DESIGN IDEAS ing spikes ruin an otherwise clean output. Efficiency of the circuit is detailed in Figure 5. Efficiency reaches 73% at a 50mA load, and is above 70% at a 200mA load. Larger inductors with less copper resistance can be used to increase efficiency, although such inductors are more expensive than the Murata units specified. 90 80 EFFICIENCY (%) low noise that can be achieved with a ceramic capacitor may be corrupted by noise spikes if proper layout practice is not followed. To illustrate this point, output and switch waveforms from Figure 1’s circuit, with a 10µ F ceramic output capacitor and 200mA load, but with D1’s cathode arbitrarily connected to the ground plane, are shown in Figure 4. 60mV switch- 70 60 50 40 10 30 100 LOAD CURRENT (mA) 3 300 1610 TA02 Figure 5. 5V to –5V converter efficiency reaches 73%. 4.5ns Dual-Comparator-Based Crystal Oscillator has 50% Duty Cycle and Complementary Outputs by Joseph Petrofsky and Jim Williams Figure 1’s circuit uses the LT1720 dual comparator in a 50% duty cycle crystal oscillator. Output frequencies of up to 10MHz are practical. Resistors at C1’s positive input set a DC bias point. The 2k–0.068µ F path furnishes phase-shifted feed- back and C1 acts like a wideband, unity-gain follower at DC. The crystal’s path provides resonant positive feedback and stable oscillation occurs. C2, sensing C1’s input, provides a low skew, complementary output. A1 compares band-limited versions of 2.7V–6V 2k 1MHz–10MHz CRYSTAL (AT-CUT) 220Ω 620Ω + GROUND CASE C1 1/2 LT1720 the outputs and biases C1’s negative input. C1’s only degree of freedom to respond is variation of pulse width; hence, the outputs are forced to 50% duty cycle. The circuit operates with AT-cut fundamental crystals from 1MHz to 10MHz, over a 2.7V–6V power supply range. 50% duty cycle is maintained at all supply voltages, with output skew below 800 picoseconds. Figure 2 plots skew, which is seen to vary by about 800ps over a 2.7V–6V supply excursion. OUTPUT – 1000 100k 2k 0.1µF – 680Ω 0.1µF 100k OUTPUT SKEW (ps) 0.068µF 800 + A1 LT1636 600 400 200 + C2 1/2 LT1720 OUTPUT – 0 2.5 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 6.0 AN70 F52 Figure 1. Crystal oscillator has complementary outputs and 50% duty cycle. A1’s feedback maintains output duty cycle despite supply variations. 36 Figure 2. Output skew varies only 800ps over a 2.7V–6V supply excursion. Linear Technology Magazine • August 1998