While designing USB Isolator I needed a 5V supply to power the device side. I picked Linear Technology LT1376-5 part which proved to be pretty reliable and mellow converter. I’ve had used this part for 3.5 years now, in a product and also in the kit and I haven’t had any serious issues with it. Some time ago I decided to put together another kit based on the same controller – at this time containing only PTH components to make assembly easier for beginners. This kit is now available at the store for $20 plus shipping and the following is a short overview of the capabilities.
The kit contains a PCB plus all parts necessary to build a DC-DC step-down converter. Voltage setting resistors to make output of 3.3V, 5V and 12V are included. The converter is capable of supplying 1A at any of the output voltages over 6-25V input voltage range ( 23Vmax for 12Vout), and can be pushed up to 1.3A if input and output voltages are close enough. Output voltage ripple is 50mv@1A.
The biggest advantage of switch mode supplies is efficiency. It is possible, for example, to step down from 24V to 3.3V without generating excessive heat even at 1A load. Also, switchers are fascinating circuits, fun to build and play with.
In addition to full kit, a bare PCB is offered for makers who prefer to source parts themselves. Eagle CAD files of the circuit are also available.
120ma Inductorless DC-DC converter
Today’s electronic projects often require more than one supply voltage. 5V is often used to power MCUs, while sensors and peripheral interface controllers usually need 3.3V – if both types are used in your circuit, both voltages must be provided. Typically, one of the voltages is produced from the main supply (battery, wall wart, etc.) and the second voltage is then derived from the first using either linear LDO regulator if the second voltage is lower than the first or a boost converter if the second voltage is higher.
The boost converter I’ve designed uses Microchip MCP1253 charge pump controller. It doesn’t use an inductor and is capable of providing selectable 5V or 3.3V output voltage and up to 120ma of output current. The converter is half-inch long and weighs just 0.5g. All through-hole pads are placed on 0.1″ grid making it breadboard-friendly. Thanks to inductorless desgn, the converter is inexpensive, stable, and easy to use while providing automatic switching between step-down and step-up modes, and offering extra options, such as external shutdown and supply monitoring. The following picture shows board connections (click on it to make it bigger):
- Vin, Vout, GND are the only connections necessary out of the box (they are marked on the other side of the PCB). Input voltage can be 2.0-6.0V, output voltage is set to 5V. The converter automatically switches from buck to boost, therefore it is possible, for example, to have regulated 5V output in the whole discharge range of a battery of four alkaline cells (3.6-6V), or 3.3V from a single LiPo (if you switch output voltage to 3.3V, see below). The output current is 120ma, maintained down to 2.8 Vin at 5V Vout
- 3-pad Vout set jumper is set to 5V by a shorting middle pad to the right one. To switch output voltage to 3.3V remove the short using solder wick and short middle pad to the left one
- Shutdown The converter can be stopped by pulling this signal low. By default, it is hardwired to Vin, in order to use it, you need to cut a trace inside Shutdown enable jumper. The jumper can be later closed, if necessary, similarly to output voltage selector
- Power Good This signal falls low when output is out of regulation, which can indicate low battery or excessive load. In order to use this signal, a 100K 0603 resistor must be soldered on vacant pads indicated by the arrow Power Good Resistor
Design files are available for download. Fully populated and tested boards are offered for sale at the store. If there is enough interest, I can also offer blank PCBs as well as boards preconfigured for 3.3V output voltage. Please leave a comment if you’d like these options added.
In August of last year, I wrote an article describing a design based on Texas Instrument’s TPS61200 low input boost converter. Soon after that, Sparkfun expressed interest in producing and selling this design. In just six short months, a LiPower has become available from Sparkfun store. It is a switchable 5V/3.3V boost converter designed to run from single cell Lithium-Polymer battery.
The TPS61200 converter is extremely versatile. It will start into full load from 0.5V, it can output decent current, it can operate when input voltage is higher than output, it can be programmed to switch off at certain input voltage level, preventing rechargeable battery from going into polarity reversal. It can also do many other neat things – to get an idea, take a look at Application Notes section on TI site. However, it is very hard to make a product capable of all this neatness at the same time. Out of the box, LiPower is exactly what is stated on the product page – the output can be switched from 5V to 3.3V and the undervoltage lockout (UVLO) is set to 2.6V, which is minimum safe voltage for LiPos. In this article, I will show you how to modify converter’s UVLO threshold to make it suitable for other types of batteries. I will start from very simple mod which eliminates UVLO completely and then explain more advanced modification, where UVLO can be set to a certain voltage.
1. Simple UVLO eliminator
tps61200 UVLO-off mod
If you want to use primary cells, such as alkaline or non-rechargeable lithium, you don’t want your switch to turn off at 2.6V. On the contrary, you want your supply to run until the last drop of juice gets sucked out of the battery. To set UVLO to the minimum possible value, which is 250mV, UVLO pin must be tied to Vin; the easiest way to do it is to place a short across resistor R3 (see schematic
). A picture on the left shows how to achieve this – the green asterisk marks the place on the boards where mod is located.
Take a 4-6″ piece of bare thin wire, tin one end of it. If you have a vise, clamp the board so that JST connector is on the left and component side is on top. Apply some liquid flux to R3 and C2. Take wire in the left hand and place tinned end between R3 and C2. Heat up with soldering iron until soldered. Cut the excess wire leaving a little extra in case you later decide to revert to the original configuration.
To demonstrate that you don’t need to be NASA-certified electrical assembly technician to solder SMT parts I tried to produce as sloppy a soldering job as I possibly can – despite somewhat scary look modified circuit works just fine. Look closely and you will notice that I inadvertently de-soldered R3 from its place. Should this happen to you, solder it back somewhere so you won’t lose it.
