Today's multivoltage portable system designs demand a variety of devices for managing power conversion from switched-capacitor charge pumps to inductive dc/dc converters. But when designers want a compact, simple-to-implement solution that delivers a reliable power source at low cost and requires few external components, a low-dropout regulator (LDO) more often than not is the part of choice.
Why? Primarily because LDOs offer a highly attractive compromise among cost, footprint, noise and efficiency. Clearly, a linear regulator will not supply as power-efficient a voltage conversion as a switched device. Efficiency in some LDOs can be as low as 35 to 40 percent. Moreover, the heat loss in these devices can pose a major problem in space-constrained portable designs. But LDOs also deliver high ripple rejection and low noise, making them an excellent fit for noise-sensitive circuits in applications such as RF or audio. For circuits that require relatively minor voltage step-down conversions, those low-noise attributes more than offset poor power efficiency.
Weighing specs
In most cases, basic LDO specifications such as input voltage range, output voltage, load current range and power dissipation will drive product selection. But some specialized application requirements may place a higher priority on other specs. In some space-constrained portable applications, for example, quiescent current or package size can play a bigger role. Designers in the wireless handset market may need chip-scale packaging to meet space limitations and supply easy assembly. Other applications may be able to accommodate a 3 x 3-mm SOT-23 or a more compact SC-70 package. For designs requiring a particularly low profile, package options such as a thin SOT or QFN often are best.
Over the past few years, LDO manufacturers have been tracking the needs of portable-system designers with an array of devices in smaller footprints. A case in point is the recently announced NCP590 from ON Semiconductor (Phoenix). The dual-output device delivers up to 300 mA per output in a small, low-profile, 2 x 2 x 0.9-mm DFN-8 package.
Similarly, the 500-mA MAX8902A and MAX8902B from Maxim Integrated Products (Sunnyvale, Calif.) deliver up to 92-dB power supply rejection ratio (PSRR) at 5 kHz, 16-µVrms output noise and only 100-mV (max.) dropout voltage at full load, yet come in a tiny eight-pin, 2 x 2-mm TDFN package. That combination makes the new devices ideal for noise-sensitive circuits in space-constrained GPS units, MP3 players, PDAs and cell phones.
When quiescent current is important, make sure the LDO is specified over all load, temperature and process variations. LDO designers continue to make major advances in this arena. Just last month, Texas Instruments (Dallas) announced the new TPS780xx family of 150-mA LDOs. Offering dual-level voltage output for MSP430 microcontroller-based portable devices, the LDOs extend battery life by offering an extremely low, 500-nA quiescent current. The device's voltage-select pin allows designers to switch between two voltage levels to customize or reduce power consumption by half during operation. Typically, the LDO can be used to dynamically switch to a lower voltage level when the microprocessor goes into sleep mode. The two voltage levels are preset at the factory using a new EPROM-based architecture.
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New LDOs like the TPS780xx from Texas Instruments extend battery life by allowing designers to switch
dynamically between two voltage levels to reduce power
consumption during operation. |
Other applications, such as those in the RF or audio arena, may place more importance on power supply ripple rejection ratio or noise. This is particularly true for modern handset topologies such as CDMA, with its strict adjacent-channel power requirements and high sensitivity to supply noise. For noise-sensitive instrumentation, RF, DSP and logic supply systems, for example, the LT1965 1.1-A LDO from Linear Technology (Milpitas, Calif.) limits output noise to just 40 µVrms. Low-dropout voltage is 300 mA (typical) at a 1.1-A load.
Minimizing compromise
In many cases, the trick for LDO designers targeting portable applications is to deliver low quiescent current without compromising other key specs. The MIC5306 from Micrel (San Jose, Calif.), for instance, features a quiescent current of just 16 µA (typical) and a shutdown current of 0.01 µA. It also offers excellent transient response and low output noise without compromising PSRR performance. Low dropout is 60 mV (typical) at 150 mA, and the device supports a wide input voltage range, of 2.25 to 5.5 V.
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ON Semiconductor's NCP590 LDO helps reduce the footprint of handsets, PNDs and other portable devices by supplying two outputs in a highly compact, 2 x 2 x 0.9-mm DFN-8 package. |
All LDOs require an input and output capacitor to ensure stability. The equivalent series resistance (ESR) of the capacitor plays a key role in the stability of the LDO. Generally larger, lower ESR capacitors will improve the LDO's PSRR, noise and transient performance. But ESR is usually a poorly controlled value and often not guaranteed by capacitor manufacturers. The ESR of tantalum capacitors, for instance, varies significantly over temperature and can adversely affect LDO performance.
More recently, many LDO manufacturers have tweaked their architectures to offer LDOs with smaller output capacitor values to support the use of lower-cost ceramic caps. Devices such as the LP38500-ADJ FlexCap LDO from National Semiconductor (Santa Clara, Calif.), for example, use a unique compensation scheme to achieve stability with ceramic tantalum or aluminum capacitors regardless of ESR. Similarly, the ADP330X family from Analog Devices (Norwood, Mass.) combines an innovative design with an enhanced process to require only a 0.47-µF output capacitor for stability. This low rating allows designers to achieve stability with virtually any capacitor type.
By integrating LDO regulators with multiple functions, designers are also achieving significant savings in board space. With its recently introduced EMIF06-SD02F3 memory card transceiver, STMicroelectronics has mounted the five key functions required for memory card interfaces in mobile phones, GPS navigation devices, digital cameras and other consumer products. The single chip integrates a 200-mA LDO with signal conditioning, bidirectional level shifting, electrostatic discharge protection and EMI filtering. The LDO's dropout voltage is 100 mV max with a 200-mA load. The highly integrated device is available in a 24-bump, 400-micron pitch flip-chip.
John H. Mayer (jhmayer@ix.netcom.com) is a freelance writer based in Belmont, Mass.
