New generation ASSP optimizes power management in handheld devices

Last year, consumers bought more than a billion cell phones, 220 million laptops, 140 million MP3 players, 90 million digital cameras (DSCs), and 10 million personal navigation devices (PNDs). All of these devices have certain commonalities in terms of their internal system architecture. First, they are all battery powered, usually using some type of lithium-ion battery (Li-Ion) as the primary power source , and another input power source for backup or charging. Second, they all have some kind of built-in storage device, usually including some type of ROM, RAM or NAND flash memory, and in many cases a hard disk drive (HDD) or SDIO card. According to the latest research from technology research firm IDC, 161 billion GB of digital information was produced worldwide last year. That's equivalent to needing 2 billion iPods to store all this information.

However, another category of products not mentioned above is products that combine two or even three of the above product functions, such as portable media players (PMP) or digital media broadcasting (DMB) products. These products also use lithium-ion batteries as the main power source and have a large storage capacity. They are becoming important playback devices in the consumer electronics field.

A key advantage of PMP or DMB products is that they can play both MP3 and MP4 formats. Therefore, music and movies from DVD-CD or downloaded from the website can be enjoyed with one device. Typically, the storage media of the device can store more than 150 hours of video or 1200 hours of music. However, like any other handheld device that relies on battery power, manufacturers of these PMP devices face unprecedented pressure to integrate many functions into a structure with limited size and appearance, while also providing longer operating time.

Since most PMPs have video and MP3 playback functions, the internal circuits require multiple low- voltage rails with different power levels . The reason is clear, because most digital large-scale integrated circuits operate at 1.5V or lower. At the same time, the voltage required for memory and I/O is 2.5~3.3V. Therefore, it is not practical to use multiple load point (POL) DC/DC converters to directly convert the voltage from the lithium-ion battery, and system designers must adopt more integrated solutions.

Most battery-powered handheld devices utilize a custom integrated circuit (ASIC) to handle battery charging, power channel control, multiple supply voltages, and protection functions such as true output open circuit and precision USB current limiting. The purpose of this approach is clear: one device can meet all power management needs. However, this approach also has some disadvantages. First, ASICs are manufactured using a specific wafer manufacturing process, and it is very difficult to achieve the best performance for each function. Second, the long delivery cycle caused by the definition and development of the ASIC becomes more important for designs with short dynamic design cycles. Generally speaking, it takes more than one and a half years from concept to delivery for a power management ASIC. During this cycle, a specific product design may have been changed three times or more.

Application-specific standard products for power management

Most battery-powered handheld devices can usually be powered by an AC adapter, a universal serial bus (USB), or a lithium/polymer battery, but how to achieve power path control between these power sources is a big technical challenge. Until recently, designers have tried to use a discrete approach, that is, using a group of MOSFETs and operational amplifiers to achieve this function, but they face big problems, such as hot plugging and large transient currents, which can cause big system problems.

There is a certain commonality in functionality and performance among various types of battery-powered handheld devices where application-specific standard products (ASSPs) can be used without the performance tradeoffs associated with IC manufacturing in single-wafer manufacturing processes. Linear Technology has recently developed a new generation of such products, the LTC3555, which represents a new level of performance and functionality in such applications.

The LTC3555 seamlessly manages power flow between the AC adapter, USB and Li-Ion battery, compliant with USB standards, all packaged in a 4×5mm QFN. As if that weren’t enough, it also comes with a full-featured Li-Ion/Polymer battery charger capable of providing up to 1.2A of charge current, plus three high-efficiency synchronous buck converters to generate the low voltage rails required by most USB peripherals . In addition, the LTC3555 provides a constant 25mA low dropout linear regulator to power the real-time clock (RTC) and low-power logic circuits. The entire device can be controlled via a simple I2C interface or simple I/O ports.

Figure 1: Simplified block diagram and schematic of the LTC3555.