This is it – the modified LiPower will work as long as input voltage is higher than 250mV. Note, however, that maximum output current is going to be low as well. According to figure 1 of the datasheet, it will be around 50ma depending on output voltage.
One last thing about this mod. Since R4 – the lower resistor of the voltage divider is left in place, some current will flow through it all the time. The amount of current depends on input voltage and can be calculated using Ohm’s law; for example, for 3V input the current through R4 is 13uA (it’s micro-amperes). If such amount of current is of concern, remove R4 from the board (it is located on the right of R3) and make sure its pads are not shorted together.
Continue reading TPS61200 board modifications. Part 1 – changing undervoltage lockout
1.2V to 5V DC_DC converter based on TI TPS61200
About a year ago, while researching low startup voltage DC-DC converters I ran into Texas Instruments’ TPS61200. This monolithic synchronous rectifier boost converter has several nice features. First, the input voltage range starts at 0.3V; therefore, it’s possible to run the converter from low-voltage source such as single solar cell or supercapacitor. Second, the converter is powerful – up to 1.8A for certain input/output voltage combination. (“Certain” is a key word here – see below for explanation). Another nice feature is the ability to down-regulate the output when Vin exceeds Vout; for example, you can configure the converter to run from single-cell Li-ion/Li-poly battery and output stable 3.3V over the whole 4.2V-2.7V Li-Poly range. In addition to all that, the controller has built-in undervoltage lockout feature – minimum input voltage, below which controller would shut itself off can be set with simple voltage divider. This feature comes handy when rechargeable battery is used as power source. TPS61200 also has pins for enabling/disabling and power-saving mode on/off. Device is manufactured in 2 fixed Vout configurations – 3.3V, 5V, plus adjustable variant. Maximum working input voltage is 5.5V, minimum output voltage for adjustable part is specified at 1.8V.
After much prototyping and testing I came out with a layout that works well. The result can be seen on title picture – Arduino Duemilanove board (running USB keyboard polling sketch ) , USB Host Shield, LCD display and USB keyboard all powered by single 1.2V NiMH AA cell. The circuit that makes it possible can be seen on the left side of the picture connected between the battery and USB B connector, which is used here as 5V power connector to the Arduino.
The run time of this setup from freshly charged 1800ma NiMH cell is 6 hours which makes it quite practical. It should be noted that when input voltage is much lower than output voltage, efficiency suffers ( see figure 8 in the datasheet. 3 NiMH cells in series or single-cell Li-Poly is much better for 5V output, even 4 NiMHs will work if load is light – in down conversion mode power losses in the converter increase and I found that the chip gets very warm with output current of 200ma or more while down converting.
Continue reading Designing DC-DC converters using TI TPS61200 controller
Magnetic probe amplifier connected to external trigger input
I made (hopefully) last iteration of magnetic probe amplifier board. Schematic remains the same. Layout, however, is slightly different. First, I made it more narrow to better fit Tektronix 7000-series time base plugins external trigger input, as can be seen on the title picture. Second, the amplified probe output is made via SMx type connector – PCB-mounted SMA and SMB all have the same footprint. I used straight SMB since I have a surplus of Tektronix P6041 cables. The board layout permits soldering right-angle connector here as well. This arrangement is much handier than previous one.
Since publishing initial design I haven’t seen much interest in it, so instead of ordering a bunch of PCBs I made this board available on BatcPCB Marketplace. Schematic and board layout in Eagle 5.x format are also available. I built one board and haven’t found any errors on copper or silkscreen – if you find any, please let me know.
Magnetic probe amplifier connected
Recently, I was researching low-noise DC-DC converters and while reading Linear Technology Application Note 70
found this clever and useful circuit, designed by Jim Williams. The idea is to sense current in power inductor of the converter with another inductor, placed within short distance from the first one. The sensing inductor is connected to a circuit which amplifies and conditions the signal and generates nice clean square wave pulses which can be used to trigger oscilloscope sweep. The probe is isolated from the circuit preventing measurement corruption. As a bonus, analog output of probe amplifier allows observing current waveform through power inductor.
As is often the case with application notes, circuit description and build details are somewhat brief; I’m posting my notes hoping that the information will be helpful for other builders. Also, since BatchPCB doubled my order, I have extra PCBs; if anyone wants to build this circuit on a professionally made PCB with just couple green wires, e-mail me – the PCB can be yours for the price of postage.
Continue reading Magnetic probe amplifier
3.3V to 5V DC-DC converter
During development of Arduino USB Host Shield I designed small and simple boost converter to provide 5V to Vbus from 3.3V input. The circuit, built around Linear Technology’s LTC3426, worked so well, that I decided to release a standalone version. There are many uses for such converter – LCD contrast bias being most typical. Another example when 3.3V to 5V converter could be handy, is old style Arduino shields. This photo shows my converter sitting on empty space of 3.3V-only Arduino Pro board.
Maximum output current of the converter is 650ma. It can be loaded up to 700, but inductor becomes warm. Output ripple is around 25mv at 500ma. If less ripple is desired, 3.3uH inductor can be used. Output capacitor can be increased also, hovever, benefits are marginal. Measured efficiency of this circuit is around 90%.
Project files, including Eagle schematic and layout files, as well as Gerbers, are available from Downloads section. Design rules are pretty relaxed and board is routed almost on a single side (the other side is a ground plane), so making one at home should not be a problem. Also, PCBs and assembled and tested converters are available in the store.