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Many AC parameters affect RF system performance including transient response, power supply ripple rejection and self noise. Power supply ripple rejection, PSRR, is the regulator's ability to reject input noise at a specific frequency. It is a measure relative to frequency and it is always expressed in dB. The importance of this parameter in a battery-powered application is thus: batteries give an ideal voltage source when there are no perturbations in that voltage source. Lithium-ion batteries have a reasonable amount of output impedance compared to other batteries. The output voltage will show a large variation based upon the current drawn from that battery. In GSM, transmit and receive cycles in the RF draw a huge amount of current, causing a large change in voltage on the output of the battery. That change in voltage is seen throughout the system and on inputs to all of the system regulators. Regulators powering noise-sensitive circuits, such as RF chipsets, must be able to reject large change in supply voltage so that RF circuitry doesn't modulate that noise into the RF output and cause interference between adjacent operating channels, or users. As seen in Figure 1, MIC5305 PSRR prevents interference; the RF will function without problems.

When designing with a lithium-ion battery system, the usable range of the battery is between 3.0 V to 4.2 V. The output impedance of that battery goes up when the voltage drops and at low temperatures. Meaning, at low supply voltages and cold temperatures, the changes in the output voltage of the battery, with the same change in current caused by the transmission of data through the power amplifier of a GSM system, will be much larger. With the same PSRR level, the output of the regulator will have more noise than at nominal battery voltage and nominal temperature. Hence, the necessary PSRR level required to maintain good RF sensitivity under extreme conditions is very high.

All power supplies take some finite amount of time to respond to changes. When input voltage to a regulator changes, the output voltage starts to move because, virtually instantaneously, all bias points and steady state conditions change. The output will start to change as well. The transient response of the regulator determines the amount of time it takes for the LDO to realize that its output has changed, combined with the amount of time it takes the LDO to start pulling the output back into nominal regulation. This factor depends on bias currents, slew rates of amplifiers, and the amount of output capacitance the loop is designed to work with. The more output capacitance, the less voltage change the output of the regulator will see. Likewise, for a change in output current. When the output of the regulator sees a large change in output current, the output capacitor discharges until the regulator reacts to changing current and voltage on the output and pulls the output back into regulation.

In order to achieve wide large signal bandwidth, the regulator will have to consume a significant amount of current in order to broadband the amplifiers as well as giving the loop significant slew rate capability. Normally, when designing a low-power regulator, the currents scale down significantly and the load transient response scales down as well. The MIC5305 load transient response and MIC5235 load transient response, as shown in Figures 2 and 3, show the transient response of a 150 mA regulator with 90 uA of quiescent current (MIC5305) vs. the transient response of a 150 mA regulator with 18 uA of quiescent current (MIC5235). The 18 uA regulator has a significant amount of output capacitance yet has more voltage droop because it requires a significantly larger amount of time to respond to load changes. The MIC5305 regulator has a significantly faster transient response and has less voltage droop even though the output capacitor is a lower value than that of MIC5235.

Various AC parameters will always affect RF system performance. With careful design considerations and IC solutions, designers can get the best possible LDO performance for their products.

John McGinty is strategic applications engineer at Micrel Inc. He has more than eight years high-tech experience in the semiconductor industry. He is responsible for designing Micrel's analog products throughout Europe as well as for new product definitions. He holds a Bachelor of Science Degree in Engineering Physics from John Carroll University, OH and his first high-tech patent is pending.

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