Design Feature: September 26, 1996

The impetus for the development of low-voltage systems comes from shrinking IC geometries and the limited voltages available in battery-operated equipment. These factors have spurred the development of a generation of compact, power-efficient dc/dc converters. These power sources serve to drop voltages in mixed-voltage systems or to boost them in battery-powered systems.

It makes eminent sense to locally convert power-supply voltages in mixed-voltage systems. In a system using both 5V logic and a 3.3V µP, for example, you can either supply both voltages from the board's edge connector, or you can convert the 5V supply to 3.3V at a location near the µP. If the µP is a current glutton, the choice of routing the 3.3V across the board may be inappropriate, owing to the resistive losses in the copper traces on the board.

Assume, for example, a separate 3.3V line has 50-m[ohm] resistance, a reasonable assumption for a long copper run. A 10A current consumption would produce a 0.5V drop, which means you would have to supply 3.8V at the edge connector to compensate for the drop (and the 5W power loss). Locally converting the 5 to 3.3V from a hefty embedded power plane both reduces power losses and eliminates the need to compensate for the voltage drop.

A third alternative exists for supplying low-voltage circuitry. A linear regulator can drop the 5V supply voltage to 3.3V and provide excellent line and load regulation to boot. However, this alternative is impractical when the load draws appreciable current because of high power losses in the regulator. Figure 1 shows a comparison of the power wasted in a linear regulator (Figure 1a) vs that in a switching regulator (dc/dc converter) (Figure 1b), for a 10A load operating at 1.8 and 3.3V.

Clearly, for such a high-current load, the linear regulator is inappropriate. The regulator itself dissipates 17W for a 33W load and 32W for an 18W load. These figures do not even take into account the quiescent current (I_Q) of the regulator. For a regulator using a pnp pass transistor (for a low-dropout characteristic), this current can add appreciable wattage to the regulator's already- high internal dissipation. The 74 and 85% power-conversion efficiencies of the dc/dc converter are typical data-sheet figures for 1.8 and 3.3V converters, respectively. I_Q is not a consideration in the power calculations, because the efficiency figures take that current into account.

However, for low-current applications, don't discount linear regulators. If the load draws only a few milliamperes, the power wasted in the regulator amounts to only a few milliwatts. In these cases, a linear regulator—with its ease of use, minimal need for external components, and total lack of switching noise—could be the optimum voltage-conversion device.

Powering the low-voltage µP

Burgeoning transistor counts and shrinking feature sizes are responsible for the lower-than-5V operating voltages of recent µPs. Considerations of both breakdown voltage and power consumption mandate lower supply voltages than the traditional 5V rail. However, these lower voltages do not translate to lower power. These several-million-transistor ICs use CMOS technology, which essentially consumes no dc quiescent current. At high frequencies, however, it's a different story. These several million transistors are basically a collection of tiny capacitors that require charging and discharging currents. So, at high switching rates, the net current consumption can be substantial.

These hefty, high-speed current demands pose several challenges for designers and users of power sources for µPs. First is the need for small size—nobody wants a converter the size of a shoebox sitting next to a small IC. Second, the nanosecond-range edge rates require high-quality "flywheel" capacitors to serve as a reservoir for the switching currents, because no conceivable power-source output stage can cope with these heavy, fast signal currents.

A final complication for the power-source designer or user is the device-to-device uncertainty in operating voltage for certain µPs, such as the Pentium Pro. These µPs meet their speed specs only at specific operating voltages, which can range from 2 to 3.5V for the Pentium Pro. This µP comes with a 4-bit operating-voltage code, which selects one of 16 discrete voltages from 2 to 3.5V. So, any dc/dc converter destined to power this processor must offer either an adjustable or a code-selectable output voltage.

Linfinity Microelectronics addresses the need for a selectable supply voltage with its $29.10 (1000) LXM1600 Series of dc/dc-converter modules. These 3.1×1.5×1-in. modules accept a 4-bit code from the Pentium Pro to produce a voltage of 2 to 3.5V in 16 discrete steps. Figure 2 shows a typical application. The LXM1600 is available in two versions, one with a 12V-only input supply and the other with a 5V input for the power converter and a 12V supply for the control-logic bias. Both versions offer either 11.2 or 12.4A maximum output current. Efficiency is typically 80% for the 5/12V model and 85% for the 12V-only version.

