DESIGN IDEAS Low Noise LT1614 DC/DC Converter Delivers –5V at 200mA from 5V Input by Steve Pietkiewicz The inverting DC/DC converter function is traditionally realized with a capacitor-based charge pump. Although simple, the output impedances of the best charge pump solutions are in the 5Ω to 10Ω range, resulting in significant regulation issues when the load current increases beyond a few tens of milliamperes. The LT1614 inductor-based inverting DC/DC converter uses closed-loop regulation to obtain an output impedance of 0.1Ω, eliminating output voltage droop under load. Figure 1 details the 5V to –5V converter circuit. The LT1614 contains an internal 0.6Ω switch rated at 30V, C3 1µF L1 22µH VIN 5V allowing up to 28V differential between input and output. Quiescent current is 1mA and the device contains a lowbattery detector with a 200mV reference voltage. The device switches at 600kHz, allowing the use of small, inexpensive external inductors and capacitors. In fact, the total cost of the components specified in Figure 1 (excluding the LT1614) is approximately $0.70 in 10,000-piece quantities. The LT1614 operates by driving its NFB pin to a voltage of –1.24V, allowing direct regulation of the negative output. This converter topology, which consists of inductors in series with both input and output, results in low output noise and also in low reflected noise on the 5V input supply. The output and switch nodes are shown in Figure 2. Output ripple voltage of 40mV is due to the ESR of the tantalum output capacitor C2. Ripple voltage can be reduced substantially by replacing output capacitor C2 with a 10µ F ceramic unit, as pictured in Figure 3. In layout, be sure to tie D1’s cathode directly to the LT1614’s GND pin, as shown in Figure 1. This keeps the switching current loops tight and prevents the introduction of high frequency spikes on the output. The L2 22µH D1 SW VIN + SHDN VC VOUT –5V/200mA R1 69.8k LT1614 C1 33µF V OUT 100mV/DIV AC COUPLED NFB GND R2 24.9k + 100k C2 33µF 1nF D1: MBR0520 L1, L2: MURATA LQH3C220 C1, C2: AVX TAJB336M010 C3: AVX1206YC105KAT (CERAMIC, X7R) Figure 1. 5V to –5V DC/DC converter uses an inverting topology and delivers 200mA. V OUT 100mV/DIV AC COUPLED V SW 5V/DIV 500ns/DIV Figure 2. LT1614 output and switch node with a 33µF tantalum capacitor and 200mA load current V OUT 100mV/DIV AC COUPLED V SW 5V/DIV V SW 5V/DIV 500ns/DIV 500ns/DIV Figure 3. LT1614 output and switch node with a 10µF ceramic output capacitor and 200mA load current Figure 4. Improper placement of D1’s cathode results in 60mV switching spikes at output, even with a 10µ F ceramic output capacitor. Linear Technology Magazine • August 1998 35 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