our prototype. If the output voltage or current seems to be dropping more than this, check that your USB supply is operating within its limits. It might have its own internal current limiting. If the voltage at CON1 is not being maintained near 5V, that is a sign that the supply you are using is not handling the load. It is a good idea to check the voltage going to the boost module at the IN+ and IN− pads. If this is much less than the voltage at CON1, the low-voltage cut-out is operating. That may be due to voltage drops in the cable or the USB supply sagging under load. Adjust VR1 and check that the current limit changes. You might need to increase the load (decrease the resistance) by adding extra parallel resistors. Set the current to your desired value and connect your desired load. Then, confirm that it works as expected. Using other boost modules We don’t recommend this unless you are experienced. Finding the siliconchip.com.au You can clearly see the wire from the FBPIN pad to the trimpot above. feedback (FB) pin can be tricky if your boost module is not labelled. A good place to start is the centre pin of the adjustment trimpot, although we have seen some modules that do not follow that trend. The FB pin is brought out on the XL6009 IC, and most boost controllers should have an external feedback pin, so it makes sense to start looking there. On the XL6009, it is the rightmost of the small pins (pin 5). On the modules we have tried, it is the smaller pin closest to the voltage adjustment trimpot. You could solder a wire directly to this pin, although it won’t be as neat as connecting to the adjustment trimpot terminal. Instead, you can use a multimeter set on continuity mode to find another more accessible (eg, through-hole) solder joint with a nearzero resistance to the feedback pin. If in doubt, look for a data sheet for the switchmode controller chip on your module. Using it We tried the Mini LED Driver with one of the large 70W LED panels we used in June with the Buck-Boost LED Driver (available from our Online Shop, Cat SC6307 or SC6308). We connected the LED panel after setting the output to 12V with no load and winding the current limiting to its minimum. It drove the panel at 480mA, with the output voltage being 11.1V. Slowly increasing the current limit increased the panel current and brightness. To confirm that the current is being adequately regulated, disconnect the LED panel and check that the output voltage rises by at least half a volt; this means that there is headroom for the Mini LED Driver to regulate its current. We found that the panel would dim and sometimes flicker after the current was set past a certain point, meaning that the USB power supply had reached its limit. Another symptom of overloading is a high-pitched sound from the boost module when under load. If this occurs, wind the current limit down to prevent SC damage to the USB supply. Parts List – Mini LED Driver 1 double-sided PCB measuring 72mm x 24mm, coded 16106221 1 DC-DC boost module based on XL6009 controller with red PCB (MOD1, see text) [SC6546] 1 2-way, 5.08mm screw terminal block (CON1) AND/OR 1 mini-USB socket (CON2) 1 2-way, 5.08mm screw terminal block (CON3) 5 20mm lengths of 1mm diameter solid core wire or component lead offcuts (see text) Semiconductors 1 ZXCT1009 high-side current shunt monitor, SOT-23 (IC1) 1 BAT54 (or BAT54C or BAT54S) schottky diode, SOT-23 (D1) 2 BC847B NPN bipolar transistors, SOT-23 (Q1, Q3) 1 BC857B PNP bipolar transistor, SOT-23 (Q2) 1 PMV50EPEA or AO3407 P-channel Mosfet, SOT-23 (Q4) Capacitors 2 100μF 25V electrolytic 1 100nF 50V X7R M3216/1206 SMD ceramic Resistors (all 1206/M3216 1/8W unless specified otherwise) 1 47kW 3 10kW 1 4.7kW 1 2.2kW 1 1.5kW 1 220W 1 5kW top adjust multi-turn trimpot (VR1) 1 15mW 2512/M6432 3W current shunt resistor [SC3943] Kit (SC6405 SC6405 – $25): has the PCB and all onboard parts, including the XL6009 module. 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