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.
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