Ultra-low quiescent current power management IC extends operating time in portable applications

Technical Background

The trend of miniaturization and high integration of today's portable electronic devices requires longer battery life, lower power consumption, and limited size of printed circuit boards occupied by ICs and related peripheral components inside the devices. This trend has been accelerated by advances in battery technology that extend battery life and reduce the size of the final product. Rechargeable prismatic lithium-ion batteries/polymer lithium batteries have been used as advanced chemical batteries to power mid-range to high-end portable electronic devices, providing the most suitable form factor, usable voltage range, power density (capacity) and battery life. Therefore, when people once noticed the toxicity of nickel-cadmium (NiCd) batteries, people are looking for new ways to use lithium-ion batteries for high-current applications (such as power tools). And new lithium-ion chemical batteries (such as lithium phosphate batteries) are emerging, which provide lower usable operating voltage ranges, lower series resistance and higher safety compared to traditional lithium batteries/polymer lithium batteries.

Recently, new disposable lithium batteries (such as Energizer "e2" batteries) have been introduced that are the same size as alkaline batteries. Their extended battery life, small size, convenience and low price make them ideal for portable devices that have previously used alkaline and nickel batteries (such as digital cameras and handheld GPS). These batteries extend the range of choices for portable electronic device engineers beyond rechargeable lithium-ion batteries, polymer prismatic cells, alkaline AA or AAA batteries, and rechargeable nickel metal hydride (Ni/MH) batteries. The advantages of this battery over other non-rechargeable alkaline or nickel batteries are more important than the disadvantage of higher initial cost. In contrast, rechargeable NiMH batteries have relatively low energy density, but are a mature technology, low cost, and non-toxic, so they have been used in many portable electronic devices. Rechargeable and non-rechargeable alkaline batteries have always been popular in low-end electronic devices, with low self-discharge and low cost characteristics to compensate for their relatively low battery life.

Design Challenges

It is undeniable that designers of today's battery-powered portable electronics face considerable design challenges. These designers are bound to require high-performance power management structures in order to adapt to the trend of increasing system complexity, power budgets, and thermal management. Such systems should achieve the best balance between battery operating time, compatibility with multiple power sources , high power density, small form factor, and efficient thermal management. Careful selection of batteries and connecting other power sources (USB, wall adapter, etc.) to power the system must be done, because battery life and battery operating time are obviously important considerations.

At the same time, the increasing number of functions in the system forces the increase of system power consumption, which naturally reduces the battery operating time when the device is working. For rechargeable batteries, subsequent charging and recharging cycles will limit the battery life, especially when the recharging frequency is high. In the case of battery power, the high battery consumption, high quiescent current (IQ, no load) and low power conversion efficiency of the power management IC will have a negative impact on the battery operating time. Therefore, in both rechargeable and non-rechargeable battery cases, designers must weigh product performance, quiescent current and operating current, system power consumption and conversion efficiency in order to provide long battery operating time to the end user.

All of this is a thing of the past, thanks to new low-power power management integrated circuits (PMICs) that provide efficient power to the system with minimal external components, significantly reduced size, and greatly improved performance compared to other traditional high-power and high-heat PMICs. In addition, these new PMICs can greatly simplify the design and reduce the solution size compared to cumbersome, low-performance discrete IC solutions. Simple Solution - PMIC with Ultra-Low IQ and High-Efficiency Switching Regulator

ICs with low quiescent and operating currents over a wide load range and high-efficiency switching regulators help preserve battery operating time in portable electronic devices. PMICs from Linear Technology with PowerPath control, ultra-low quiescent and standby currents, and state-of-the-art integrated functional unit circuits (such as high-efficiency programmable synchronous buck-boost and buck switching regulators) solve these design challenges simply and easily. This is possible because of a different approach taken in PMIC development, using a higher level of optional integration to provide a compact solution without sacrificing performance.

For example, the LTC3554 is a micropower multifunction PMIC for Li-Ion/Polymer battery applications. The LTC3554 includes a USB-compatible linear PowerPath manager, an independent battery charger, two high-efficiency synchronous buck regulators, and a push-button control circuit in an ultra-thin (0.55mm) 3mm x 3mm UTQFN package. A selectable standby mode pin (see Figure 1) reduces battery drain current to only 10μA while all outputs remain in regulation.

Figure 1. LTC3554 battery drain current in standby mode

When the LTC3554 provides up to 400mA of battery charging current from a USB port or 5V wall adapter, its PowerPath manager automatically prioritizes the loads, seamlessly managing the switchover between multiple input sources to power the loads. The input current limit is pin-selectable and internally set to meet USB power specifications (external resistors required). The LTC3554 allows input voltages up to 5.5V, and its absolute maximum transient voltage is 7V for increased robustness. The IC 's "instant-on" operating feature ensures that the battery can power the system even when fully discharged. Its autonomous operating capability simplifies design, eliminating the need for an external manager to terminate charging. The LTC3554 includes two built-in synchronous step-down regulators that operate at 100% duty cycle and each regulator can provide up to 200mA of output current with adjustable output voltages as low as 0.8V. For added flexibility, the two regulators can be independently enabled and disabled, and their oscillation frequency and corresponding slew rate are pin-selectable (1.125MHz or 2.25MHz), allowing the application circuit to dynamically trade off efficiency and EMI performance. The high switching frequency characteristics of the IC also allow the use of small, low-cost capacitors and inductors. Its built-in low RDS(ON) switches can increase efficiency by up to 93% and maximize battery operating time. In addition, Burst Mode optimizes efficiency at light loads, with a quiescent current of only 25μA per regulator (<1μA in shutdown mode), see Figure 1 for details. In addition, the regulator is stable when using ceramic capacitors, achieving very low output voltage ripple.

