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REVIEW: 300-mV input-voltage boost converter throttles micro-fuel cells

TI Boost Converter Enables Solar and Micro-Fuel Cell Innovation in Portable Electronics

0.3-V Input Power Management Chip with 0.5-V Start-Up Allows Battery-Driven Devices to Tap New Wave of Low-Power Energy Sources

DALLAS—Texas Instruments Incorporated (TI) introduces the industry's lowest input voltage DC/DC boost converter, which will enable portable electronic end-equipment to draw power from new energy sources, such as solar and micro-fuel cells. The tiny power circuit can operate with input voltages lower than 0.3 V with high efficiency, allowing designers to overcome the low-voltage design barrier of incorporating these alternative energy sources in applications, such as mobile phones, portable medical devices and media players.

See: www.ti.com/sc07062.

The new TPS61200 step-up converter with integrated 1.5-A switch supports input voltages of 0.3 V to 5.5 V during normal operation, and continues to manage power down to 0.0 V if the under-voltage lockout pin is connected directly to the output voltage. The converter provides an extremely low 0.5-V start-up capability in any load condition, and can operate with more than 90 percent efficiency. In contrast, today's best step-up converters can only support an input voltage beginning at 0.7 V with start-up at 0.9 V—good for primary re-chargeable battery cells or main supplies, but not low enough to support new applications using energy harvesting power sources.

The TPS61200's ability to operate from a single solar cell eliminates the need for multiple solar cells in series, and eliminates the required protection circuitry associated with series connection. This opens the door to new potential 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. “It's not certain what we will see first, but the ideas are endless. What is certain is that designers can use the TPS61200 to achieve low-voltage operation to connect these new low-power energy sources with the ICs in their system.”

Protecting the System at Low Voltages

The TPS61200, which comes in a 10-pin, 3 mm x 3 mm QFN package, offers several features that are critical during low-voltage operation, including output short-circuit protection, programmable under-voltage lockout and a unique “down-conversion mode” that helps protect the device when an input voltage rises higher than the output voltage. 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.

Extends the Operating Range of Single-Cell Alkaline, NiCd and NiMH Batteries

The integrated circuit's extremely low operating voltage also eliminates many of the design challenges that occur when operating a single-cell Alkaline, nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) batteries to power anything from toys to portable medical devices. The TPS61200 extends the operating time of many pulsed-load applications that experience pass-load conditions at low-voltage inputs.

SwitcherPro and TINA-TI 7.0 Design Tools

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

Pricing and Availability

The TPS61200 boost converters are available today in volume from TI and its authorized distributors. The device comes in a 3 mm x 3 mm, 10-pin QFN package. Suggested resale pricing in quantities of 1,000 units is $1.68. The TPS61200EVM-179 evaluation module, application notes and TI's new Power Management Selection Guide are available through power.ti.com.

These unusual DC-to-DC boost converter chips from Texas Instruments (TI) not only stack up as precision high-efficiency devices, but also pack hefty FET pass elements that can supply as much as 1.5-A if need be. But, they typically sip less than 55-µA of quiescent current, and only about 0.5-µA in a shut-down mode. That's in lockstep with low-voltage operation and emerging low-power energy sources such as micro-fuel cells.

A micro-fuel cell is a power source that converts chemical energy into electrical energy by means of an electrochemical reaction of a fuel in the presence of a catalyst.

Some micro-fuel cells use methanol as fuel, for example, releasing carbon dioxide and water vapor as byproducts. It's expected that high power-to-volume ratio micro-fuel cells will last longer than today's batteries, and will be able to be re-charged simply by popping in a new methanol cartridge.

The amalgamation of fab technologies in TI's chips is noteworthy. Part of it accrues to the company's well known BiCMOS process technology that uses CMOS switches for the chip's start-up circuitry. It's married with area-efficient low RDS ON LDMOS (lateral DMOS) series-connected power transistors in the device's synchronous rectifier blocks.

No Extra Discretes

The IC's start-up circuitry also means you can use the same external discrete devices and inductor element as the main power circuit. The IC functions as a 2-step inductive boost converter, with a seamless transition between the start-up circuits and the switcher.

Not mentioned in TI's press release (on the left) is the fact that three flavors of these ICs are available, for both fixed output and adjustable output applications. The TPS61200 part number the fully adjustable converter that accommodates 1.8-V to 5.5-V loads. The TPS61201 is a fixed 3.3-V device, and the TPS61202 is a fixed 5-V device.

Let's look a bit more closely at the versatile TPS61200. In operation, its output voltage is programmed by means of a resistive voltage divider. Alternatively, output voltage can be fixed at a given setting, within the IC's range, on-chip.

Zero Current

Significantly, a converter can be totally disabled to stop any current from being drawn from a system's battery. Essentially, the system is completely disconnected from the load.

When so disconnected, there's no leakage current. That's important in applications where you want to squeeze every bit of energy out of a source such as a small solar panel or a micro-fuel cell.

Boost Topology

Back to the TI silicon. As a boost topology converter, the TPS61200 uses a fixed-frequency PWM (pulse-width-modulated) design with synchronous rectification. Three on-chip N-channel MOSFET s handle power conversion under all possible operating conditions.

This enables the device to operate over a wide input-voltage and output power range. Moreover, the device maintains greater than 90-percent efficiency, delivering 600-mA of output current at 3.3-V with an input of 1.2-V, or 600-mA at 5-V with an input of 3.3-V.


Click to view graph

Synchronous conversion is quite efficient, but at light loads, the converter also enters a Power Save mode to maintain efficiency over a wide load current range. The transition between boost mode and the down conversion mode is automatic. Notably, the Power Save transition can be disabled if you want to force operation at a fixed frequency.

The Power Save mode can be disabled by logic, but in the device's down conversion mode, the Power Save mode is always enabled. That means the chip can't be forced into fixed-frequency operation at light loads.

Get Down To It

A word about down conversion: most boost converters only regulate output voltages that are higher than the input voltage. But, the TPS61200 operates differently.

For example, it can regulate 3-V at its output with two alkaline cells at the input having a total cell voltage of 3.2-V. You could, for example, power white LED s (white LEDS operate with a forward voltage drop of 3.6-V) from a charged Li-ion cell that has an output of 4.2-V.

In these applications, if the input voltage reaches or exceeds the output voltage, the converter automatically changes to the down-conversion mode, and control circuitry changes the behavior of the device's MOSFET switches.

In a solar cell application (curve, above), the output voltage at a no-load condition is slightly higher than 0.5-V, and the required start-up voltage for a connected DC-to-DC converter is 0.5-V. Maximum power is available between 0.4-V and 0.5-V of cell voltage, so the required minimum operating input-voltage for a connected DC-to-DC converter is an amazingly low 400-mV. TPS61200s to the rescue!

The Elusive Eval Module

Finally, TI's press statement mentions the TPS61200EVM-179 eval module. Unfortunately, the release of the ICs doesn't appear to coincide with the release of the module.

At this time the TPS61200EVM-179 is in a so-called preview state. That is, it's been announced by TI, but it isn't in production, and samples may or may not be available. However, TI's Analog eLab Design Center is available, and presumably, support for the TPS61200 will be forthcoming in step with availability of silicon.

Click here for a datasheet (in Adobe Acrobat .PDF format).

For more details contact Texas Instruments Inc., 12500 TI Blvd., Dallas, Texas 75243-4136. Phone: 800-336-5236 or 972-644-5580.

Texas Instruments , 800-336-5236, www.ti.com

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