How to select LDO in portable applications

Ground current or quiescent current (IGND or I _Q ), power supply ripple rejection ratio (PSRR), noise, and package size are often factors in determining the best LDO for portable applications. When selecting a low-dropout linear regulator (LDO), basic issues to consider include the input voltage range, expected output voltage, load current range, and the power dissipation capability of its package. However, portable applications require more considerations.

Input, Output, and Step-Down Voltages
Choose an LDO with an input voltage range that can accommodate the power supply. The following table lists the voltage ranges for popular battery chemistries used in portable devices.

When determining whether an LDO can provide the desired output voltage, its voltage dropout needs to be considered. The input voltage must be greater than the desired output voltage plus the specified voltage dropout, that is, VIN > VOUT + VDROPOUT. If VIN drops below the required voltage, the LDO is said to be "drooping" and the output is equal to the input minus the RDS(on) of the pass element times the load current.

It is important to note the change in performance during voltage dropout. The error amplifier driving the pass transistor is fully turned on or "cocked" and therefore does not contribute any loop gain. This means that line and load regulation is very poor. In addition, the PSRR also degrades significantly during voltage dropout.

Select the LOD that provides the expected output voltage as a fixed option to save the cost and space of the external resistor divider, which is generally used to set the output voltage of the adjustable device. With an adjustable LDO, the output can be set to provide an internal reference voltage, which is generally around 1.2V, by simply connecting the output to the feedback pin. Please confirm with the manufacturer whether this feature is available.

Load Current Requirements
Consider the amount of current the load requires and select the LDO accordingly. Note: an LDO rated at, say, 150mA may provide much higher current for a short period of time. Check the minimum output current limit specification or consult the manufacturer.

Battery voltage

Packaging and Power Consumption
Portable applications are inherently space constrained, so solution size is critical. Bare die can be minimized but lacks many of the advantages of packaging, such as protection, industry standards, and the ability to be easily adopted into existing assembly architectures. Chip scale packaging (CSP) offers the size advantages of bare die with many of the advantages of packaging.

Driven by the market demand for wireless handsets, CSP products are constantly evolving. For example, the Texas Instruments (TI) 200mA RF LDO (see Figure 1) in a 0.84 x 1.348-mm CSP, expected to be available in September, uses technology that enables easy assembly and high board-level reliability.

Other small packages include the popular 3x3mm SOT-23, the small 2.13x2.3mm SC-70, and sub-1-mm-height packages, ThinSOT, and quad flat no-lead (QFN) packages. QFNs offer better thermal characteristics due to a thermal pad on the underside that creates an efficient thermal contact between the device and the PC board.

Be careful not to exceed the maximum power dissipation rating of the package. Power dissipation can be calculated using P _DISSIPATION = (V _IN -V _OUT )/(I _OUT + I _Q ). Generally speaking, the smaller the package size, the lower the power dissipation. However, the QFN package provides excellent thermal performance, which is comparable to many packages that are 1.5 to 2 times its size.

[page]LDO topology and IQ _{To maximize the battery run time, it is necessary to select an LDO with a low quiescent current IQ}
relative to the load current . For example, considering that _IQ only increases the battery consumption by a negligible 0.02%, it is generally reasonable to use an _IQ of 200μA at a 100mA load .

It is also important to note that due to the characteristics of battery discharge, voltage drop can have a decisive impact on battery life in some cases. Since alkaline batteries discharge more slowly, their supply voltage can provide more capacity than NiMH batteries under voltage drop conditions. A careful trade-off must be made between I _Q and voltage drop to obtain the maximum capacity over the battery life, so lower I _Q does not always guarantee long battery life.

It is important to note how I _Q behaves in a bipolar topology. Not only does I _Q vary greatly with load current, but it also increases under voltage drop conditions.

Also, pay attention to how I _Q is specified in the data sheet. Some devices are specified at room temperature or only provide a typical curve showing the relationship between I _Q and temperature. Although these conditions are useful, they do not guarantee the maximum quiescent current. If I _{Q is important, you need to choose a device that guarantees I Q} under all load, temperature and process variations , and you need to choose a MOS type bypass device.

Output Capacitors
Typical LDO applications require the addition of external input and output capacitors. Selecting an LDO that does not require capacitors for stability can reduce size and cost, or eliminate these components entirely. Note that utilizing larger capacitors with lower ESR generally improves PSRR, noise, and transient performance overall.

Ceramic capacitors are usually preferred because they are low-priced and their failure mode is open circuit, while tantalum capacitors are expensive and their failure mode is short circuit. Please note that the equivalent series resistance (ESR) of the output capacitor will affect its stability. Ceramic capacitors have lower ESR, which is on the order of 10 milliohms, while tantalum capacitors have ESR on the order of 100 milliohms. In addition, the ESR of many tantalum capacitors varies greatly with temperature, which can adversely affect the performance of the LDO. Tantalum capacitors can be used instead of ceramic capacitors if the temperature variation is not large and an appropriate resistance (usually 200m) is added in series between the capacitor and ground. The LDO manufacturer needs to be consulted to ensure correct implementation.

RF and Audio Applications
Finally, consider the power requirements of specialized circuits used in portable applications.

RF circuits, including LNAs (low noise amplifiers), boost/buck converters, mixers, PLLs, VCOs, IF amplifiers, and power amplifiers, require LDOs with low noise and high PSRR. Great care should be taken in designing modern transceiver systems to ensure low noise and high linearity.

Power supply noise increases the phase noise of the VCO and can enter the receive or transmit amplifier. In the case of popular mobile phone technologies such as W-CDMA, which have strict requirements on spectrum regrowth and adjacent channel power, even a small amount of power supply noise entering the base/gate or collector/drain supply of the amplifier can generate adjacent channel noise or spurious signals.

To meet the audio requirements in portable devices such as mobile phones, MP3, games, and multimedia PDA applications, a 300-500mA LDO may be required. Moreover, in order to obtain good audio quality, this LDO should be low noise and provide high PSRR at audio frequencies (20Hz~20kHz).