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.
Another non-obvious characteristic of this controller is maximum possible output current to input voltage relation. Figure 1 in the data sheet shows that for input voltages up to 0.6V output current is really small – less than 50ma. This is the current one should expect while running the converter from single 0.55V solar cell. Voltage drop due to battery discharging shall also be considered. For example, fresh Li-Poly cell has 4.2V terminal voltage. According to Figure 1, this gives output current of 1500mA. However, at the end of discharge (2.7V) max.output current drops to ~800ma and this current should be taken as maximum for the whole range.
The controller has built-in over-current and over-temperature protection so experimenting with heavy loads won’t kill it. I found it running not too hot with output current up to 1A. More current is quite possible when input to output voltage ratio is small and cooling is adequate.
Now let’s talk about choice of power sources. A pair of AA or AAA alkaline batteries will make good source for 3.3V generation. 3 will work well for 5V. As little as one can be used if weight and/or volume of a circuit has to be reduced. Absolute max.input voltage for TPS61200 is specified at 7V, therefore, for maximum run time a series of up to 4 AA cells can be used with big loss of efficiency at the beginning of discharge, until input voltage drops below output voltage. Also, solar cells and super capacitors can be used when loads are really light. Rechargeable batteries are another good option; however, since TPS61200 is capable of working with input voltage down to 250mv, special precautions must be taken when using rechargeables with this controller.
Draining rechargeable cell below its end-of-discharge voltage may cause damage to the cell. As a result, a cell may become non-rechargeable and Lithium cell may even overheat and catch fire. For NiMH cell, end-of-discharge is 0.8V; for Li-Poly cells recommended end-of-discharge is 2.5-2.7V. To work properly with rechargeable power sources TPS61200 has built-in undervoltage lockout circuitry – when voltage on UVLO pin drops below 250mV, the converter turns off. Therefore, desired end-of-discharge voltage can be easily set with voltage divider ( R3, R4 of the schematic). UVLO pin also has hysteresis of 50mv and this hysteresis gets multiplied by dividers’ ratio; for example, when UVLO is set to 2.5V, minimum turn-on voltage of the converter will be 3V. Capacitor C4 takes care of this issue by shorting resistor R3 during turn-on.
The closeup picture shows TPS61200-based DC-DC converter designed for 5V output and Li-Poly input. Blue part marked “0″ close to the low right corner of the PCB is a jumper across R5 (I’m using fixed-otput TPS61202 here) and two resistors placed along the edge of a board make UVLO divider. JST connector polarity is compatible with Sparkfun single-cell Li-Po batteries.
Project files can be downloaded from Downloads section. Also, PCB is available at BatchPCB. TPS61200 comes in 10 pin QFN package with thermal slug on the bottom of the package, which has to be soldered on the PCB for proper cooling. Other parts are size 0603 and 1206. Both surface mount and through-hole JST connectors can be used on the input side.
I’m getting ready to start producing this converter in 3.3V and 5V variants with 0.8, 1.6, and 2.5V end-of-discharge voltages. Please let me know if any other output or end-of-discharge voltages are necessary. Also, any other comments about this design are most welcome!
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