The Future of Power Supply Design for Portable Applications

Today, with the increasing integration of functions, mobile phones can also be used as portable media players (PMP), digital cameras, handheld computers (PDAs), and even global positioning systems (GPS). How to obtain more realistic displays, break through thermal bottlenecks, and manage power efficiently and intelligently are the challenges currently faced by system designers.

National Semiconductor provides a variety of power management solutions for portable applications: (1) intelligent WLED backlight drivers; (2) dual-mode SuPA for RF power amplifiers (RF PAs); and (3) advanced power management units (PMUs).

1 Intelligent WLED backlight driver

In the past few years, liquid crystal displays (LCDs) have greatly improved their size and resolution. To make movies and images more realistic and attractive, backlighting has gradually become a focus of optimization and improvement. With the development of high-speed 3G data streaming services, LCD sizes have become larger and larger, and to meet the growing demand for multimedia content, LCD power consumption has also increased. How to extend system standby time and video playback time is a current concern for system designers.

The backlight of a traditional small-panel LCD display is realized by a pair of white light-emitting diodes (LEDs). For small-panel LCDs, white LEDs are used in parallel and powered by a charge pump. The main reason is that the cost of the charge pump is low and the brightness uniformity requirements for each LED are not very strict. Large-panel LCDs use 6 or more LEDs, and their uniformity and efficiency requirements are very strict. In this case, the charge pump cannot meet its requirements, and the boost converter has obvious advantages in this regard. The new LM3530 launched by National Semiconductor can not only power up to 11 series white LEDs, but also control the current. Figure 1 shows the typical application circuit of the LM3530.

The LM3530 features a 1 A current limit and an input voltage range from 2.7 V to 5.5 V, making the device a versatile backlight power supply that is well suited for the voltage range of a single lithium battery. The maximum LED current is adjustable from 5 mA to 30 mA through an I2C-compatible interface. Unlike traditional WLED driver topologies, the LM3530 has a true shutdown function to improve efficiency. In addition, it also has an overvoltage protection function of up to 43 V. The most powerful feature of the LM3530 is its ability to support ambient light sensing (ALS) and dynamic backlight control (DBC).

ALS: Ambient light sensing, commonly used to measure external light. In brighter environments, the backlight requires more current to illuminate the display, and in darker environments, the backlight can be lowered. In traditional ALS, the processor needs to constantly poll the output status of the ambient light sensor to determine whether to increase or decrease the brightness.

DBC: Dynamic Backlight Control. In this system, the display driver analyzes the content and outputs the settings requested by the backlight driver. This is called Content Adapted Backlight Control (CABC). Brightness is adjusted based on the ambient lighting, resulting in better contrast and dark levels.

The LM3530 combines these two signals and intelligently determines the correct backlight setting for different situations. The entire process does not require processor involvement. The performance of the ALS can be programmed by the user to customize it for different applications. Considering that the LED current can be adjusted through a logic-level pulse width modulation (PWM) signal, PWM brightness control can be enabled using the I2C interface. Integrated ALS and CABC support can save up to 50% of power.

As can be seen from the typical application circuit, the LM3530 integrates a dual-input ambient light sensing interface (ALS1 and ALS2) that converts the analog output ambient light sensor signal into a user-specified brightness level. The ambient light sensing circuit has 4 programmable boundaries, defining 5 ambient brightness areas. Each ambient brightness area corresponds to a variable programmable brightness threshold. The programmable ALS interface is used to receive the minimum and maximum voltages from ALS1 or ALS2, or to select to receive ambient light information from ALS1 or ALS2. After the area is changed, the LM3530 will issue an interrupt signal, and the processor can adjust the keyboard LED or perform other operations; the ALS input can also use a proximity sensor. When a call comes in and the user answers the call, the mobile phone can let the LM3530 choose to receive the proximity sensor output. The LM3530 will lock the display and reduce the backlight accordingly to save power and prevent false triggering of the touch panel. In addition to ALS and DBC, the 12-bit dimmer can easily smooth the dimming work. At the same time, at any given maximum LED current setting, LED brightness can be further adjusted using 127 exponential or linear dimming levels.

2 SuPA for RF Power Amplifiers (RF PAs)

Currently, 3G mobile phones are being promoted rapidly around the world. At the same time, the higher data rate also brings about current consumption and heat dissipation issues. Current power amplifiers are all powered directly by batteries, which can easily implement the system, but the linear power amplifiers manufactured according to this standard can only achieve low efficiency in the entire transmission power spectrum. How to extend talk time and battery life with the same high-performance battery is an urgent problem for system designers to solve.

Figure 2 shows the loss and inefficiency architecture of an RF power amplifier. At RF IN, the power is 28 dBm, and for the Class III power requirement at the antenna, the maximum output power is 24 dBm. Therefore, at maximum power, the power added efficiency (PAE) is only 40%. In addition to PAE, another important specification for RF power amplifiers is the adjacent channel power/leakage ratio (ACPR/ACLR). PAE and ACPR/ACLR are often used to characterize the distortion of power amplifiers and other subsystems because they have a tendency to interfere with the radio channel or system. To ensure high linearity, it should meet the specifications under both normal and extreme conditions.

