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