Chip Boosts Solar Cell Prospects in Portable Designs

Apr 1, 2007 12:00 PM
By David Morrison, Editor, Power Electronics Technology

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Asynchronous stepup converter IC from Texas Instruments lowers the minimum input voltage required for both normal operation and startup, making it possible — perhaps for the first time — for some portable applications to run off of novel energy sources. The TPS61200, which includes an integrated 1.5-A switch, can operate with input voltages lower than 0.3 V with high efficiency. But even more impressive is its ability to start up into full load from a 0.5-V input.

That low-voltage startup capability enables the converter to boost the output of a single solar cell or micro-fuel cell, something that conventional boost converter ICs cannot do. At present, existing boost converters require at least 0.85 V of input for startup — a requirement dictated by their bandgap voltage references. Moreover, that number is often specified as a “typical” minimum for startup, while the guaranteed minimum startup voltage may be up to 1 V.

The TPS61200 owes its performance to a two-stage boost circuit. The first stage is an inductive boost converter that starts up without regulation and charges a small capacitor connected to the V_AUX pin. Once that capacitor is charged, the second boost stage starts up and powers the main power switch, which powers the load.

The ability to operate from a single solar cell eliminates the need to string multiple solar cells in series as well as the associated protection circuitry. This opens the door to new, potentially innovative designs such as a built-in solar-powered cell-phone charger that uses indoor ambient lighting to help provide an infinite amount of standby time.

“Portable equipment designers, including those in the portable medical, wireless and portable audio fields, are continuing to look at how to apply solar- and fuel-cell technology into their devices to extend battery life and differentiate their products,” said Uwe Mengelkamp, director of TI's dc-dc converter product line. The IC's low operating voltage also eliminates many of the design challenges that occur when operating off a single-cell alkaline, NiCd or NiMH battery.

The TPS61200 supports input voltages of 0.3 V to 5.5 V during normal operation and continues to manage power down to 0 V if the undervoltage lockout pin is connected directly to the output voltage. The converter operates with more than 90% efficiency at 600-mA output with V_IN ≥ 1.2 V and V_OUT = 3.3 V or with V_IN ≥ 3 V and V_OUT = 5 V.

In addition to its boost capability, the chip can operate in a down-conversion mode using one of the two internal switches. The TPS61200 automatically transitions between boost and down-conversion modes when the input voltage moves above or below the output voltage.

The TPS61200 features an adjustable 1.8-V to 5.5-V output. However, two fixed-output versions also are available for applications that need only a 3.3-V (TPS61201) or 5-V (TPS61202) output.

Housed in a 10-pin, 3-mm × 3-mm QFN, the TPS61200 offers several features that are critical during low-voltage operation, including output short-circuit protection, and programmable undervoltage lockout. In addition, the converter can be disabled to further minimize battery drain. While the end equipment shuts down, the TPS61200 protects the system from receiving any additional power from the battery.

Designers can simplify power design with the TPS61200 by leveraging TI's Analog eLab Design Center, www.ti.com/analogelab, which features TI's SwitcherPro power-supply design tool and a new release of the TINA-TI 7.0 simulation environment, which includes switch-mode power-supply simulation with acceleration.

The TPS61200 costs $1.68 each in quantities of 1000. An evaluation module, the TPS61200EVM-179, and application notes for this chip are available at power.ti.com.

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