To improve system performance and extend battery life, National Semiconductor has introduced a power management technology for RF power amplifiers (PAs), called SuPA. SuPA can be used in the switch unit of the power amplifier, which is a dedicated DC/DC converter that provides the power supply voltage to the PA. Figure 3 shows the principle of improving the RF power amplifier subsystem.

As the transmitted power increases, Vout also increases. Therefore, to maintain the ACLR specification, the output voltage of the SuPA must be changed to ensure that no distortion occurs and linearity is not affected. The LM3212 reduces the input voltage of 2.7 V to 5.5 V to a dynamically adjustable output voltage of 0.6 V to 3.4 V. The output voltage is set using the VCON analog input. The dynamically adjustable output voltage ensures efficient operation at all power levels of the RF power amplifier. In shutdown mode, the device shuts down and reduces battery consumption to 0.02 μA.

When used in GSM mode, the LM3212 can support currents up to 2.5 A. With internal synchronous rectification, the LM3212 can achieve efficiencies up to 95%. National Semiconductor developed the LM3212 to address efficiency issues while meeting the requirements of GSM/EDGE (2G) and WCDMA/EVDO/TDSCDMA (3G) standards. Battery current is reduced by 50% at 30 dBm. Vcon can be controlled by a DAC output from the baseband or by a filtered variable duty cycle PWM signal generated by the GPIO.

3 Advanced Power Management Unit PMU

When developing battery-powered devices (such as mobile phones, PMPs, and DSCs), if the power system design is not reasonable, the entire system architecture, software design, and power distribution architecture will be affected. At the same time, in system design, designers should consider more about how to save battery power. In order to reduce battery current, today's portable product processors usually have multiple working modes (standby, sleep, deep sleep), that is, when the user system does not need to run at full load, the processor can enter a low-power mode with less power consumption.

From the development trend of portable product power management, system designers need to consider the following aspects: high flexibility, high integration and high efficiency. A common way to solve these problems is to choose a programmable power management device to manage the power system. The LP8720 launched by National Semiconductor is a very smart and efficient power management solution. It is a multifunctional programmable power management device with a size of 2.5 mm × 2.0 mm × 0.6 mm and a micro surface mount component (SMD) package. The device integrates an efficient 400 mA step-down DC/DC converter based on dynamic voltage regulation (DVS), 5 low-noise low-dropout (LDO) regulators and a 400 kHz I2C compatible interface.

The LP8720 with interrupt feature will issue a thermal shutdown warning interrupt signal and initiate thermal shutdown. The device has a control input DELSEL for setting the default voltage and startup sequence, and must be firmly connected to the battery (BATT) or firmly grounded (GND), or left floating (Hi-Z) for specific applications. When powering the application device, the processor LP8720 is used as a sub-PMU and can be first programmed (set) through I2C in standby mode to obtain the expected voltage and sequence, which is quite useful for different multimedia processors. The LP8720 also has another control input IDSEL for setting the slave address of the serial interface, which must also be firmly connected to the battery (BATT) or firmly grounded (GND), or left floating (Hi-Z) to obtain different I2C addresses.

If the sum of the load current of the LDO and the buck converter does not exceed 5 mA, the user can set the LP8720 to sleep mode. In sleep mode, the quiescent current of the PMU will be further reduced to 100 μA with the buck converter and LDO1 enabled. In sleep mode, the LDO and buck converter cannot load large currents. The LP8720 can enter sleep mode in two cases: one is controlled by the serial interface; the other is controlled by the DVS- pin.

The buck converter of the LP8720 supports dynamic voltage scaling through I2C and GPIO, which means that system designers can use I2C or GPIO to implement DVS according to their preference. A small inductor of 2.2 μH can be used by switching frequency of 2 MHz. Automatic PFM/PWM switching ensures that the buck converter can operate efficiently under different load conditions. The buck converter output voltage can be set by selecting the external feedback resistor network, as shown in Figure 4. Vout can be adjusted to make the voltage at the FB pin equal to 0.5 V. The resistance from the FB pin to ground (RFB2) is about 200 kΩ, which helps to keep the current flowing through the resistor network to a minimum, but still large enough not to be affected by noise. For output voltages greater than or equal to 0.8 V, the zero transfer function should be added by attaching capacitor C1.

There are 5 LDOs on LP8720, namely A-type LDO (LDO 2, 3), D-type LDO (LDO 1, 5), and LO-type LDO (LDO4). The A-type LDO is optimized for powering analog loads and has ultra-low noise (15 μVRMS) and excellent PSRR (70 dB) performance; the D-type LDO is optimized for good dynamic performance to power fast-changing (digital) loads; the LO-type LDO is optimized for low output voltage and good dynamic performance to power fast-changing (digital) loads. The specially designed LO-type LDO can use the output of the buck converter as the input voltage to achieve higher efficiency than traditional linear regulators. For the LDO, it has an Iq of 10 μA. When Vin=1.8 V, Vout=1.5 V, and Iout=150 mA, the LILO LDO efficiency can be calculated as follows:

If LILO uses a 3.6 V battery as input power, the efficiency is only 42%. Although there is some efficiency loss in the buck converter, the improvement is still considerable. Therefore, this technology can maximize the battery life of portable application equipment and minimize heat loss.