The world around us is going mobile. It seems like a new electronic gadget finds its way into our daily life routinely, from numerous wireless communication gear to notebook computers, medical monitoring devices, etc. This portable and battery-operated equipment is becoming more sophisticated with multiple functionality, and the manufacturers of these devices rely heavily on smaller and lower-cost integrated circuits without any performance compromises. Longer battery life, or longer time between charges, has become the differentiating feature for such devices.

One of the main integrated circuit functions used in virtually all electronics equipment is the regulator. For years linear type regulators have been the building block of any circuit, however, over the past several years linear regulators have been replaced by a device more suitable for battery-operated devices. The low-dropout regulator, better known as LDO, is a special type of regulator where the minimum required voltage between the input-output voltage (the dropout voltage) is significantly smaller than predecessor parts. The lower the LDOýs dropout voltage the longer the battery life as the battery can be discharged all the way down to a few hundreds of mV of the desired output voltage.

The LDOýs ease-of-use, smaller footprint and lower system cost have made it the primary choice for the designers of portable electronics when compared to any other type of regulators. Many battery-operated products use multiple LDOs to power the digital and analog circuitry. For example, 4 to 10 separate LDOs are used in a typical cellular telephone. This explosion in demand for LDOs has attracted many IC manufacturers into the market.

This article is aimed at increasing the awareness of the "specmanship" used by many LDO manufacturers in their product datasheets, which could be potentially confusing or misleading. Current and potential LDO users need to know the truth and the whole truth or the consequences could be very costly.

Output Capacitor

All LDOs require an output capacitor for stability. Improvements in technology and the topology of LDOs designs have allowed some manufacturers to offer LDOs with relatively smaller output capacitor values, typically between 0.47 µF-10 µF for most popular LDOs. Many manufacturers claim stable operation a with low-value output capacitor on the front of their data sheets. However, by investigating the test conditions used to guarantee their productýs electrical specification, one can discover the real output capacitor value needed. In addition to the value of the output capacitor, the capacitorýs parasitic "Equivalent Series Resistance" (ESR) plays an important role. Most LDOs rely heavily on the ESR value for stability. The basic problem with such LDOs is that the ESR, being a parasitic term, is not well controlled and not guaranteed by capacitor manufacturers, specifically at cold temperatures. As a result, such LDO manufacturers are forced to carefully limit the capacitor ESR to certain typical zones (see Fig. 1.)

Figure 1. Zoned Load Capacitor ESR Can Make an LDO Applications Nightmare.

The manufacturers of LDOs with strong dependencies to the ESR provide such typical charts to assist the LDO user in selecting an output capacitor that confines ESR to the stable region. Types of capacitors such as OS-CON or multi-layer ceramic capacitors (MLCC) have ESR values which are too low for use. As a result the LDO user is limited to bulky and expensive tantalum types that are undesirable for space-restricted handheld devices. In addition, some of the newer LDOs claim stability with low-value MLCC-type capacitors; however, they forget to mention that these LDOs are not stable with higher-value capacitors.

The only exception I know to the above is the patented LDO topology of Analog Devices ADP330X family which provides virtually-insensitive operation to the output capacitorýs value, type or ESR.

LDO Total Accuracy

The LDOýs output voltage accuracy is becoming more and more important as more sophisticated digital and RF parts are powered by LDOs, many of which require highly-regulated voltages for optimum performance. At times design engineers use high-accuracy LDOs as a higher-power, lower-cost and smaller alternative to voltage references for such components. The initial accuracy of LDOs is not an adequate measurement as accuracy varies over temperature range, load current and line voltage. True LDO accuracy should be shown as total accuracy over temperature, line and load.

Overhead

Overhead voltage is not a well-known specification and is seldom mentioned by most LDO manufacturers. The LDOýs overhead voltage is defined as the minimum (Vin-Vout) voltage at which the accuracy is guaranteed. Most LDO manufacturers specify an overhead minimum voltage of 0.5 V to 1.0 V. An example (see Fig. 2) is intended to clarify the importance of this specification. In a cellular phone that operates off of a fully-discharged 1-cell LiIon or a 3-cell NiMH battery pack, these Analog Devicesý LDOs provide an accurate output of 2.7 V from a 3.0 V (ý10%) input. Competing LDOs require a 4th NiMH battery to obtain the same minimum output voltage.

Figure 2 Better Headroom Management Equals Smaler Battery Stacks.

Power Handling

One of the most appealing features of LDO regulators is the availability of such power products in standard small-outline (SO8) or SOT-23 type packages. For example, various manufacturers are now offering 50 mA, 100 mA, and even 200 mA, devices in a tiny SOT-23 package. The ability to source higher current is only half of the story. To truly understand the power handling capabilities of these LDOs in SOT-23 or SO-8 packages, we should look at these devices in a specific application and the effect on the junction temperature. The example below compares a typical SOT-23 package vs. an Analog Devicesý proprietary Chip-on-Lead SOT-23 (see Fig. 3.)

Figure 3. ADI's Proprietary Chip-on-Lead™ Package.

Calculating Junction Temperature, requires the device power dissipation to be established from,

PD = (VIN ý VOUT) ILOAD + (VIN) IGND

where, ILOAD and IGND are the load and ground currents, and VIN and VOUT are the input and output voltages.

Assuming worst-case operating conditions with ILOAD = 200 mA, IGND = 4 mA, V_IN = 4.2 V and V_OUT = 3.0 V, the device power dissipation is:

PD = (4.2 V ý 3.0 V) 200 mA + (4.2 V) 4 mA = 257 mW

The proprietary package used on ADIýs ADP3330 LDO has a thermal resistance of 165ºC/W on a 4-layer board. A standard SOT-23 package has a thermal resistance of 230ºC/W. So, for

Standard SOT-23 package:     T_JA = 0.257 W x 230ºC/W = 59.1ºC
ADI Chip-on-Lead SOT-23:     T_JA = 0.257 W x 165ºC/W = 42.4ºC

To limit the junction temperature to 125ºC, the maximum allowable ambient temperatures would be,

Standard SOT-23 package:     T_A(MAX) = 125ºC ý 59.1ºC = 65.9ºC
ADI Chip-on-Lead SOT-23:     T_A(MAX) = 125ºC ý 42.4ºC = 82.6ºC

The example above clearly shows that enhanced packages can handle over 30% more power than standard SOT-23 packages.