By Tobias Buehler, austriamicrosystems AG

Approximately ten years ago when the first cellular phones emerged, they contained a number of discrete power components. The overall form of the first cell phones was very bulky, and their high power consumption only allowed for short standby and talk times. In response to increased consumer demand for smaller, higher performance wireless electronics, engineers have developed advanced design techniques, that allow saving PCB (printed circuit board) real estate and provide a faster and more efficient performance.

The introduction of simple, dedicated Power Management Unit ICs (PMUs) offered significant improvements, saved a lot of PCB space, and reduced overall costs. The technologies used for these devices were 1.2µm CMOS, BiCMOS or in some cases simple bipolar technology. In these early designs, the pass transistor of the high current LDOs had to be external, due to weak transistor performance, resulting in poor regulation characteristics and higher power drain, compared to today's more efficient designs.

Only when submicron CMOS processes were introduced could low impedance power transistors be integrated with justifiable silicon area and digital functionality could be increased with more complex state machines. Additionally, simple DC/DC converters, chargers and backlight switches became integrated into the PMUs.

Most vendors' preferred BiCMOS processes due to its better analog performance, but this came with much higher manufacturing costs compared to standard CMOS processes. In response, dedicated engineering efforts were undertaken in CMOS designs to yield special design techniques with nearly equal performance as BiCMOS.

Today, the demands on Power Management Unit ICs increase every day and new trends indicate that many or all features not currently covered by the chip set will eventually be integrated into the PMU. Very often considered an ASIC, these function-rich PMUs enable cell phone manufacturers to decide with very short notice to add special functionalities, demonstrating great flexibility. The more modern 0.35 μm/0.25 μm technologies allow for complex digital parts with up to 100k gates.

Another important issue is the 5 V compliance of the process, due to the direct battery connection. Fully charged Li-Ion Batteries exhibit 4.2 V, and even regulated AC adapters output 5 V. This will become one of the main challenges for future power chips, because the latest 0.18 μm/0.13 μm CMOS technologies are only available with 3.3 V options.

Current Solutions

Highly integrated PMUs are being introduced to the market. One device is manufactured in a 0.35 μm CMOS technology and housed in a thermally enhanced 6 × 6 mm 48 pin QFN package.

Figure 1.

Features of the device include 10 programmable high performance linear regulators, two highly efficient DC/DC step up/ step down converters, a 2 × 0.5 W stereo audio power amplifier, a complete chemistry independent battery charger and eight programmable general purpose I/Os for interrupt, hardware enable and high current LED supply.

The LDOs offer unparalleled regulation characteristics, comparable with the best available discrete components. The typical output noise is less than 30 μV (100 kHz bandwidth), which allows direct connection to ultra sensitive RF blocks such as VCOs and reference oscillators. The line and load regulation is better than ۫ mV static and ± 10 mV transient. Any battery noise will be suppressed effectively at the LDO output. Especially TDMA systems produce a high battery ripple due to the RF power amplifier (GSM: 217 Hz) that is periodically turned on and off. Peak currents of up to 2 A and parasitic battery resistance of 0.3 Ohms lead to a 0.6 V ripple. The LDO has the task of properly isolating this noise from the locked PLL circuit. To achieve this analog performance, the LDO is designed as a two-stage amplifier, with a high gain, low bandwidth outer loop and a nested high bandwidth low gain inner loop.

Figure 2.

Figure 2 displays a simplified symbolic schematic to show the two loops. (The realization on silicon is slightly different and much more complex.) Dynamic biasing reduces the current consumption down to 50 μA. The internal 300mA current limitation protects the device in the event of short circuits. The integrated PCH 1 Ohm pass transistor guarantees low dropout voltages of 150 mV at 150 mA. The resistor divider is programmable in 50 mV steps from 1.85 V to 3.4 V.

Digital circuits are less critical concerning noise and regulation characteristics. Therefore digital regulators can be designed with a much lower power drain. Memories and baseband processors also require lower supply voltages down to 0.8 V.

Depending on the applications running on the CPU, the battery life for portable devices can be increased by dynamic voltage frequency scaling (DVFS). The clock speed and the actual supply voltage of the processor are dynamically adjusted to meet the required MIPS. The dependency power consumption to supply voltage is quadratic; thus even small voltage reductions will lead to significantly lower power demand.

Therefore, the digital LDOs of the device can be programmed "on the fly" from 0.75 V to 2.5 V in 50 mV steps. This feature helps to run large applications on mobile battery powered devices and maintain an acceptable battery life.

To further increase efficiency, a DC/DC step down converter should be used to act as a pre-regulator for the digital LDO. In this case, the input voltage of the LDO will be only a few 100 mV higher than the output voltage.

The DC/DC step down converter works at 1MHz to allow the use of small and inexpensive external components. Only a 4.7µH inductor and a 4.7µF capacitor are required — no external Schottky diode. The achievable efficiency with low ESR coils is 95% at 100 mA.

An increasingly popular feature in cellular phones is polyphonic ring tones. Users enjoy listening to music samples and want to move beyond the simple buzzer sounds of previous generation phones. To achieve this, it is necessary to drive a small speaker at high power. The required 0.5 W to 1 W power in an 8 Ohm speaker can only deliver by a battery powered audio amplifier, and fits perfectly into a PMU. High power supply rejection, especially in TDMA systems, is also important for audio amplifier (217 Hz).

More and more cell phones are equipped with color displays requiring white LEDs for backlights. The typical dropout voltage of white LEDs is in the 3.5 V to 4.2 V range and therefore needs a step up converter. Display manufacturers integrate this LED into the display module and connect them in series. Three to four LEDs require 10 V to 16 V total supply voltage, typically generated by a DC/DC step up converter. The output voltage is regulated to the actual voltage drop of the white LEDs in order to achieve the best efficiency.

Another common function of PMUs is the battery charger. The requirements come in three stages of charging: trickle charge for empty batteries, constant current and finally constant voltage charging. All parameters should be programmable to meet different charging algorithms. Additionally, a real battery current integrating "fuel gauge" helps to provide an accurate and linear battery indicator for the user.

Next Generations

The typical partitioning of a cellular phone shows the digital baseband processor, the analog baseband, memories, the radio and the power chip. The analog and digital baseband are also manufactured in CMOS as is the PMU. Using the latest technology (such as 0.13 μm) shrinks digital components by a factor of two (compared to 0.18 μm), but analog components are reduced by only a few percent. The engineering effort required for a digital shrink is limited, but analog cells have to be completely redesigned. Therefore, the optimum partitioning for future platforms will be the marriage of analog baseband and power management units. As mentioned before, the 0.25 μm CMOS technology with the high gates density allows for increased digital functionalities. All baseband Sigma Delta Converters need significant digital pre- and post filtering, but it is feasible using this technology.

Necessity of increased battery lifetime will demand additional DC/DC converters (e.g. RF-power amplifier supplies). Combining the different blocks, high power switching components, and low noise A/D converters on one piece of silicon will be a tough challenge for PMU designers. Careful floor-planning and layout, as well as unique on-chip isolation techniques, will enable design engineers to develop stable, reliable highly integrated PMUs.

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