Karl R. Volk, Senior Corporate Applications Manager, Maxim Integrated Products, Sunnyvale, Calif.

The name of the game in cellular phone design is high integration for low cost and small size. For this reason, custom power management ICs (PMICs) handle most power-supply requirements in a modern handset. However, discrete ICs continue to find homes wherever highest performance or new features and innovations are required. A good example of this can be seen in the RF radio section of the phone, where low-dropout linear regulators (LDOs) are preferred for low-noise voltage regulation. Step-down (buck) DC-DC converters are also used in innovative ways to power the latest generation of transmit power amplifiers (PAs) for improved battery-life.

A typical cellular radio uses three LDOs to power the receiver, transmitter, and voltage-controlled oscillator (VCO). Usually, these LDOs are not integrated in the PMIC due to noise and isolation concerns. The use of discrete LDOs provides superior power-supply rejection and transmit-to-receive isolation while maintaining individual on/off control for each radio section. In most handsets, the three LDOs are chosen as small SOT23 or SC70 packages, which simplifies layout routing.

However, depending upon the RF chipset's sensitivity to crosstalk, multiple LDOs may be combined in a single IC, which may provide some cost and space-saving advantages. For example, a part such as the MAX8882 combines two 150mA low-noise LDOs in a single SOT23 package and provides over 60dB of crosstalk isolation. Going one step further, a triple LDO, such as the MAX8890 in 12-pin QFN package, may be used. Because some recent RF chipsets provide above-average transmit-to-receive isolation, a few radios are now replacing two of the LDOs with a single, higher-current LDO, for example, the MAX8887 300mA low-noise LDO in SOT23.

In the RF section, the voltage -controlled oscillator (VCO) requires a very high performance LDO to reduce phase-noise. Phase-noise is a measure of the spreading of the VCO's output frequency and is primarily caused by the VCO's limited power supply rejection. Typically, there is a maximum allowed amount of phase-noise, depending upon the communications standard. Although phase-noise in a particular radio design is easy to measure, the exact performance required from the LDO is not obvious.

Despite the uncertainty in the performance required, there are three LDO specifications that will generally lead to sufficiently low phase-noise:

• output noise of about 30uVrms between 10Hz and 100KHz

• power-supply-rejection-ratio (PSRR) typically over 60dB up to about 10KHz

• good line-transient response — usually shown in the form of an oscilloscope plot, but really a measure of the PSRR versus all frequencies.

Of these specifications, the output noise affects the VCO phase-noise during a quiet battery situation, while the power supply rejection ratio (PSRR) and line-transient response affect the phase-noise in a real system when other circuits are causing voltage ripple on the battery. Further reducing the phase-noise to levels below those required by the particular communications standard is desirable for improved radio tuning sensitivity. Therefore, companies such as Maxim are developing LDOs specifically targeted for noise-critical VCO applications. For example, the MAX8510, , outputs 80 mA with less than 10 µVrms output noise, 80dB PSRR below 5kHz and more than 74dB PSRR up to 10kHz.

During the last two years, there have been increased innovations and design activities with respect to using a switch-mode power supply to power the transmit PA. These designs can be divided into two major categories: efficiency improvement only and amplitude modulation of the RF signal envelope through efficient means.

In the first case, a transmission standard lowers the RF transmit signal level in response to the handset's proximity to the basestation. Because of this, designers increase talk-time by using a step-down (buck) converter to efficiently lower the PA's Vcc supply voltage to the minimum headroom above PA saturation. In the second case, the PA is deeply saturated with RF frequency and phase signals, while the buck converter modulates the PA supply to impart amplitude envelope information onto the RF signal, thus increasing efficiency and simplifying other circuits in the radio.

One of the first buck converters targeted directly at PA power was designed specifically for use in WCDMA handsets. However, with the delayed rollout of WCDMA, attention to improved efficiency has refocused on NCDMA. Today, nearly all WCDMA and many NCDMA design projects include a buck converter in the radio architecture. The earliest efforts had focused on achieving the highest possible efficiency via a "continuous" scheme that adjusts Vcc for every 1dBm change in transmit power.

Two step approach

However, it quickly became apparent that the buck converter and inductor were somewhat large and the software, lookup table, and calibration needed for this adjustment were fairly complex. With NCDMA's well defined urban and suburban transmit probability-density-functions, a "two-step" method quickly became the standard.

In the two-step solution, the PA is powered from the handset's battery via a MOSFET switch during high-power transmission and from the buck converter's output during low-power transmission. This scheme not only offers the lowest possible dropout in high-power mode, but also allows for a very small buck converter (in SOT23) and tiny inductor as they need to supply only one quarter as much output power compared to the continuous scheme.

Furthermore, with a single switchover point, software and calibration are greatly simplified. Typically, the buck converter switchover point is chosen to match the probability density function and, in may cases, to simply provide the highest talk-time number, as printed on consumer packaging. In a few cases, designs are implementing "multi-step" solutions to further improve actual talk-time without the full complexity of the continuous adjustment scheme.

Because small size and integration are so important, new-generation switching regulators will integrate a high-power-mode MOSFET switch. Each of these devices controls its internal MOSFET switch with an LDO to facilitate smooth entry and exit from dropout, allowing operation in two-step, multi-step, or continuous adjustment schemes.

And, there is some innovative work being done with envelope remodulation and polar-loop modulation schemes for popular GSM-based standards. In these designs, the buck converter's output is dynamically modulated to impart amplitude information onto the RF signal via the PA's Vcc supply. This makes the buck converter an actual part of the radio as a class-D or class-S amplifier, rather than a power supply. Although this simplifies some of the radio design, there are several extremely challenging aspects for a buck converter used in this way.

First, the modulation bandwidth may be several hundred kilohertz with a very low distortion and phase lag requirement. Second, because the PA is saturated, the buck converter's output ripple must be extremely low. Also, the buck converter's output must be essentially rail-to-rail (from ground to the battery voltage, which requires 0% to 100% duty-cycle operation). When considered together, the optimum solution is something like a multi-phase ("multiple inductor") delta modulator running at many megahertz switching speed to keep ripple low. The actual switching speed needs to be low enough to prevent losing too much efficiency due to switching losses and high enough to reduce the number of phases (inductors) required to meet the output ripple demand. Although serious research and development work for this approach is ongoing, it remains to be seen if this type of solution can be made cost effective enough for a handset.