Ultra-low dropout LDOs provide the perfect regulation for a wide range of devices
Charles Kuo, Worldwide LDO Marketing Manager, Diodes Incorporated
Product design demands an understanding of power management requirements and how to meet them. The latest LDOs from Diodes help meet the needs of almost any application.
Power management is one of the key building blocks of product design and is often bespoke to the application. The range of power management solutions is vast; AC-DC converters include primary side regulation controllers and secondary side controllers and switches, as well as active ORing controllers. DC-DC conversion is equally important; in some applications it can be necessary to drop 40V to as low 0.6V. In battery powered applications, battery management systems include charging solutions that provide a single-chip solution designed to work with a common interface such as a USB port and, as rechargeable batteries can be potentially hazardous, it is important to include the right level of protection in devices that use rechargeable battery packs.
Irrespective of the source of power, one of the most critical devices in power management is the Low Dropout Regulator (LDO) and solutions include single and dual LDOs. The development of ultra-low power nodes intended to form part of the IoT, many of which will be battery powered and expected to operate for many years without changing the cell, is creating increased demand for LDOs with micropower quiescent currents. In some cases, nodes will be powered by energy harvested from their environment, which puts even greater pressure on developers to deliver the most energy-efficient design possible. As a linear regulator, the LDO can be the best option for these kinds of applications, particularly as they can be extremely small and low cost. However, not all LDOs are created equal, so it is important to understand the key parameters of an LDO in order to select the right solution.
As the name suggests, an LDO is designed to dropout at a predesigned point, that is it will stop regulating the voltage delivered. As such, the dropout point of an LDO is perhaps the most important parameter of the device. An LDO is basically designed to provide a voltage regulated to within the specified limits. When it operates beyond its specification the manufacturer cannot guarantee operation. The dropout voltage is a figure that represents a difference between the input voltage and the output voltage. It is either expressed as a voltage or a percentage, but is always included in the data sheet. If the difference between the input and output voltages reaches this point, the LDO’s internal circuitry can no longer regulate the output voltage, at which point the output voltage is likely to fall below its specified level (dropout). As such the dropout figure is an indication of how well the LDO can sustain the specified output voltage under various conditions, including changes in the input voltage and load.
To understand how this might happen under operating conditions it is worth looking at how an LDO operates. The schematic in Figure 1 depicts a typical LDO configuration which includes three basic elements; a pass device, a reference voltage generator, and an error amplifier (which acts as a regulation controller). Under normal operating conditions the output voltage is controlled by the voltage drop across the pass device (in modern integrated LDOs the pass device is implemented using a MOSFET). The current flowing through the pass device is controlled by the voltage on its Gate terminal, which is in turn set by the regulation controller.
Figure 1: A typical LDO circuit featuring a reference voltage generator, error amplifier and pass device
As stated above, the regulation controller is effectively an operational amplifier providing an error between the output and the reference voltage, which forms the Gate voltage for the pass device. The LDO will be specified to operate within a given input voltage range, to deliver a specified output current and within these limits the pass device is essentially a variable resistor. If either the input voltage or output current meets (or attempts to exceed) its specified limits, the regulator controller could drive the pass device into its Fully Enhanced mode, at which point the voltage drop across the device will be determined by the device’s RDS(ON) and the output current being drawn through it. As a result, the output voltage will be controlled not by the regulation controller but by the load conditions. It is at this point that the device is said to be in ‘dropout’; if the load current continues to increase, the load voltage will continue to drop.
Many integrated LDOs use a PMOS as the pass device, in which case the Gate-Source voltage operates in the same direction as the output voltage; the lower the output voltage, the lower the Gate-Source voltage and, as a result, the higher RDS(ON). This results in limiting the output current and voltage in dropout conditions. However, devices that implement an NMOS as the pass device can effectively avoid the issue of being driven into Fully Enhanced mode, even if the output voltage drops significantly. An example of an LDO with an NMOS pass device is the AP7176B, an LDO designed to deliver up to 3.0A with a variable output voltage (Figure 2 shows the functional block diagram of the AP7176B). Its dropout voltage for an output voltage of 2.5V is 0.33V (typical) when delivering its full 3.0A output current, with a junction temperature of 25°C. Even across its full operating temperature range of -40°C to +125°C the dropout voltage (at 2.5Vout) will only reach 0.53V(Max).
Figure 2: Functional block diagram of the AP7176B 3A ultra-low dropout LDO
While the dropout voltage can be low, it will never by zero. As a function of the output current it is effectively limited by the physical dimensions of the MOSFET and in this respect, it also determines the maximum output current an LDO can deliver while providing regulation. As some applications will need more current that others, choosing the right LDO will partly depend on the dropout voltage required but also the system’s overall demands. In most modern applications, smaller devices are often better, so if a single LDO cannot deliver the right voltage/current requirements for all of the components on a board it may be necessary to use more than one LDO. Diodes offers a wide range of dual LDOs to meet this need. This highlights another important parameter for LDOs, the quiescent current they draw under low or no-load conditions. Compared to other linear regulator topologies the LDO is highly efficient but it is, by design, always on. It is important therefore that LDO manufacturers achieve the lowest possible quiescent current, particularly if multiple devices need to be used in portable devices such as wearables or sensor nodes targeting the IoT. The AP7350 is a perfect example of a highly accurate LDO with an ultra-low quiescent current; based on a CMOS process it offers a quiescent current as low as 0.25μA and a typical standby current of just 0.02μA.
Many applications today use integrated circuits with high power requirements across multiple voltage rails, such as microprocessors, DSPs and FPGAs. This further increases the need for highly efficient LDOs that are able to operate under dynamically changing load conditions. When a microprocessor increases its clock rate to complete a complex task, for example, the power it demands can change significantly and rapidly. To protect the LDO under these conditions manufacturers implement current limiting, as well as thermal shutdown circuitry. The diagram shown in Figure 3 shows the current limiting and thermal shutdown features of the AP7343, which also includes an enable input that allows the device to be put into an ultra-low power mode when not in use.
Figure 3: Like many of Diodes’ LDOs, the AP7343 offers current limiting and thermal shutdown protection
The ability for an LDO to react rapidly to changing load conditions is perhaps best illustrated by this scenario; modern microprocessors can dynamically vary their clock frequency and voltage levels in order to conserve system power under low processing conditions but rapidly ramp them up when demand increases. LDOs must react quickly to such changes in load current, as shown in Figure 4. The diagram depicts the transient response of the AP7361C, a 1A ultra-low dropout LDO with adjustable and fixed output voltage, under a load change of 400mA. As the upper trace shows, the output voltage deviates by less than 20mV and fully recovers within 100μs of both load changes. Figure 5 shows the dropout voltages for the AP7361C under various operating conditions, which offers an output voltage accuracy of ±1% and a Power Supply Rejection Ratio of 75dB @ 1kHz. With a low quiescent current of just 60μA (Typical), it also features current limiting, thermal shutdown and the ability to disable the device when not in use.
Figure 4: The AP7361C offers fast transient response to changes in load current
Diodes offers a wide range of single- and dual-LDOs for a variety of applications, from portable and wearable devices to printers, PCs, TVs, Set-top boxes and many other home electrical appliances, as well as industrial applications. Most of the LDOs have been designed for use with low-cost multi-layer ceramic capacitors with low equivalent series resistance (ESR). With features including fast transient response and rapid start-up times, they can significantly help extend the lifetime of battery powered applications, while providing highly accurate and stable supply voltages for the latest microprocessors, FPGAs and DSPs.
Please visit https://www.diodes.com/products/power-management/low-dropout-regulators/ to see our full range of LDOs.