Figure 2. LTC3554 battery drain current in burst mode

Under light load and no-load conditions, the LTC3554’s buck regulator automatically switches to a power-saving hysteretic control algorithm that intermittently operates the switches to minimize switching losses. This is called Burst Mode operation, in which the buck regulator cycles the power switches enough times to charge the output capacitor to a voltage slightly above the regulation point. The buck regulator then enters a sleep mode that reduces quiescent current . In this state, power dissipation is minimized while the output capacitor supplies the load current. As soon as the output voltage drops below the regulation point, the buck regulator wakes up from sleep mode and adjusts the switches again until the output capacitor voltage is again slightly above the regulation point. The sleep time therefore depends on the load current, since the load current determines the discharge rate of the output capacitor. If the load current increases beyond approximately 50mA, the buck regulator reverts to constant frequency operation.

When the STBY pin is driven high, all switching regulators are allowed to operate in standby mode. In this mode, the regulators maintain the output in regulation, while the quiescent current of the individual buck regulators is reduced to only 1.5μA. This mode is suitable for applications with micro-power standby, sleep or memory "keep-alive" modes. The load capability of each regulator can be reduced to 5mA. Standby mode is used in extremely light load conditions, where even the low quiescent current of Burst Mode operation is considered excessive.

Each buck regulator is shut down when the input of its respective PWR_ON pin is tied low. In shutdown, each switching regulator consumes only a few nA of current loss from the power pin (BVIN). Each disabled regulator can also have a 10kΩ pull-down resistor at its output from its switch pin to ground.

The LTC3554 provides an integrated push button interface that enables power-up and power-down of the application circuitry with a single push button, with user input signals sent through the PBSTAT output. The initial push button press sequences the power-up of the buck regulators and supplies power to the application circuitry. Subsequent push button presses are indicated by a low signal on the PBSTAT output. By monitoring the PBSTAT signal, the application microprocessor can change operation or perform a power-down operation in response to a push button command. The push button interface also features a “hard reset” state that can be reached by pressing the button for more than 5 seconds. The hard reset will power down both buck regulators and place the LTC3554 in an ultra-low current (<1μA) state, saving battery run time (see Figure 1). The hard reset can be used to power down the application circuitry or to recover from a software lockout of the application microprocessor. Ultra-Low IQ PMIC

It is common to power low- to mid-range portable electronics with non-rechargeable batteries (e.g., alkaline or Energizer “e2” lithium) or even batteries that are charged external to the end product (e.g., nickel or rechargeable alkaline). To meet the needs of such devices, Linear Technology has developed a micropower PMIC, the LTC3101. Typically, in applications powered by two AA batteries, a boost regulator is used to generate the 3.3V rail. However, when using e2 lithium batteries, VBAT is higher than VOUT due to the higher battery voltage, so a boost solution either does not work or is very inefficient (depending on the type of boost converter used). However, since the LTC3101 uses an internal buck-boost converter to generate the 3.3V rail, it has no input voltage limitations and can easily handle e2 lithium batteries. In summary, the buck-boost converter is not only effective because it provides the ability to operate from a USB/Li-Ion battery/5V wall adapter input, but it must also operate efficiently with all possible two-AA battery inputs.

The LTC3101's "always on" VMAX and LDO outputs are responsible for powering critical functional circuits or additional external regulators. Internal sequencing circuits and independent enable pins provide flexible power-up and power-down options. In addition, the IC's PowerPath control circuitry uses a low-loss PowerPath control topology to achieve seamless and automatic power flow management between these multiple input power supplies. The input of each switching power supply has an additional switching MOSFET, one FET connected to the BAT input and the other FET connected to the USB input. This enables the IC to automatically select the input it will use (if both USB and battery are present) and optimize efficiency when operating with either input power supply .

The LTC3101's buck-boost regulator can continuously deliver up to 800mA of current (when the input voltage is above 3V) and is ideal for efficiently regulating a 3.0V or 3.3V output over the full input voltage range of 1.8V to 5.5V. Both of the LTC3101's buck regulators feature 100% duty cycle operation, each delivering up to 350mA of output current with an adjustable output voltage as low as 0.6V. Its internal low RDS(ON) switch achieves up to 95% buck-boost efficiency and up to 93% buck regulator efficiency, maximizing battery run time. In addition, Burst Mode operation optimizes efficiency at light loads, with total quiescent current of only 38μA (when all regulators are enabled) and 15μA (in standby mode with the LDO and MAX outputs running); see Figures 3 and 4. The high switching frequency of 1.27MHz allows the use of tiny, low-cost capacitors and inductors with a height of <1mm. In addition, all regulators are stable with ceramic output capacitors, resulting in very low output voltage ripple.

Figure 4. Total quiescent current of the LTC3101 in standby mode.

When the PWM pin is forced low, both buck regulators automatically switch between Burst Mode operation when the load is light enough (less than about 10mA) and PWM mode operation when the load is heavy. In dropout operation, the active P-channel switch is continuously on to maximize battery life, as shown in Figure 4.