How to choose the right LDO for portable products
Ground current or quiescent current (IGND or IQ), power supply ripple rejection ratio (PSRR), noise, and package size are often the factors that determine the best LDO choice for portable applications. The basic issues to consider when selecting a low-dropout linear regulator (LDO) include the input voltage range, the expected output voltage, the load current range, and the power dissipation capability of its package. However, portable applications need to consider more issues. Input, output, and voltage reduction Select the input voltage range to accommodate the LDO of the power supply. The table below lists the voltage ranges for popular battery chemistries used in portable devices. The voltage drop needs to be considered when determining if the LDO can provide the expected output voltage. The input voltage must be greater than the sum of the expected output voltage and the specified voltage drop, ie VIN †VOUT + VDROPOUT. If VIN drops below the required voltage, we say that the LDO has a “voltage drop†and the output is equal to the input minus the pass element RDS(on) multiplied by the load current. Need to pay attention to the performance changes when the pressure drop. The error amplifier driving the bypass transistor is fully turned on or "cocked" so no loop gain is produced. This means that line and load regulation is poor. In addition, the PSRR is also significantly reduced in terms of voltage drop. An LOD that provides the expected output voltage is used as a fixed option to save the cost and space of the external resistor divider. An external resistor divider is typically used to set the output voltage of the tunable device. The output can be set with an adjustable LDO to provide an internal reference voltage, typically around 1.2V, simply by connecting the output to the feedback pin. Check with the manufacturer to see if this feature is available. Load current requirement Consider the amount of current required by the load and select the LDO accordingly. Please note: LDOs with a current rating of 150 mA, for example, may provide much higher currents in a short period of time. Please check the minimum output current limit specification or consult the relevant manufacturer. battery voltage Package and power consumption Portable applications inherently have space limitations, so the size of the solution is critical. Dies can be minimized in size but lack the benefits of packaging such as protection, industry standards, and the ability to be easily adopted by existing assembly architectures. Chip-scale packaging (CSP) offers many of the advantages of packaging while providing the size advantages of a die. Driven by the demand for wireless handheld terminals, CSP products are constantly being introduced. For example, the Texas Instruments (TI) 200mA RF LDO with a 0.84 x 1.348-mm CSP features technology for easy assembly and high board reliability. LDOs in chip-scale packages offer both die size and package advantages compared to SOT-23 and SC-70 packages Other small packages include the popular 3x3mm SOT-23, the small 2.13x2.3mm SC-70, and the sub-1-mm-height package, ThinSOT, and leadless quad flat package (QFN). The QFN provides better heat dissipation characteristics due to the use of 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 rating of the package. Power consumption can be calculated using PDISSIPATION = (VIN-VOUT)/(IOUT + IQ). In general, the smaller the package size, the smaller the power consumption. But the QFN package provides excellent thermal performance, which is comparable to many packages that are 1.5 to 2 times the size. LDO topology and IQ In order to maximize 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 insignificant battery consumption by 0.02%, it is reasonable to use 200μA IQ at 100mA load. In addition, it should be noted that due to the discharge characteristics of the battery, the voltage drop in some cases will have a decisive influence on the battery life. Due to the slower discharge rate of alkaline batteries, their supply voltage can provide more capacity than NiMH batteries in the case of voltage drop. A careful trade-off between IQ and voltage drop is required to achieve maximum capacity over the life of the battery, so a lower IQ does not always guarantee long battery life. Note the performance of IQ in a bipolar topology. IQ varies not only with load current, but also with voltage drop. In addition, you need to pay attention to how the IQ is specified in the data sheet. Some devices are specified at room temperature or only provide a typical curve showing the relationship between IQ and temperature. Although these conditions are useful, the maximum quiescent current is not guaranteed. If IQ is important, you need to choose a device that guarantees IQ for all load, temperature, and process variables, and you need to choose a MOS-type bypass device. Output capacitor Typical LDO applications require the addition of external input and output capacitors. Selecting LDOs that do not require capacitor stability can reduce size and cost, and completely eliminate these components. Note that large capacitors with lower ESR generally improve overall PSRR, noise, and transient performance. Ceramic capacitors are usually preferred because they are inexpensive and the fault mode is open, compared to tantalum capacitors and their failure modes are short circuits. Note that the equivalent series resistance (ESR) of the output capacitor affects its stability. Ceramic capacitors have a lower ESR, which is about 10 ohms, while tantalum capacitors have an ESR of 100 ohms. In addition, the ESR of many tantalum capacitors varies greatly with temperature, which can adversely affect LDO performance. If the temperature does not change much, and an appropriate resistor (typically 200 m) is connected in series between the capacitor and the ground, a tantalum capacitor can be used instead of the ceramic capacitor. LDO vendors need to be consulted to ensure proper implementation. RF and audio applications Finally, consider the power requirements of dedicated circuits used in portable applications. RF circuits, including LNAs (low noise amplifiers), step-up/step-down converters, mixers, PLLs, VCOs, IF amplifiers, and power amplifiers, require LDOs with low noise and high PSRR. Care should be taken when designing modern transceiver systems to ensure low noise and high linearity. Power supply noise increases the phase noise of the VCO and enters the receive or transmit amplifier. In the case of popular mobile phone technology such as W-CDMA, which imposes strict requirements on spectrum regeneration and adjacent channel power, a very small amount of power supply noise entering the base/gate or collector/drain power supply of the amplifier will generate adjacent channel noise or spurious signals. In order to meet the audio requirements in portable devices such as mobile phones, MP3s, games, and multimedia PDA applications, an LDO of 300 to 500 mA may be required. Moreover, in order to achieve good audio quality, this LDO should be low noise at the audio frequency (20 Hz to 20 kHz) and provide high PSRR.
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