NATIONAL Semiconductor, represented in Australia by Arrow Electronics has released dc-dc converters optimised for powering radio frequency (RF) power amplifiers from a single lithium-ion battery. The LM3200 saves power and reduces board space in smart phones, wireless LANs, PDAs and other portable wireless devices.
It enables mobile phone makers to reduce power consumption by up to 80%, while increasing talk time by as much as 90 minutes. A high switching frequency allows it to use small external components such as a 2.2µH inductor. It has an analogue voltage-control input that controls the output voltage of the device depending on the power requirements of the power amplifier. Its bypass operation allows it to support the power amplifier's power requirement during peak usage periods to keep the phone running longer. The converter steps down an input voltage from 2.7V to 5.5V to a variable output voltage from 0.8V to 3.6V, while output voltage is set using a voltage-control analogue input for adjusting power levels and efficiency of the RF power amplifier.
The LM3200 offers four modes for mobile phones and similar RF PA applications. Fixed-frequency pulse-width modulator (PWM) mode minimises RF interference and provides efficient performance under typical operating conditions. Forced bypass mode turns on an internal bypass switch to power the PA directly from the battery. Automatic bypass mode minimises dropout by turning on the bypass switch when the battery decays close to the output voltage. Shutdown mode turns the device off and reduces battery consumption to 0.1μA (typ).
The LM3200 is available in a 10-pin lead-free micro SMD that measures 2.2mm x 1.8mm. A switching frequency of 2MHz allows the use of tiny surface-mount components. One 2.2μH inductor and two 10μF ceramic capacitors are needed to complete the application circuit.