DESIGN IDEAS Low Noise Boost VCO Power Supply for Portable Applications by Ted Henderson Introduction Many portable RF products use voltage controlled oscillators (VCOs) to generate the RF carrier frequency. These applications often require low noise VCO power supply voltages that are greater than the primary battery supply. A DC/DC converter powering a low noise linear regulator is often used. Unfortunately, there are several disadvantages to this solution. The DC/DC converter tends to produce noise that may not be rejected by the regulator, resulting in regulator output noise far greater than the thermal noise levels. The linear regulator may require a large output compensation capacitor with specific ESR requirements. The board area for both devices and support components can be large. The LTC1682 charge pump DC/DC voltage converter has been designed to minimize these issues. The charge pump and linear regulator have been mutually optimized for minimum regulator output noise. The linear regulator was designed to operate with several different types of output capacitors including small, low value, low ESR ceramic capacitors. The charge pump voltage converter and low dropout linear regulator are combined on one SHUTDOWN 8 CPO VOUT 4.7µF 7 VIN 2.5V TO 4.4V 6 C+ 1 4.7µF 5 1000pF 2 SHDN 36k LTC1682 VIN FB C– GND 3 0.22µF 4.7µF 4.2V 4 B P VCO MURATA MQE001-902 M C VCO OUTPUT 902MHz 100k VC 1µF 15k 1k 1000pF 1000pF 4.7µF Figure 1. 4.2V VCO power supply CENTER = 902MHz SPAN = 100kHz SWP = 10s RESBW = 1kHz VBW = 30Hz REF = 0dBm AMPLITUDE 10dB/DIV die and assembled in an extremely small MS8 package. Both fixed and adjustable output voltage versions are available to cover the widest possible output voltage range. VCO Power Supply Figure 1 shows the LTC1682 generating a 4.2V low noise power supply for a 900MHz VCO with an input voltage range of 2.5V to 4.4V. Figure 2 shows the close-in phase noise of the VCO operating open loop and Figure 3 shows the typical peak-to-peak noise voltage at VOUT. Conclusion The new LTC1682 family of charge pump DC/DC voltage converters represents a complete low noise solution for VCO power supplies. A wide input voltage range of 1.8V to 4.4V, low dropout voltage, low quiescent current, low external parts count and small board area make these devices ideal for portable applications. VOUT 200µV/DIV COUT = 4.7µF IOUT = 10mA VOUT = 4.0V BW = 10Hz–2MHz 100µs/DIV Figure 2. Close-in phase noise Figure 3. Output noise voltage For more information on parts featured in this issue, see http://www.linear-tech.com/go/ltmag Linear Technology Magazine • September 1999 37