If you can afford somewhat more power loss, a recent low-dropout linear regulator from Semtech provides an adjustable output to meet the needs of Cyrix 6x86, AMD K5, and Pentium P54C and P55C processors. The $3.20 (100) EZ1585D provides 6A output current and has a 1.3V maximum dropout voltage. Its typical line and load regulation are 0.015 and 0.05%, respectively.

Life becomes somewhat simpler if you're specifying a power source for a 3.3V-only system. You need simply choose a dc/dc converter with adequate output-current capability from one of a large number of converter manufacturers. However, if future upgrades of your system are conceivable, with attendant higher current demands, you face a thorny decision. First, you could overspecify (and overpay for) the power source and select one whose current capability greatly exceeds your current needs. Alternatively, you could select a converter family that has various current ratings in the same footprint for future replaceability.

You have a third alternative with Power Trends' master/slave 3A switching regulators. These modules, measuring 2×0.62×0.38 in., allow you to increase current to any desired value in a system in 3A increments. Figure 3 shows a typical master/slave arrangement, in which three PT6435 3A slave modules add 9A to the output of either a 3A PT6425 or an 8A PT6501 master module. Both the 3A master and slave modules cost $15.65 (1000); the 8A PT6501 master module costs $22 (1000).

In Figure 3, resistors R₁ through R_N serve to equalize the voltage drops in all the modules. For example, with all 3A modules, 10-m[ohm], ¼W resistors are appropriate. When 3A slaves work with an 8A master, you should use a 10-mV, 1W resistor for the master and 27-mV, 1/2W resistors for the slaves. A remote-sense pin in the master module equalizes the current in the connected modules and compensates for the small voltage drops in the series resistors. The 3A master/slave converters have typical efficiencies of 85% at 1.5A output and 80% at 3A output.

µPs are not the only users of low-voltage supplies. Bus structures, such as the Futurebus (2.1V) or the Gunning-transceiver-logic (GTL) bus (1.2V), need beefy power supplies for termination. Power Trends provides the beef with its $12.90 (1000) PT6405 stand-alone (not master/slave), 3A dc/dc converters. Four models—PT6405, PT6406, PT6407, and PT6408—provide output voltages of 3.3, 1.8, 2.1, and 1.2V, respectively. The respective typical efficiency figures for the four models are 80, 68, 72, and 57%.

Roll your own supply?

If you have tight pc-board real-estate restrictions, severe budget constraints, or particular technical requirements, you may elect to configure your own dc/dc converter. Harris Semiconductor can help you in this endeavor, with its family of synchronous half-bridge ICs. The $2.25 (OEM) HIP5015/5016 SynchroFET ICs (Figure 4) incorporate control circuitry and two power-DMOS transistors. The DMOS transistors operate in synchronous mode. The PWM controller (Figure 4) is available from several IC manufacturers; the TL5001 from Texas Instruments is one example.

A standard buck converter uses a power MOSFET and a Schottky diode. The MOSFET transfers energy from the input, and the Schottky diode blocks reverse current. When the MOSFET turns off, the Schottky diode conducts. In the HIP5015/5016, a MOSFET replaces the Schottky diode. The voltage drop accruing from the MOSFET's on-resistance is lower than the forward voltage of the Schottky diode. Consequently, a buck converter using synchronous rectification can achieve 90 to 95% efficiency vs the 80 to 85% inherent in converters using a Schottky diode.

A buck-converter IC from Temic Semiconductors allows you to configure a synchronous dc/dc converter, using external P and NMOS FETs. The $1.75 (100,000) Si9140CY is a switch-mode controller that accepts a 3 to 6.5V input and operates at frequencies exceeding 1 MHz. In addition to the two MOSFETs, the device requires 13 resistors, 10 capacitors, one diode, and one inductor in the application circuit recommended in the data sheet.

A recent monolithic dc/dc converter from Elantec incorporates synchronous-rectifier MOSFETs; moreover, its output voltage complies with the previously described Pentium Pro 4-bit protocol. The $10.67 (1000) EL7560C integrates the PWM control logic with two 12.4A MOSFETs in a 28-pin SOIC power package. The IC provides 2.1 to 3.5V output in 16 incremental steps of 0.1V each, depending on the 4-bit code from the µP.

