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Revised USB Charger Regulator With Low Battery Cut-Out This revised version of the tiny USB charger module presented in the July 2015 issue now has extra circuitry to prevent any USB device such as a permanently connected dash-camera from discharging the car’s battery below 12.15V. We’ve boosted the continuous output current from 2.5A to 3A and as well as being installed in a motor vehicle, it could be built as a portable USB device charger or for many other purposes. By Nicholas Vinen W E’VE HAD a good response to our article in the July 2015 issue on installing USB charging points in a car. One of the benefits of that approach is that the USB sockets are powered even when the vehicle ignition is off, allowing phones and similar devices to be left charging while the car is parked. That brings up the risk of draining the vehicle battery if those devices are left plugged in long-term, which was discussed in the July article. Basically, the stated solution was to avoid connecting anything permanently if it draws a lot of current from the USB socket on a continuous basis. One reader asked why we didn’t simply incorporate a low-battery cutout in the circuit to address this. The simple answer is that we were trying to minimise both the complexity of the design and the cost of building the Features & Specifications Maximum sustained input voltage: 15V Low battery cut-out: 12.15V Battery cut-in voltage: 12.86V High efficiency: typically >90%, 0.5-2A Output voltage range: 0.8-15V, typically 5V (must be at least 2V below input) Output current: up to 3A Quiescent current: approximately 1mA Current with low battery cut-out engaged: typically <10µA Output ripple and noise: ~5mV RMS <at> 1.2A Load regulation: ~150mV/A, 0-250mA; ~75mV/A, 250-3000mA Line regulation: <1mV/V Transient response: output stabilises within ~20μs for a ±1.2A load step Other features: transient voltage suppression, no heatsinking necessary, soft start, output short circuit protection, output over-current protection, overheating protection 64  Silicon Chip unit. Had the switchmode regulator IC used in that project incorporated a programmable under-voltage lock-out feature (as some do), we would have used it but unfortunately the RT8299A, despite its other good points, does not. The RT8299A does have an enable input (pin 6) but this is designed to be driven with a logic signal and it has a low and ill-defined threshold voltage; thus we can’t use a resistive divider to set any kind of accurate threshold. An external voltage reference and comparator are therefore required. Circuit description The revised circuit is shown in Fig.1. The top part is basically identical to the original USB charger circuit shown on page 37 of the July 2015 issue, with one small change we’ll get to later. The added section below is derived from the Battery Lifesaver circuit shown on page 66 of the September 2013 issue. The incoming 12V supply connects to linear low-dropout regulator REG2 which provides both the power supply and an accurate reference voltage to micropower comparator IC1. REG2 has siliconchip.com.au 100nF 50V X7R D1 SSA33L A 8 2 K 6 12V INPUT K 1 + – 2x 10 µF TVS1 PGOOD SW FB GND 4 A CON1 BO O T REG1 RT8299A EN 25V X5R 15V 2 7 Vcc VIN 1 3 6.8k 3 OUT IN 2 1.3k 10M ZD1 16V 1 µF SC 1 µF 2 IC1 4 A 20 1 5 7 3 1M Added protection ZD1 and its associated 1.3kΩ resistor are not shown on the photo of the siliconchip.com.au RT8299A, MCP6541 IN GND 8 OUT 4 1 6 IC1: MCP6541 MINI 12V USB POWER SUPPLY MK2 an output voltage tolerance of ±0.4%. The battery voltage is divided by two resistors, 1.43MΩ and 1MΩ. These values are chosen so that the voltage at the non-inverting input (pin 3) of IC1 drops below the 5V reference at pin 2 once the supply voltage drops below 12.15V. The 10MΩ resistor provides about 0.75V hysteresis, so that if the unit switches off due to a low vehicle battery voltage, it won’t switch back on until the battery rises above 12.86V, ie, the next time the engine is started and the battery starts to charge. So the output of IC1 is high when the battery voltage is sufficiently high and low otherwise. This output goes to the enable pin (pin 6) of REG1 to shut the regulator down when the battery voltage is low. The 100kΩ pull-up resistor originally provided for the EN pin is no longer needed although we’ve left the pads on the PCB. This allows the original circuit to also be built on this PCB, in which case REG2, IC1 and their associated components are simply omitted. VBUS D– D+ GND MCP1703 K A 1 K SSA33L, ZD1 1.43M GND 1 2 3 4 16V X5R +5V VBUS D– D+ GND CON2b 2x 22 µF 100pF 50V COG REG2 MCP1703– 5002–E/CB 1 2 3 4 OUT– 100Ω 1.3k CON2a 50V X7R 5 2x USB TYPE A OUT+ 100nF L1 6.8 µH Fig.1: the circuit is based on an RT8299A switchmode step-down regulator (REG1). TVS1 protects the regulator from transient voltage spikes, while diode D1 provides reverse polarity protection. Comparator IC1 shuts down REG1 if the battery voltage falls below 12.15V. prototype. They were added to the final version to better protect REG2 against supply spikes which are common in vehicles. This is necessary because while TVS1 clamps REG1’s supply below its 24V maximum, this is too high for REG2 to handle, with its maximum rating of 18V. Since ZD1 has a breakdown voltage of 16V, it will not conduct with normal automotive battery voltages (12-15V) but will protect REG2 during the worst spikes. Its leakage current at normal operating voltages is negligible. When the EN pin of REG1 is pulled low and its output is shut down, it draws less than 3µA. REG2 consumes around 2µA and IC1 around 0.7µA. There’s a further 5µA through the battery sense resistive divider for a total of around 10µA. This is well below the self-discharge current of a car battery and a tiny fraction of the load a typical modern vehicle puts on its battery with the ignition switched off. Note that 1.43MΩ seems like an odd value for a resistor but it is in the E96 series and is easy enough to get. Failing that, you can parallel 5.1MΩ and 2MΩ resistors (both E24 values). Paralleling SMD resistors is easily done since they can be soldered on top of one another. Step-down regulator In the July 2015 issue, we explained in detail how switchmode regulator REG1 works. In brief, the voltage at pin 3 (SW) toggles between 0V and the incoming supply voltage (ie, that at pin 2, VIN). When pin 3 is high, current flows through inductor L1 into the output filter capacitors and the load, charging up L1’s magnetic field. When pin 3 is driven low, this magnetic field begins to collapse and as as result, current continues to flow into the load but this time it’s pulled from ground via pin 4. The duty cycle of the square wave output at pin 3 is controlled so that the average voltage at the load is very close to 5V. This is determined by sampling the feedback voltage at pin 5 (FB), which comes from a 6.8kΩ/1.3kΩ resistive divider across the output. When the output voltage is 5V, the feedback voltage is 0.8V and this matches REG1’s internal reference. If the feedback voltage is too low, REG1 increases the duty cycle and if it’s too high, the duty cycle is reduced. September 2015  65 + CON1 1.3k REG2 D1 K 1 µF L1 100Ω 6R8 ZD1 1 µF 1.3k 1.43M 6.8k 1 22 µF OUT+ OUT+ 22 µF OUT– OUT– CON2 10M IC1 100pF 100nF 1 REG1 RT8299 100nF 10 µF 1M MCP6541 10 µF DUAL TYPE A USB SOCKET FOR CON2 (VERTICAL MOUNTING) + − K TVS1 Once again, you will have to pick the closest value you can actually get. 12V 12V − SCREW TERMINALS OR SIL HEADER FOR CON1 – + – + 18107152 Fig.2: follow these top and bottom layout diagrams and the photos to assemble the PCB. Take care with the orientation of REG1, TVS1 & D1 on the top and REG2, IC1 & ZD1 on the bottom. Note that the photos show a prototype PCB. The capacitor between pin 3 and pin 1 (BOOT) is used to generate a voltage of around 10V (the output voltage plus 5V) which REG1 uses to drive the gate of its internal Mosfet in order to pull pin 3 high. The series capacitor and resistor from pin 3 to ground form a snubber to reduce the rate of voltage change at this pin, cutting down on EMI. Increased output current The literature provided for REG1 suggests a filter inductor value of around 2.2µH. Choosing switchmode inductor values can be quite tricky as there are trade-offs. The advantage of low values such as the 2.2µH suggested is that since they require fewer turns of wire, the wire can be shorter and thicker, thus lowering resistive losses. They can also be physically smaller both due to less wire and a smaller core. However, lower inductance does mean more output ripple voltage, while changing the inductor value changes the time constants in the switchmode feedback loop and can affect stability and transient response. The 10µH inductor specified for the original version of this circuit has a continuous current rating of around 2.5A. This time, we tried a similarly-sized 6.8µH inductor with a 3A continuous current rating and a saturation current of 3.9A. Thus we can now obtain the full 3A output specified for REG1 while the ripple level is still very low. Along with the PCB, we’ve been supplying a kit of parts for the USB charger project that