Inverting DC/DC Controller Converts a Positive Input to a Negative Output with a Single Inductor David Burgoon There are a number of ways to produce a negative voltage from a positive voltage source, including using a transformer or two inductors and/or multiple switches, but none are as easy as using the LTC3863, which is elegant in its simplicity, has superior efficiency at light loads and reduces parts count when compared to these solutions. The LTC3863 can produce a –0.4V to –150V negative output voltage from a positive input range of 3.5V to 60V. It uses a single-inductor topology with one active P-channel MOSFET switch and one diode. The high level of integration yields a simple, low parts count solution. The LTC3863 offers excellent light load efficiency, drawing only 70µ A quiescent current in user programmable Burst Mode® operation. Its peak current mode, constant frequency PWM architecture provides positive control of inductor current, easy loop compensation and top-notch loop dynamics. The switching frequency can be programmed from 50kHz to 850kHz with an external resistor and can be synchronized to an external clock from 75kHz to 750kHz. The LTC3863 offers programmable soft-start or output tracking. Safety features include overvoltage, overcurrent, and short-circuit protection including frequency foldback. –12V, 1A CONVERTER OPERATES FROM 4.5V–16V SOURCE The circuit shown in Figure 1 produces a –12V, 1A output from a 4.5V–16V input. Operation is similar to a flyback converter, storing energy in the inductor when the switch is on and releasing it through the diode to the output when 20 | October 2013 : LT Journal of Analog Innovation the switch is off, except that with the LTC3863, no transformer is required. To prevent excessive current that can result from minimum on-time when the 45.3k Figure 1. Inverting converter produces –12V at 1A from a 4.5V–16V source output is short-circuited, the controller folds back the switching frequency when the output is below half of nominal. 100k 0.47µF 16V CLKIN RUN CAP 27nF 390pF SENSE SS Q1 D1 GATE 14.7k 61.9k LTC3863 L1 10µH ITH 1.21M FREQ SGND VFBN 68pF PGND Figure 2. Switch node voltage, inductor current and ripple waveforms at 5V input and –12V output at 1A VSW 10V/DIV 100µF 20V VIN 4.5V TO 16V 16mΩ PLLIN/MODE 0.1µF 10µF 25V ×2 VIN + VFB 33µF 16V ×2 Figure 3. Switch node voltage, inductor current and ripple waveforms at 5V input and –12V output at 30mA in pulse-skipping mode VOUT 50mV/DIV (AC-COUPLED) VOUT 50mV/DIV (AC-COUPLED) IL 1A/DIV VIN = 5V VOUT = –12V IOUT = 1A 1µs/DIV VOUT –12V 150µF 1A 16V ×2 D1: DIODES PDS540 80.6k L1: TOKO 919AS-100M Q1: VISHAY SI7129DN-T1-GE3 VSW 10V/DIV IL 1A/DIV + ADVANCED CONTROLLER CAPABILITIES 1µs/DIV VIN = 5V VOUT = –12V IOUT = 30mA PULSE-SKIPPING MODE design features The LTC3863 can produce a –0.4V to –150V negative output voltage from a positive input range of 3.5V to 60V. It uses a single-inductor topology with one active P-channel MOSFET switch and one diode. The high level of integration yields a simple, low parts count solution. The LTC3863 can be programmed to enter either high efficiency Burst Mode operation or pulse-skipping mode at light loads. In Burst Mode operation, the controller directs fewer, higher current pulses and then enters a low current quiescent state for a period of time depending on load. In pulse-skipping mode, the LTC3863 skips pulses at light loads. In this mode, the modulation comparator may remain tripped for several cycles and force the external MOSFET to remain off, thereby skipping pulses. This mode offers the benefits of smaller output ripple, lower audible noise, and reduced RF interference, at the expense of lower efficiency when compared to Burst Mode operation. This circuit fits in about 0.5in2 (3.2cm2) with components on both sides of the board. Figure 2 shows switch node voltage, inductor current, and ripple waveforms at 5V input and –12V output at 1A. The inductor is charged (current rises) when the PMOSFET is on, and discharges through the diode to the output when the PMOS turns off. Figure 3 shows the same waveforms at 30m A out in pulse-skipping mode. Notice how the switch node rings out around 0V when the inductor current reaches zero. The effective period stops when the current reaches zero. Figure 4 shows the same load condition with Burst Mode operation enabled. Power dissipation drops by 36% at this operating point, and efficiency increases from 72% to 80%. Figure 5 shows waveforms with the output shorted. The switching frequency is reduced to about 80kHz in this condition to prevent excessive current that could otherwise result. Figure 4. Switch node voltage, inductor current and ripple waveforms at 5V input and –12V output at 30mA in Burst Mode operation Figure 5. Switch node voltage, inductor current and ripple waveforms at 5V input with the output shorted HIGH EFFICIENCY Figure 6 shows efficiency curves for both pulse-skipping and Burst Mode operation. Exceptional efficiency of 89.3% is achieved at 1A load and 12V input. Notice how Burst Mode operation dramatically improves efficiency at loads less than 0.1A. Pulse-skipping efficiency at light loads is still much higher than that obtained from synchronous operation. CONCLUSION The LTC3863 simplifies the design of converters producing a negative output from a positive source. It is elegant in its simplicity, high in efficiency, and requires only a small number of inexpensive external components to form a complete converter. n Figure 6. Efficiency in normal and Burst Mode operation 90 VSW 10V/DIV VSW 10V/DIV 85 80 EFFICIENCY (%) IL 1A/DIV VOUT 50mV/DIV (AC-COUPLED) VOUT 50mV/DIV (AC-COUPLED) 75 70 65 60 55 50 IL 1A/DIV 500µs/DIV VIN = 5V VOUT = –12V IOUT = 30mA Burst Mode OPERATION VIN = 5V SHORTED OUTPUT 5µs/DIV 45 0.01 VIN = 5V, Burst Mode OPERATION VIN = 12V, Burst Mode OPERATION VIN = 5V, PULSE-SKIPPING MODE VIN = 12V, PULSE-SKIPPING MODE 0.1 ILOAD (A) 1 October 2013 : LT Journal of Analog Innovation | 21