The LTC3555 application circuit diagram is shown in Figure 1, which shows the multifunctional implementation principle. DC/DC conversion is a relatively simple step-down conversion. The three on-chip step-down converters of the LTC3555 all operate in current mode control with efficiencies up to 95%, with I2C or chip-selected burst mode or automatic burst mode. The switching frequency of the DC/DC converter is 2.25MHz, allowing the use of very small external capacitors and inductors. The continuous output currents of these step-down converters are 1A, 400mA and 400mA respectively, and the output voltage is programmable between 0.8-3.6V.

The LTC3555's power delivery method is different from existing batteries and power management ICs. It is actually a charge-fed system. In general power management ICs, the external power supply does not directly power the load. Instead, the battery is charged by the AC adapter or USB port, and then the load is powered. In the case where the battery is over-discharged or has no power at all, there will be a delay in powering the load. This is because the power cannot be taken directly from the battery until the battery has the minimum amount of charge required. With the LTC3555, this delay can be eliminated and the handheld device can be powered immediately as soon as the wall adapter or USB is plugged in. In addition, the chip can take the unused power of the load and use it to charge the battery.

These two advantages (i.e., eliminating charging delays and simultaneously charging and powering the load) extend the effective operating time and accelerate charging when connected to USB. Another advantage of this power management technique is that it improves efficiency when AC or USB power is available. In this case, the unnecessary conversion stage (for battery charging) can be eliminated.

High efficiency Switching power supply Channel controller

Unlike the previous generation LTC3455, which had a linear power path controller, the LTC3555 has a high efficiency switch mode power path controller. Designed specifically for USB applications, the LTC3555’s power path controller incorporates a precision average input step-down switching regulator that maximizes the available USB power. Because power is conserved, the LTC3555 allows the load current on VOUT to exceed the current drawn by the USB port, but not exceed the USB load specification. The power path switching regulator communicates with the battery charger to ensure that the input current does not exceed the USB specification limit. Furthermore, an ideal diode from BAT to VOUT ensures that power is always delivered to VOUT, even when there is insufficient power or no power at all on VBUS.

Figure 2: LTC3555 power channel block diagram.

When VBUS is available and the power path switching regulator is activated, power can be delivered from VBUS to VOUT through SW (see Figure 2). VOUT drives a mixed load consisting of the external load (switching regulators 1, 2, and 3 in Figure 1) and the battery charger. If the mixed load does not exceed the programmed input current limit of the power path switching regulator, VOUT will track 0.3V (above the battery voltage ). By keeping the voltage on the battery charger low, efficiency is optimized because the power lost to the linear battery charger is minimized, resulting in the optimization of the available power delivered to the load.

If the combined load at VOUT is large enough to cause the switching power supply to reach the programmed input current limit, the battery charger will reduce the charge current by the required amount to meet the value required by the external load. Even if the battery current is set to exceed the allowed USB current, it will not exceed the USB specification because the switching regulator will always limit the average input current to ensure that this does not happen. Furthermore, the load current on VOUT is always prioritized, and only the remaining power is used to charge the battery.

If the battery voltage is less than 3.3V, or if the battery is not present and the load demand does not cause the switching regulator to exceed the USB specification, VOUT will drop to a value between 3.6V and the battery voltage. If the battery is not present and the load exceeds the available USB power, VOUT will drop to ground.

The LTC3555 includes an ideal diode (see Figure 2) and a controller for an optional external ideal diode. The ideal diode controller is always on, responding quickly when VOUT falls below the battery voltage. If the load current increases beyond the power allowed by the switching regulator, power will be drawn from the battery through the ideal diode. In addition, if power to VBUS (USB or wall adapter) is unplugged, all application power will be provided by the battery through the ideal diode. The conversion from input power to battery power at VOUT is very fast, allowing only a 3uF capacitor to prevent VOUT from dropping. This is possible because the ideal diode includes a precision amplifier that turns on a high-power on-chip P-channel MOSFET transistor when the voltage on VOUT is approximately 15mV below the battery voltage (VFWD) . The resistance of the internal ideal diode is approximately 180mΩ, which can be reduced to 50mΩ using an external resistor.

Clearly, designers of battery-powered handheld devices have many options to ensure that battery life is optimized for their specific application. An optimally performing, multi-function ASSP can provide the voltage or power levels required for optimal system function while ensuring that power drain from the battery is minimized during normal operation.