includes all the SMDs. We’ll do the same for this re66  Silicon Chip vised version and it will include the new inductor plus the extra components for the low-battery cut-out. Changing the voltage thresholds The 12.15V threshold will suit most lead-acid, AGM or SLA/gel cell batteries and should leave sufficient charge to start a motor. However, it’s possible some constructors will want to change this, eg, if powering the unit with a Li-ion, LiPo or LiFePO4 battery instead. The easiest way to do this is to replace the 1.43MΩ resistor with a different value – if necessary, by parallel­ing two standard values. For a desired cut-out threshold Vco, calculate the required value Rdiv in ohms as: Rdiv = (Vco -5V) x 200,000 and pick the nearest value available. For example, for a 12.5V threshold, use a 1.5MΩ resistor. Calculate the threshold from the chosen resistor value as: Vco = (Rdiv ÷ 200,000) + 5V The cut-in threshold Vci is then: Vci = (Rdiv ÷ 181,818) + 5V If you need to lower this (ie, reduce hysteresis), increase the value of the 10MΩ resistor; 15MΩ, 22MΩ and 33MΩ SMD resistors are available. Alternatively, to increase hysteresis, lower the value of the 10MΩ resistor. The hysteresis voltage Vh for a feedback resistor Rfb is calculated as: Vh = (Vco - 5V) x 1,000,000 ÷ Rfb For the value specified, this gives 0.715V. If you need a particular cut-in voltage, calculate the resistor value thus: Rfb = (Vco - 5V) ÷ (Vci - Vco) x 1,000,000 Construction Fig.2 shows the new component overlay diagrams. The top side is virtually identical to that shown in the July issue; all the extra parts have been added to the bottom. The revised PCB is coded 18107152 and measures 16 x 51mm. It’s easiest to fit all the components to the top side of the board first. Start by fitting REG1. While an SOIC-8 package is generally easy to solder, this one has a thermal pad on the underside which is also supposed to be soldered to the board. To do this properly, you need to use a hot-air rework station. These are available from eBay sellers for around $50 (eg, search for “Atten 858d”). If you have one of these, simply apply some solder paste to each pad, place the IC on top, check its orientation carefully (pin 1 to upper left) and then heat the IC and its leads until the solder reflows. Be sure to continue heating it long enough for the solder on the thermal pad to melt also; you can usually see fumes from the flux escaping under the IC. While we recommend this method and it’s how we built this prototype, it is possible to solder the chip by hand. To do this, first place a small amount of non-conductive (silicone-based) heatsink paste on the central pad and clean the residue off the other pads. Then tin one of the eight remaining pads, carefully place the IC in position and reheat that pad while pressing down gently on the IC until its lead contacts the PCB. Once it’s in place, check the alignment, then solder the remaining seven pins and add some solder to that first pin to refresh the joint. Any solder bridges between pins can then be easily cleaned up using solder wick. Note that it’s best to avoid moving the IC by much during soldering, so that the heatsink paste is not spread around. Also, don’t clean the board using any solvents as these are likely to wash the paste away. We used the hand-soldering method successfully when building the original version shown in our July issue. One of the most common problems with soldering an IC like this is that it’s possible to get solder on a pin without it actually flowing onto the corresponding pad. As a result, it’s best to check all eight leads under a magnifying lamp siliconchip.com.au to make sure the solder fillets have properly formed. With REG1 in place, inductor L1 is next. This is a little tricky due to its high thermal inertia. There are various methods but the simplest is to treat it like a large chip component. This involves adding a fair bit of solder to one of the pads, enough that it’s visibly built up, then heating this solder while sliding L1 into place along the surface of the PCB. It’s easiest to do this while holding it with angled tweezers. As soon as L1 hits the solder, some of it will cool and solidify. You will have to hold the iron in place while L1 heats up and the solder will then re-melt. Once that happens, you can finish sliding L1 across into the correct position between the two pads. You can then flow solder onto the opposite pad. Note that it’s best to do this immediately before L1 cools down. Note also that it will take a little while to apply enough heat to form a good joint. You will then