Alan Elbanhawy and Zaki Moussaoui, Exar
Current, Power Supplies, Safety Standards, Voltage Regulator
Driving consumer devices cleanly and efficiently is more important now than ever before, because of the growing amount of functionality being put into products today. From our telecommunications infrastructure to power recliners, people are getting used to an increased level of functionality and interoperability in the things around them while expecting even more functionality and connectivity in the future.
The twin issues of power management and signal integrity in consumer devices from sophisticated subsystems, the increase in wireless ”smart” households, and intelligent web-based white goods, demand better power solutions. For example, a typical processor-based board needs multiple rails to supply core voltages for all of the chips and components at voltages from 1.8V down to 0.6V, with load requirements up to 3A. Add to that requirement the rails for memory, with voltages from 1.8V for DDR2 to 1.35 for DDR4 (see Figure 1). Many of these systems use PWM switching buck converters, but when low noise and space saving are required, a low dropout regulator (LDO) can be a better solution.
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Figure 1: A power solution must address all expected voltage rail conversions and currents
Overcoming some of the previous limitations of using LDOs in these point-of-load applications poses a number of design challenges. Since these devices may need to take an input voltage as low as 1.1V ±5%, much of the internal control circuitry must be able to operate below 1V. These operations of course include all critical functionality such as overcurrent and short-circuit protection, as well as a stable control loop. To handle challenging current ratings as high as 2A and 3A means that the design still needs to be accurate even at these very low input voltages, so soft-start and current-limiting performance continues to work. Don’t forget that with very little voltage overhead, the transient response of the LDO must work extremely well to keep the transient ripple under control and within specifications. Properly done, a 1.2V rail can be down-converted to 1.1V at up to 91.6% efficiency.
For example, Exar’s latest ultra-low-dropout LDOs can address all of these concerns and performance requirements. Exar’s XRP6274/75 family challenges the industry with an ultra-low dropout voltage of only 75mV at 2A and 138mV with 3A loads over their entire operating temperature range. Well-suited for high-current and extremely tight input/output voltage conversions, the XRP6274/75 family can address applications previously unthought-of, such as applications requiring conversion between 1.35V to 1.2V at up to 3 amps.
Since many devices often operate at 2.5V, a set of XRP6274/75 devices can create a complete low-voltage power solution very efficiently, economically, and reliably implemented without the EMI problems or larger PCB space requirements associated with switch-mode supplies. Even without a bias supply, these LDOs offer excellent transient response, as illustrated in Figure 2. Other solutions cannot match this performance without routing an external bias and adding components on already-dense and crowded boards.
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Figure 2a: Transient response at 20mA-3A, 1.5VIN, 1VOUT
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Figure 2b: Transient response at 1A-3A, 1.5VIN, 1VOUT
In addition to having a very good noise performance of 200μVrms from 10Hz to 100kHz, the XRP6274 and XRP6275 challenge the industry in being able to operate from a single supply rail of 1.045V to 2.625V without the need for a noise generating charge pump or an extra voltage supply. This next-generation performance enables the creation of extremely efficient power conversion solutions that can surpass the current crop of switch-mode regulators at the same input/output voltages.
With its low drop out, single-input rail operating down to 1.045V, the XRP6275 Ultra LDO can deliver up to 3A of output current at high percentage efficiencies (as high as the mid-90’s), making it very well suited as a solution for high current and extremely tight input/output voltage conversions. Since the solution does not require a separate bias voltage or charge pump for the analog control circuits, combined with its small 3mm x 3mm x 0.8mm package, the XRP6275/74 can be placed in some of the tightest spaces existing on almost any PCB.
Suitable for a wide range of applications, the XRP6274/75 ultra low drop out LDOs provide single-rail operation at voltages from 1.045V to 2.625V in very small footprints, with an extensive array of protection features to ensure high performance operation along with the extreme high reliability required by the latest crop of applications.
Figure 3 shows a typical power solution application block diagram. Usually LDOs aren’t known for high power-conversion efficiencies, but an LDO-based solution with as little as 5 external passive components can deliver conversion efficiencies as high as 88.9% for applications from 1.35V to 1.2V at up to 2A or 3A with a board footprint measuring only 0.15” x 0.4”. Achieving this level of performance along with the ability to fit into a very small footprint gives the design engineer to place these LDOs in the immediate vicinity of the loads nearly anywhere on the PCB easily, without sacrificing power conversion performance or taking up excessive board space.
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Figure 3: The device block diagram showing the major building blocks and safety features
Many applications require high levels of performance, such as driving graphics processors, PC cards, low-voltage digital ICs, and advanced microprocessors and microcontrollers. 1. The output voltage on the XRP6274/75 ultra low drop out LDOs can be set as low as 0.6V with ±0.5% accuracy, making them very well suited for devices such as FPGAs, DSPs, ASICs, and other SOCs.
Keeping it safe
Protection features are also important for performance and safety, so the XRP6275 integrates output current limit protection to protect the main power supply, the regulator itself, and the load being driven from excess current. In short circuit conditions, the device will take additional steps to limit input current and power by disabling and re-enabling the pass device until the short condition is removed and normal operation can resume. Depending on the impedance of the short, this short circuit protection function may initiate operation between 10% and 50% of the targeted output voltage.
The XRP6274 also includes thermal shutdown circuitry to restrict the device’s operating temperature within a safe range. When activated (typically at 160°C), the XRP6274 is forced into a low-power reset state with a typical hysteresis of 30 degrees. Combined with the short circuit current protection, it reduces and limits the heating effects of over-temperature and/or over-current conditions on the LDO and surrounding circuits.
Furthermore, the XRP6274 features a soft-start function, which controls the output voltage ramp, and allows the regulator to gradually reach its initial steady-state operating point. This reduces current spikes and surges at start up due to output capacitor inrush current. Internal sequencing completes in 50μs, and then the reference voltage is linearly ramped over a period of approximately 2ms.
In addition to the standard protection features, these ultra-low-dropout LDOs also have reverse bias protection. Most LDOs have a parasitic body diode that provides a path from VOUT to VIN if the input is shorted to ground. The XRP6274 has a blocking diode to ensure that no high discharge currents can occur between the output and the input when the input is shorted to ground.