While power-supply efficiency and battery life have always been problematic in handheld designs, powering the portable RF circuitry -one of the most compelling features of portable devices-remains especially challenging. But the presence of low-level RF circuitry makes power-supply noise a far greater concern than in earlier portable systems. Fortunately, there are a number of techniques to help power supplies accommodate RF designs-usually without adding to system cost or size.
The major design issues for portable products that contain RF circuitry are suppressing the dc/dc converter electromagnetic interference (EMI) with minimum circuitry, dealing with high-current pulsed loads and getting the most from high-impedance batteries that are less than ideal.
Most accepted techniques for reducing noise in power-supply circuitry involve some energy-loss trade-offs. Some of the common techniques used to reduce high-frequency energy include RC networks that "snub" fast-moving signals (snubbers), LC input and output filters (also known as "traps") on the power traces and slowed switch-transition time (in dc/dc circuits). Those techniques can be effective, but often come with a reduced-efficiency price. Before resorting to such heavy-handed tools in portable systems, it's worth trying efficiency-friendly techniques first, such as clock-synchronized dc/dc conversion, "smart" inductor ringing clamps and multimode dc/dc conversion. Even thoughtful component selection can help.
One of the lowest-cost techniques for power-supply noise reduction is dc/dc converter synchronization. Spectral plots can show how effective synchronization can be. While synchronization actually doesn't reduce the total noise energy, it does confine it to frequencies dictated by the clock. The clock rate is selected to park converter noise away from sensitive frequencies. Constraining ripple frequencies might even cut costs, since less filter capacitance may be needed than without synchronization. In addition, this improves reliability if capacitance values can be sufficiently reduced to allow ceramic capacitors to replace aluminum electrolytic or tantalum ones.
In very low-power wireless devices such as pagers, sensitivity is particularly important. Nevertheless, the receiver must still coexist with switch-mode power circuits that boost a one-cell battery to the required operating voltages. At the sub-milliamp loads common to paging devices, interference may come from the high-frequency ringing waveform that occurs with switching-regulator discontinuous inductor current. The standard method for suppressing ringing is to connect a resistive snubber across the inductor. The downside, again, is that this subtracts from battery-conversion efficiency.
A better scheme, especially when integrated in the boost converter IC, is an actively switched ringing clamp. The switch, which is connected across the output of the pulse-width modulation (PWM) and its inductive filter, shunts the inductor only when the inductor's energy is nearly depleted, so there is almost no efficiency penalty. It's also worth noting that the damping of inductor ringing reduces radiated EMI without resorting to shielding, but does not reduce output-voltage ripple.
Another way to prevent inductor ringing in dc/dc converters is to force continuous coil current. An added benefit can also be a fixed (or synchronized) dc/dc converter operating frequency. Forcing continuous inductor current at light loads is only possible when synchronous rectification is used. With synchronous rectification, the output "catch" diode is either replaced or paralleled by a MOSFET switch. In low-power designs, that usually makes economic sense only if the switch is included within the dc/dc converter IC.
One of the penalties of forcing fixed switching frequency or continuous inductor current in a dc/dc converter is higher operating current at light loads. The switches continue to clock at a high rate even when the load is small, so efficiency can suffer. This deficiency is solved when the dc/dc converter IC includes multiple operating modes. Two modes frequently provided are fixed-frequency pulse-width-modulation for best noise characteristics, and pulse-frequency-modulation for best light-load efficiency. Depending on the device, modes may be pin-selected or may switch automatically at a particular load current. Such multimode power ICs provide the greatest latitude since they allow the designer to tune power- supply behavior for different system operating states.
Digital wireless designs can be hell on batteries when the RF transmitter is keyed on. The pulsed nature of the transmission is especially nasty since high power is needed in short pulses. In the GSM standard, for example, a 577-microseconds transmit burst occurs every 4.6 ms. With clever system design, the pulsed behavior can be turned into an advantage by using transmitter idle time to fill a reservoir capacitor. The RF power amp (PA) gets its peak load from this capacitor without overloading the battery. Battery current is then drawn in a continuous fashion, rather than in short peaks, so loss through the battery's resistance is minimized. Besides the battery-life benefit, this scheme can also reduce noise by allowing the dc/dc converter to turn completely off during the transmit pulse.
Perhaps the most extreme case for a wireless application is a two-way pager. In those products the power source may be only a one-cell AA battery, but there is still an RF power amp that may need current pulses of an amp or more. As in the GSM phone case, an energy reservoir saves the battery from pulse load used by the transmit burst. But since transmit duration may be several seconds, a conventional cap is inadequate for storing that much energy. Either a superhigh-value cap (0.1 farad), or a rechargeable battery is required. Circuitry for managing charging as well as other functions in these devices becomes quite complex, so highly integrated solutions are preferred, especially in light of the size constraints inherent in paging devices.
Though other RF power-amp technologies are gaining ground, gallium-arsenide (GaAs) field-effect transistor (FET) RF power amps remain the most efficient with regard to highest transmitted power for dc power-in. Consequently, they are the preferred technology as cell phones and new wireless devices attempt to stretch operating time while reducing size.
GaAs PAs provide higher gain and power efficiency at frequencies over 1 GHz as compared with conventional bipolar PA technologies. Since RF transmit circuits are usually the largest power consumers in wireless products, PA efficiency is a major factor for battery life. But GaAs efficiency gains do not come without inconveniences. GaAs FETs require a negative bias voltage in addition to their main voltage supply . And, this bias must be quiet, with less than a few mV of ripple and noise to prevent signal degradation.
GaAs FET bias ICs
The output current required for GaAs PA bias is typically 4 mA or less, so charge-pump ICs are preferred over inductor-based inverters because of the lower cost and the elimination of an inductor or transformer as an EMI source. A noise source that may be troublesome in charge pumps is the charging current that flows when the "flying"capacitor is switched. This can generate objectionable output noise unless a post regulator or filter is added. The best noise performance is provided by dedicated GaAs FET bias ICs that combine a high-speed inverting charge pump with a low-noise linear regulator to generate a very quiet negative voltage source from a positive dc input.
Output noise is kept below 1 mV peak-to-peak. A second GaAs PA requirement is that negative gate bias be applied before power connects to the PA (drain). If power is applied before gate bias, the PA can be damaged. The correct sequence can be ensured by a bias circuit monitor that controls a switch-usually a P-MOSFET-in series with the GaAs drain. The P-MOSFET will supply current to the drain only when the gate is properly biased.
The lowest-cost noise reduction comes with careful component selection and pc-board layout. Shielded coils can be effective and there are surface-mount choices for low-power designs. Ceramic capacitors, with values up to and over 10 F, offer superior equivalent series resistance and inductive characteristics in bypassing applications and dc/dc filters.