Aug 1998 Low Noise 33V Varactor Bias Supply

DESIGN IDEAS
Low Noise 33V Varactor Bias Supply
by Jeff Witt
Wideband tuning circuits, such as
those used in cable television systems, require a power supply for
driving a varactor. This bias supply is
usually at a voltage higher than the
system supply voltage, allowing a large
tuning range. The supply must have
very little noise; voltage ripple, for
example, can appear as sidebands on
a local oscillator. This circuit takes
advantage of the fixed operating frequency of the LT1317B boost regulator
to generate a low noise 33V bias
voltage.
The circuit (Figure 1) is a simple
boost regulator with its output voltage doubled by diodes D2 and D3 and
capacitor C3. With this doubler, the
circuit can generate an output voltage greater than the voltage rating of
the LT1317B’s internal power switch.
This supply can deliver 10mA at 33V
from a 3V to 6V input, allowing
operation from either 3.3V or 5V logic
rails. The high operating frequency
(600kHz) results in low, easily filtered output ripple, as shown in
Figure 2. The high frequency also
allows the use of small, low cost
external components.
D3
680Ω
150pF
D2
C3 0.1µF
VIN
L1 22µH
C1
15µF
10V
VIN
A 47Ω
D1
VOUT
33V
0mA–10mA
SW
SHDN
LT1317B
C2
10µF
35V
150k
VC
33k
B
0.1µF
0.1µF
0.1µF
FB
NODE A
OUTPUT RIPPLE
20mV/DIV
NODE B
OUTPUT RIPPLE
20mV/DIV
GND
3300pF
5.90k
1µs/DIV
D1–D3: MOTOROLA MMBD914LT1
C1: AVX TAJ156M010
C2: SANYO 35CV33GX
L1: MURATA LQH3C220
Figure 1. This circuit generates a low noise bias supply for varactor-based
tuning circuits.
DESIGN IDEAS
Low Noise 33V Varactor Bias Supply
................................................... 34
Jeff Witt
Low Noise LT1614 DC/DC Converter
Delivers –5V at 200mA from 5V Input
................................................... 35
Steve Pietkiewicz
8ns Dual-Comparator-Based
Crystal Oscillator has 50% Duty Cycle
and Complementary Outputs
................................................... 36
for
the latest information
on LTC products,
visit
www.linear-tech.com
Figure 2. The output ripple of Figure 1’s supply as it
delivers 5mA at 33V from a 5V input; traces A and B show
ripple before and after the RC output filter, respectively.
For further information on any
of the devices mentioned in this
issue of Linear Technology, use
the reader service card or call
the LTC literature service
number:
1-800-4-LINEAR
Ask for the pertinent data sheets
and Application Notes.
Joseph Petrofsky and Jim Williams
Battery Charger IC Doubles
as Current Sensor ....................... 37
Craig Varga
34
Linear Technology Magazine • August 1998
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