Ground current or quiescent current (IGND or IQ), power supply ripple rejection ratio (PSRR), noise, and package size are often factors that determine the best LDO choice for portable applications. When choosing a low-dropout linear regulator (LDO), the 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 need to consider more issues.
Input, output and reduced voltage
Select the input voltage range to suit the LDO of the power supply. The following table lists the voltage ranges of popular battery chemicals used in portable devices.
When determining whether the LDO can provide the expected output voltage, its voltage drop needs to be considered. The input voltage must be greater than the sum of the expected output voltage and a specific voltage drop, that is, VIN》 VOUT + VDROPOUT. If VIN drops below the required voltage, then we say that the LDO has a “voltage drop†and the output is equal to the input minus the RDS (on) of the pass element times the load current.
It is necessary to pay attention to the performance change during pressure drop. The error amplifier driving the bypass transistor is fully turned on or is in a "cocked" state, so it does not generate any loop gain. This means poor line and load regulation. In addition, PSRR will be significantly reduced during the pressure drop.
Choose the LOD that can provide the expected output voltage as a fixed option to save the cost and space of the external resistor divider. The external resistor divider is generally used to set the output voltage of the adjustable device. With adjustable LDO, the output can be set to provide an internal reference voltage, which is generally around 1.2V, and only the output needs to be connected to the feedback pin. Please confirm with the manufacturer whether this function is available.
Load current requirements
Select the LDO based on the amount of current required by the load. Please note: An LDO with a rated current of, for example, 150mA may provide much higher current in a short period of time. Please check the minimum output current limit specification, or consult the relevant manufacturer.
battery voltage
Packaging and power consumption
Portable applications have inherent space limitations, so the size of the solution is critical. Bare die can minimize the size but lack the many advantages of packaging, such as: protection, industry standards, and the ability to be easily adopted by existing assembly architectures. Chip scale packaging (CSP) can provide the advantages of die size while also bringing many packaging advantages.
Driven by the market demand for wireless handheld terminals, CSP products are constantly being updated. For example, Texas Instruments (TI) 200mA RF LDO with 0.84 x 1.348-mm CSP uses technology that enables easy assembly and high board-level reliability.
Compared with SOT-23 and SC-70 packages, LDOs using chip scale packaging have both die size advantages and packaging advantages
Other small packages include the popular 3x3mm SOT-23, small 2.13x2.3mm SC-70, and sub-1-mm-height package, ThinSOT, and leadless quad flat pack (QFN). QFN can provide better heat dissipation characteristics due to the use of a heat dissipation pad on the lower side that enables efficient heat dissipation contact between the device and the PC board.
Please be careful not to exceed the maximum power dissipation rating of the package. Power consumption can be calculated by PDISSIPATION = (VIN-VOUT) / (IOUT + IQ). In general, the smaller the package size, the smaller the power consumption. But QFN package can provide excellent heat dissipation performance, this performance is completely comparable to many packages whose size is 1.5 to 2 times its size.
LDO topology and IQ
In order to maximize the running time of the battery, 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 an IQ of 200μA under a 100mA load.
In addition, it should also be noted that due to battery discharge characteristics, in some cases the voltage drop will have a decisive impact on battery life. Due to the slow discharge speed of alkaline batteries, their power supply voltage can provide more capacity than NiMH batteries under voltage drop. A careful trade-off must be made between IQ and voltage drop in order to obtain maximum capacity during battery life. Therefore, a lower IQ does not always guarantee a long battery life.
Need to pay attention to the performance of IQ in bipolar topology. IQ not only varies greatly with load current, but also increases under voltage drop.
In addition, pay attention to how 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 situations are useful, they do not guarantee maximum quiescent current. If IQ is important, you need to choose a device that can guarantee IQ under 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 an LDO that does not require capacitor stability can reduce size and cost, and eliminate these components altogether. Please note that the use of large capacitors with lower ESR can generally improve PSRR, noise, and transient performance.
Ceramic capacitors are usually the first choice because they are low in price and the failure mode is open circuit. In contrast, tantalum capacitors are more expensive and the failure mode is short circuit. Please note that the equivalent series resistance (ESR) of the output capacitor will affect its stability. The ceramic capacitor has a low ESR, which is on the order of 10 ohms, and the tantalum capacitor ESR is on the order 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 a proper resistance (typically 200m) is connected in series between the capacitor and ground, tantalum capacitors can be used instead of ceramic capacitors. LDO vendors need to be consulted to ensure correct implementation.
RF and audio applications
Finally, consider the power requirements of dedicated 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. Care should be taken when designing modern transceiver systems to ensure low noise and high linearity.
Power supply noise will increase the phase noise of the VCO, and will enter the receiving or transmitting amplifier. In the case where popular mobile phone technologies such as W-CDMA have strict requirements for 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 false signals.
To meet the audio needs of portable devices such as mobile phones, MP3s, games, and multimedia PDA applications, LDOs of 300 to 500 mA may be required. Moreover, in order to obtain 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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