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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 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.
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