design ideas Nanopower Buck Converter Runs on 720nA, Easily Fits into Energy Harvesting and Other Low Power Applications Michael Whitaker The LTC3388-1/LTC3388-3 integrated synchronous stepdown regulator provides a regulated output while consuming a mere 720nA of quiescent current. It accepts inputs up to 20V and can deliver up to 50mA of load current. Eight pinselectable output voltages are offered: 1.2V, 1.5V, 1.8V, and 2.5V on the LTC3388-1 and 2.8V, 3.0V, 3.3V, and 5.0V on the LTC3388-3. The extremely low quiescent current prolongs battery life for keep-alive circuits in portable electronics and makes it a good fit for energy harvesting applications. HIGH EFFICIENCY AT LIGHT LOADS The LTC3388-1/LTC3388-3 (Figure 1) achieves its low quiescent current by entering a sleep state once the output is in regulation. In the sleep state, load current is provided by the output capacitor while the output voltage is monitored. When the output falls below a fixed hysteresis window, the converter wakes up and refreshes the capacitor. SUPPORTS ENERGY HARVESTING APPLICATIONS current required to maintain regulation approaches the DC sleep quiescent current. ENABLE AND STANDBY This hysteretic method of providing a regulated output minimizes losses associated with FET switching and makes it possible to efficiently regulate at very light loads. The total quiescent current at VIN in the sleep state is 720n A when VIN is 4V and increases to only 820n A when VIN is 20V. At light loads, the time the buck is active is miniscule relative to the time it sleeps, so the average quiescent 2.7V TO 20V 1µF 6V 2.2µF 25V CAP LTC3388-1/ LTC3388-3 100µH SW VOUT 47µF 6V VOUT VIN2 4.7µF 6V Figure 1: Low quiescent current buck converter VIN The LTC3388-1/LTC3388-3 is especially well suited for energy harvesting applications where only low amounts of energy are available. Figure 3 shows the LTC3588-1 piezoelectric energy harvesting power supply harvesting ambient vibration energy with a piezoelectric transducer and producing a 3.3V output. The LTC3388-3 is powered from this output and is configured to provide a −3.3V rail, producing a low power dual output supply. The LTC3388-3’s low quiescent current, combined with the LTC3588-1’s (continued on page 43) 100 90 EN PGOOD STBY D0, D1 GND 2 OUTPUT VOLTAGE SELECT VOUT = 1.8V, L = 100µH 80 EFFICIENCY (%) Two pins, EN and STBY, control enable and standby functions on the LTC3388-1/ LTC3388-3. When EN is low, the buck is turned off and only 520n A of quiescent current appears at VIN . When EN is high, the STBY pin places the LTC3388-1/ LTC3388-3 in standby. In this mode the buck is prevented from switching, resulting in a quiet supply. The PGOOD pin, which is high when the output is in regulation, remains active while in standby. PGOOD transitioning low can serve as an indicator that the output has fallen and that the LTC3388-1/LTC3388-3 should leave standby mode to refresh the output. The extremely low quiescent current of the LTC3388-1/LTC3388-3 allows for high efficiency at loads as low as 10µ A. This is especially useful for low power systems that spend a long time idling and only periodically wake up to perform a task. Figure 2 shows typical efficiency of the LTC3388-1 for the 1.8V output, which is suitable for powering low power microprocessors. 70 60 50 40 30 20 VIN = 3.0V VIN = 10V VIN = 20V 10 0 1µ 10µ 100µ 1m LOAD CURRENT (A) 10m Figure 2: LTC3388-1 Efficiency vs load current for the 1.8V output January 2011 : LT Journal of Analog Innovation | 41 design ideas The LTC3787 is a high power 2-phase single output synchronous step-up DC/DC controller, which replaces the boost diodes with high efficiency N-channel MOSFETs. This solution eliminates the heat sink normally required in medium to high power boost converters. The LTC3787 can produce a 24V at 10A output from a 12V input at up to 97% efficiency. bursts, when the circuitry is powered up to take measurements and transmit data. The LTC3105 offers an auxiliary LDO that delivers up to 6m A of output current to power external microcontrollers and sensors while the main output is charging. Once fully charged, the main output can deliver voltages as high as 5.25V with up to 100m A of output current. It can also regulate VOUT even when VIN is greater than or equal to VOUT, offering further design flexibility. In shutdown, the LTC3105 offers output disconnect, isolating VIN from VOUT, requiring only 10µ A of quiescent current. The combination of the LT3105’s 3mm × 3mm DFN package (or MSOP-12) and very small external components offers a very compact solution for energy harvesting applications. HIGH POWER POLYPHASE SYNCHRONOUS BOOST CONTROLLER ELIMINATES HEAT SINK WITH 97% EFFICIENCY The LTC3787 is a high power 2-phase single output synchronous step-up DC/DC controller, which replaces the boost diodes with high efficiency N-channel MOSFETs. This solution eliminates the heat sink normally required in medium to high power boost converters. The LTC3787 can produce a 24V at 10A output from a 12V input at up to 97% efficiency. The LTC3787’s 135µ A standby quiescent current when configured for Burst Mode operation makes it ideal for high power automotive audio amplifiers, as well as industrial and medical applications where a step-up DC/DC converter must deliver high power in a small solution size. The LTC3787 operates from an input voltage ranging from 4.5V to 38V during start-up, maintains operation down to 2.5V after start-up and can regulate an (LTC3388-1/LTC3388-3, continued from page 41) PIEZO MIDE V25W own low quiescent current results in a complete solution that draws only 1600n A with no load at both outputs. CONCLUSION The LTC3388-1/LTC3388-3 monolithic buck converter’s extremely low quiescent current makes it ideal for low power applications. A quiescent current of less than a microamp prolongs battery life for keep-alive circuits in portable electronics and enables a new generation of energy harvesting applications. n output voltage as high as 60V. The powerful 1.2Ω onboard N-channel MOSFET gate drivers are capable of slewing large MOSFET gates quickly. The device’s current mode architecture, clock output and phase modulation enables easy paralleling of multiple devices for up to 12-phase operation. The LTC3787 has a phase-lockable frequency from 75kHz to 850kHz or a selectable fixed frequency from 50kHz to 900kHz. In applications where the input voltage exceeds the regulated output voltage, the LTC3787 keeps the synchronous MOSFET on continuously so that the output voltage follows the input voltage with minimal power loss. In addition, this device features adjustable cycle-by-cycle current limit and can use a sense resistor or monitor the voltage drop across the inductor (DCR) for current sensing. Furthermore, the LTC3787 has adjustable soft-start, a power good output and maintains ±1% reference voltage accuracy over an operating junction temperature range of –40°C to 125°C. n 1µF 6V 10µF 25V 4.7µF 6V PZ1 PZ2 VIN PGOOD LTC3588-1 22µH CAP SW VIN2 VOUT D1 D0 GND 3.3V 1µF 6V 47µF 6V 2.2µF 10V 4.7µF 6V VIN PGOOD CAP LTC3388-3 * SW VIN2 VOUT EN D1 D0 22µH GND STBY 47µF 6V –3.3V Figure 3: Piezoelectric energy harvester with dual ±3.3V outputs * EXPOSED PAD MUST BE ELECTRICALLY ISOLATED FROM SYSTEM GROUND AND CONNECTED TO THE –3.3V RAIL. January 2011 : LT Journal of Analog Innovation | 43