need to go back and add some more solder and heat to the initial pad, until you get a similarly good fillet on that side; much of the flux will have boiled off during the initial soldering process. Alternatively, use solder paste and a hot air wand although you will probably need to hold the inductor in place using steel tweezers or the hot air may blow it out of position. The rest of the components are easier as they are substantially smaller but you can use the same basic idea of adding solder to one pad and then sliding the part into place. The only remaining polarised components are D1 and TVS1; in each case the cathode (striped) side goes towards the nearest edge of the PCB. Don’t get any of the different value capacitors, resistors or diodes mixed up. The resistors will have printed value codes on the top but the other components are likely to be unmarked so you will have to remove them from their packaging one at a time and immediately solder them to the PCB. Bottom side components Now you’ll need to flip the board over but it won’t sit straight due to the components sticking up, especially L1. To solve this, get two heavy objects of the same thickness (eg, timber off-cuts) and space them apart on your bench so that each end of the board can rest on siliconchip.com.au Parts List 1 double-sided PCB, code 18107152, 16 x 51mm 1 6.8µH 3A RMS (3.9A saturation) 6x6mm SMD inductor (L1) (Digi-Key ASPI-6045S-6R8MTCT-ND, element14 2309891) 1 2-way mini terminal block or pin header (CON1) (optional) 1 dual stacked vertical type-A USB socket, through-hole mounting (CON2) (element14 1841169, Digi-Key ED2984ND) OR 2 vertical or horizontal type-A USB sockets, through-hole mounting (CON2) (element14 1696534/1654064, Digi-Key UE27AC54100-ND/ UE27AE54100-ND) 1 50mm length of 20mm-diameter heatshrink tubing Semiconductors 1 RT8299AZSP 3A switchmode step-down regulator IC (REG1) (element14 2392669, Digi-Key 1028-1295-1-ND) 1 MCP1703AT-5002-/CB or MCP1703T-5002-/CB 5V LDO regulator (REG2) (element14 1439519, Digi-Key MCP1703AT-5002E/CBCT-ND) one or the other, with the components hanging down in the gap in between. Now solder IC1 in place. This is similar to REG1 but doesn’t have a thermal pad so you can simply tin one pad, slide it into place while heating that pad, then solder the rest of the pins. Be sure that its pin 1 dot is orientated as shown in Fig.2 and clean up any solder bridges between pins using solder wick and a small amount of flux paste. Next fit REG2 and ZD1, which are both in SOT-23 packages; don’t get them mixed up. It’s then just a matter of installing the remaining passives, ie, four SMD resistors and two 1µF ceramic capacitors where shown. Connectors The PCB has provision for a pin header or terminal block as the power input, or you can simply solder wires to the two pads – if in doubt of the polarity, check Fig.2. For the outputs, there is space for one or two on-board USB sockets, either a vertical or horizontal type-A 1 MCP6541-(I/E)/SN micropower comparator (IC1) (element14 1557429, Digi-Key MCP6541-E/ SN-ND) 1 3A 30V Schottky diode, DO-214AC (D1) (element14 1843685, DigiKey SK33A-TPCT-ND) 1 SMAJ15A SMD 15V 400W TVS or equivalent (TVS1) (element14 1886343, Digi-Key SMAJ15ALFCT-ND) 1 BZX84B16 16V 0.25W zener diode (ZD1) (element14 2463473, Digi-Key BZX84B16FDICT-ND) Capacitors (all SMD 3216/1206*) 2 22µF 16V X5R/X7R 2 10µF 25V X5R/X7R 2 1µF 50V X7R 2 100nF 50V X7R 1 100pF 50V C0G/NP0 Resistors (all SMD 3216/1206*, 1%, 0.25W) 1 10MΩ 1 1.43MΩ (element14 2139709, Digi-Key RHM1.43MCJCT-ND) 1 1MΩ 2 1.3kΩ 1 6.8kΩ 1 100Ω * 2012/0805-size parts are also suitable USB socket, or a vertical dual type-A USB socket. However, as explained in the July issue, many constructors will prefer to run wires from the OUT+ and OUT- pads to one or more surfacemounting USB sockets, depending on the exact application. There were also detailed instructions in the July 2015 issue on how to install the unit in the overhead binnacle in a typical modern motor vehicle. We also explained back then that, should you wish to use the PCB as a general-purpose 3A step-down regulator, you could change the 6.8kΩ feedback resistor to obtain any output voltage from 0.8V up to about 10V. Should you wish to do this, the new resistor value is simply calculated as: R = (Vout ÷ 0.8 - 1) x 1.3kΩ If you plug 5V into this formula you will see that the result is very close to the 6.8kΩ value specified. Finally, check that the unit works, ie, gives a 5V output for a 13-15V input, then encapsulate it in heatshrink tubing to protect it and prevent short SC circuits. September 2015  67