Power Electronics

DC-DC Converters Make the Most of Single-Cell Batteries

In the quest for the ultimate in user comfort, OEMs continue to reduce the size and weight of portable personal equipment. Whether it's a cordless phone, a palmtop, or an MPEG player, the battery pack continues to throw obstacles in the way of the optimum ergonomic solution.

A move from multicell to single-cell backed circuits addresses the size and weight issue head on, but the design challenge facing OEMs remains a considerable one. Portable equipment designs must accept only the highest circuit efficiency and the longest possible battery life, while remaining fully tolerant of an unavoidably descending power source — all within the usual cost constraints.

Integrated dc-dc converter chips already represent some of the greatest strides made in single-cell portable equipment design. The popular synchronous converter variety retains several weaknesses. Notably, an inability to start under full load, and an inherent cost that often makes it an unaffordable luxury. Alternative non-synchronous types also have drawbacks, namely an inability to meet efficiency targets or to handle the lower operating voltage limits.

One solution is to adopt a design methodology that combines the latest thinking in integrated dc-dc control with that of high performance discrete semiconductor device technology. This type of dc-dc converter avoids the synchronous solution drawbacks and significantly improves the performance of non-synchronous integrated solutions.

Meeting this need is the ZXSC100, designed for portable applications requiring step-up voltage conversion from a low input voltage such as that delivered by a single Alkaline, NiCd, or NiMH cell. The ZXSC100 is a non-synchronous PFM, dc-dc controller IC which, when combined with a high performance switching transistor, creates an efficient boost converter solution. Two possible applications for the device are cordless phones and LED flashlights.

Cordless Phone DC-DC Converter

For the cordless phone, the ZXSC100 based dc-dc converter solution (Fig. 1) features the ZXT14N20DX bipolar transistor and ZHCS2000 Schottky diode. All components are surface-mount type, with the controller chip's compact MSOP8 packaging half the size of SO8 devices.

Unlike alternative synchronous solutions, the dc-dc converter starts under full load without any compromise on efficiency, which is 82%. This is possible even as the battery discharges down to 1V, with operation maintained down to a minimum operating input of 0.926V.

The primary consideration in moving the cordless phone to single-cell operation is to reduce size, weight, and cost. However, it's essential to maintain talk and standby times. Battery capacity is a critical factor. Single cells offer sufficient capacity providing the optimization of the circuit efficiency. Efficiency of the ZXSC100 configuration was a paramount concern, influencing the choice of circuit components. Fig. 2, on page 34, illustrates the efficiency of the final solution. The characteristic curves demonstrate how any degradation in efficiency averts at low input voltage.

Transistor Selection

In such a circuit configuration the choice of pass transistor has a crucial impact on efficiency. Due to its very low saturation voltage, typically 45mV at a load of 1A, the SuperSOT4 ZXT14 achieves the highest conversion efficiency, even below 1V. The low VCE(SAT) minimizes the on-state losses, while the device's high gain minimizes losses due to base current. Fig. 3 shows the saturation characteristic of the ZXT14N20DX.

The transistor also has an influence on the maximum output voltage of the solution. The ZXT14 has a VCE characteristic enabling a maximum output of 20V; however, you would typically program the circuit to give the more common 3.3V output. You can program the output voltage using a pair of divider resistors (R3 and R4) — the minimum governed by the supply applied to the circuit, with the lowest possible being the minimum battery voltage itself.

To ensure circuit losses are minimized, the ideal rectifier diode should have a low forward voltage and a fast recovery time, it should also be selected so that the maximum forward current is greater than, or equal to the maximum peak current in the inductor. In addition, the maximum reverse voltage should be greater than, or equal to the output voltage.

The ZHCS2000 Schottky diode, D1 in Fig. 1, meets these design criteria and is perfect for this application. With a forward voltage of 385mV at a load current of 1A, the rectifier diode introduces minimal losses to the circuit.

There are three other components required to configure the ZXSC100 solution. The inductor L1 is chosen to ensure a core saturation current well above the peak operating current of the circuit. The value of the resistor R2 governs the peak current while R1 sets the maximum base current available to the drive transistor.

Drive Options

In other applications where matters of size or cost are even more significant, alternative pass transistors can be introduced. As examples, the SOT23-6 packaged ZXT13 offers additional space savings while the FMMT617 SuperSOT in the SOT23 package provides cost advantages.

Since a low saturation voltage characterizes these bipolar transistors, such pass transistor selections have a minimal effect on losses with an optimum circuit efficiency. For higher power solutions, you can also include a low-cost base boost transistor.

LED Flashlight

LEDs are favored over traditional incandescent bulbs in flashlight applications as they convert more of their input energy to light rather than heat. Any increase in efficiency here directly converts into an extension of useful battery life. LEDs also withstand shock, vibration, and frequent switching. Even at low battery voltage they remain bright, don't dim like a bulb, and don't alter their color with age. LED flashlights are consequently robust, reliable, and offer typical lifetimes in excess of 100,000 hr.

To drive the phosphorous-coated InGaN blue LEDs commonly used in flashlight applications, 3.6V is usually required. For single cell operation this means the use of a step up or “boost” converter. Current is typically around 20mA to 50mA, though this is variable depending on the choice of LED.

The ZXSC100 provides a solution for LED flashlights that meets the demand for smaller size and higher efficiency. With cost constraints tighter than with the cordless phone, the two solutions are somewhat different. You can see an example of LED flashlight driver circuit in Fig. 4, on page 38.

To minimize losses, you can drive the LED with a pulsed current, preventing the need for the Schottky diode and associated capacitors. This provides cost savings. With the white LED itself setting the output voltage, you don't need the base current setting and supply voltage divider resistors.

Component count reduction diminishes cost and ensures a small size. The solution only consists of a low-cost miniature inductor, the ZXSC100X8, in its small MSOP8 package, the ZXT13 SOT23-6 transistor and a surface mount resistor.

There is no compromise on efficiency, with this standard configuration achieving a peak efficiency of well over 80%. Careful inductor selection could raise this to a higher level still.

With minimal loss in efficiency, you can achieve an even smaller circuit size with the use of a high performance SOT23 transistor. For example, the FMMT617 type offers even smaller size and a reduced cost.

The inherent flexibility of the ZXSC circuit means you can drive parallel or series LEDs with a simple circuit modification to match the LED characteristics. For high-end LED flashlight applications, a rectified continuous current circuit achieves up to a 40% increase in brightness.

The range of single cell converters will continue to expand. Current plans include a linear regulator on the dc-dc output for applications requiring absolute minimum ripple, and a fixed current drive device in the ultra small SOT23-5 package, offering the optimum low cost, small size solution for LED flashlight applications.

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