The EL7560C is a buck regulator and, accordingly, needs an output inductor and capacitor (2.5 µH and 4 µF, respectively, in a typical application). The recommended connection diagram on the data sheet shows the following external discrete components: five resistors, 10 capacitors, three diodes, and two inductors. As stated, synchronous rectification yields impressive efficiency. Figure 5 shows typical efficiency vs current at three output-voltage levels. The efficiency is comfortably over 90% at medium current levels and over 85% at the maximum 12.4A output.

Converter ICs proliferate

IC-packaged dc/dc converters are appearing at an accelerating rate. For example, an $8.55 (1000) LM2825N buck regulator from National Semiconductor incorporates the external components classically required in buck-regulator circuits, in a 24-pin DIP. It's available in 3.3 and 5V, fixed-output versions; adjustable and 12V versions are slated for this year. The 1A device has 80% typical efficiency. If you don't mind adding some external capacitors, a resistor, a Schottky diode, and an inductor, National's Simple Switcher buck converters come in 1A (LM-2595/2598) and 3A (LM2596/2599) versions. Prices start at $2.75 (1000) for the 1A devices and $4.07 (1000) for the 3A converters.

One resistor, one capacitor, one Schottky diode, one PMOS FET, and one inductor are all the external components you need with Maxim's MAX1626/1627 step-down dc/dc converters (Figure 6). These ICs accept inputs from 3.3 to 16.5V and provide 3.3 or 5.5V or an adjustable output. Efficiency is greater than 90% for loads ranging from 3 mA to 2A.

As opposed to step-down converters, a great need exists for step-up converters in battery-powered applications. Semiconductor manufacturers have responded to this need with a variety of boost-converter ICs. The $1.89 (1000) Maxim MAX-608, for example, is guaranteed to start up from inputs as low as 1.8V and delivers as much as 3W from a two-cell NiCd battery. It uses a pulse-frequency-modulation (PFM) control scheme to provide more than 80% efficiency for load currents from 5 mA to 1A.

Maxim's MAX848/849 step-up converters halve the input needs of the MAX608. They can operate from one lithium-ion cell or one to three NiCd or nickel metal-hydride cells. These converters use synchronous rectification to boost efficiency, which is typically 90% with a 1.1V input. They both incorporate a two-channel, serial-output A/D converter for monitoring battery voltages. One channel monitors single-cell voltages of 0.625 to 1.875V; the other covers the 0 to 2.5V range. The MAX848 provides 0.5A output; the MAX849 supplies 1A. Prices start at $2.50 (1000).

Linear Technology's $2.80 (1000) LT1307 also operates from input sources with voltages as low as that of a single-cell alkaline battery (1V). Targeting low-power applications, such as pagers and modems, the converter provides 75-mA output current at 3.3V. Fixed, 600-kHz PWM operation allows the use of small, inexpensive ceramic output capacitors. The use of these surface-mount capacitors saves money and board space in comparison with tantalum types.

Watch for the announcement of a boost-converter chip set from Temic. The Si9160 converter and its companion application-specific Si6801 MOSFET target RF amplifiers in cellular phones. The combination yields an overall efficiency greater than 90% for the single-cell lithium-ion voltage range and more than a decade range in current.

Also targeting portable applications, a family of dc/dc converters from Impala Linear Corp offers a choice of conversion techniques. All the devices provide output currents to 100 mA at 2.5, 3.3, or 5V from input sources as low as 900 mV. The $0.82 (1000) ILC6370 uses a PWM technique at 50, 100, or 180 kHz for its conversion, and delivers 85% efficiency at 50-mA output. The $0.82 (1000) ILC6390 uses PFM and achieves high efficiency by skipping pulses according to the current demand. The $0.89 (1000) ILC6380 combines the two modulation techniques for low ripple during normal operation and high efficiency during standby or shutdown modes.

A final class of dc/dc-converter ICs is voltage inverters—circuits that change positive voltages to negative voltages. The ADP3603 and ADP3604 from Analog Devices, for example, provide 50- and 120-mA output current, respectively, at ­3V, operating from a 4.5 to 6V input. These inverters use charge-pump techniques to eliminate the need for external inductors. They operate at a 120-kHz switching frequency and, thus, allow the use of small output capacitors. Prices are $1.70 (1000) and $2.15 (1000), respectively.

You can reach Senior Technical Editor Bill Travis at (617) 558-4471, fax (617) 558-4470, e-mail b.travis@cahners.com.