design features 15V, 2.5A Monolithic Buck-Boost DC/DC Converter with 95% Efficiency and Low Noise Operation Eddy Wells Power-hungry handheld devices and industrial instruments often require multicell or high capacity batteries to support their ever-increasing processing needs. A wide voltage range, high efficiency buck-boost DC/DC converter is the ideal solution for longer battery run times and handling multiple input sources. The LTC3112 is a 2.2V to 15V input capable 2.5A buck-boost converter. The extended voltage range allows conversion from a variety of power sources such as one, two or three Li-ion cells, lead acid batteries, supercapacitors, USB cables and wall adapters to output voltages programmed between 2.5V and 14V. The LTC3112 features the latest generation buck-boost PWM control scheme, effectively eliminating jitter when crossing the barrier between buck and boost operation. Safeguards such as current limit, overvoltage protection, thermal shutdown, and short-circuit protection provide robust operation in harsh environments. For demanding applications where component size or conversion efficiency is critical, the LTC3112’s 750kHz default switching frequency can be synchronized between 300kHz and 1.5MHz. For designs where output current needs to be controlled or measured, an output current monitor pin is available. Selectable Burst Mode® operation extends the operating life when the battery-powered device is idle. The LTC3112-based converter shown in Figure 1 can generate 30W of power with a 12V output. The solution footprint is less than 200mm2, which cannot be matched by a controller-based buck-boost or complex dual-inductor SEPIC design at similar power levels. The main external components are limited to the input and output filter caps and the power inductor. The LTC3112 is offered in a thermally enhanced 16-lead 4mm × 5mm DFN or 20-lead TSSOP package. extends high efficiency operation for more than two decades of load current. Figure 1. LTC3112 based 30W solution OPERATION FROM MULTIPLE INPUT SOURCES The LTC3112’s wide operating range allows devices to be powered from multiple input sources. Figure 2 shows an application where the LTC4412 PowerPath controller (TSOT-23 package) provides a low loss selection between two input sources. The LTC4412 maintains a 20mV forward voltage across the selected P-channel MOSFET, keeping losses to a minimum. In this circuit, the LTC4412 automatically switches the greater of a single Li-ion cell or 12V wall adapter to the input of the LTC3112. Efficiency curves based on the two input sources are given in Figure 3. Peak efficiencies of greater than 90% are achieved with either input. Selectable Burst Mode operation (dashed lines) with 50µ A of typical sleep current A feedforward network (CFF, RFF of Figure 2) connected between the VIN and FB pins provides improved transient response when the wall adapter voltage is applied. Feedforward values were selected by first measuring the voltage change in voltage at COMP as VIN transitions from 3.6V to 12V. A 380mV change at COMP was observed, optimal values for VIN and RFF can now be calculated as follows: CFF = ∆VCOMP • (CFB + CP ) = 33pF ∆VIN RFF = RFB • CFB = 681k CFF VOUT regulation is maintained within 300mV or 6% during the 15µs transition with a 47µF output cap (Figure 4) and 500m A load. A falling VIN edge is about 10-times slower, resulting in an even smaller transient. A 3.6V input, 5V output load step response using the compensation components of Figure 2 is shown in Figure 5. In this case, a 250m A to 1A load step results in only a 250mV transient on VOUT with a 47µF output capacitor. Figures 4 and 5 illustrate how the LTC3112’s loop July 2012 : LT Journal of Analog Innovation | 17 The LTC3112 features the latest generation buck-boost PWM control scheme, effectively eliminating jitter when crossing the barrier between buck and boost operation. AUXILIARY P-CHANNEL MOSFET 12V WALL ADAPTER CFF 33pF RFF 681k 0.1µF VIN Figure 2. LTC4412 PowerPath controller selects highest voltage input to power the LTC3112 converter BURST PWM GATE CTL STAT response can be configured to provide excellent response to both input voltage and output current load steps. SW2 BST1 VIN BST2 VOUT LTC3112 47µF OFF ON 1µF 470k PWM/SYNC COMP IOUT GND OVP To protect data, some data systems require a short period of time to backup data when the primary power source fails. A bank of supercapacitors is often used to provide the required burst of energy. The LTC3112’s wide input voltage range and In this circuit, a stack of supercapacitors totaling 22mF is charged to 15V while the primary power source is active. A lower ESR electrolytic or ceramic cap is placed in parallel to minimize VIN ripple. The VCC supply pin is back-driven from the 5V output in this example, allowing the LTC3112’s gate drive circuits to Figure 3. 5V output efficiency from a single Li-ion cell (3.6V) or wall plug (12V) Figure 4. 3.6V to 12V input step and resulting VOUT transient CFB 680pF FB RUN ability to buck or boost make it ideal for such an application, as shown in Figure 6. 5V BACKUP SUPPLY 0.1µF 22pF LTC4412 VIN SENSE GND SW1 VCC PRIMARY P-CHANNEL MOSFET Li-ION BATTERY CELL 4.7µH 845k RFB 33k 47pF 10k TO ADC 1V PER AMP 100pF 42.2k VIN 5V/DIV EFFICIENCY (%) 90 operate efficiently with an input voltage from 15V down to 2.2V. Available energy at the input is given by: 1 2 2 EIN = • CIN • ( VINITIAL ) − ( VFINAL ) 2 22mF 2 2 = • 15 − 2.2 2 = 2.4J The results of the backup event are shown in Figure 7. A resistive network from VIN, VOUT and GND is used to drive Figure 5. 250mA to 1A load step and resulting VOUT transient 12V 3.6V Burst Mode Operation 85 75 70 VOUT 500mV/DIV VOUT 1V/DIV 80 VIN = 3.6V VIN = 12V 0.1 1 10 100 ILOAD (mA) 1A 18 | July 2012 : LT Journal of Analog Innovation 10A IL 1A/DIV IL 1A/DIV 20µs/DIV 47µF 158k 95 PWM VOUT 5V 1.5A 200µs/DIV design features The LTC3112’s ability to support large load currents make it ideal for handheld devices with increased processing power. Solution size and conversion efficiency benefit from 50mΩ N-channel MOSFET switches and thermally enhanced packages. VOUT 4.7µH 499k BAT54 0.1µF VIN 15V TO 2.2V + 22mF SUPERCAP STACK + + = 250mA • 5V • 1.7s = 2.1J The prior example can be easily scaled depending on the voltage rating of the Figure 7. Supercap discharge performance during power supply backup event BST2 VOUT LTC3112 PWM/SYNC 499k RUN 5V/DIV ILOAD 500mA/DIV 500ms/DIV 680pF VOUT 5V/500mA 33k 845k IOUT GND OVP 1µF Figure 8. Maximum output current versus VIN with VOUT = 5V and VCC back-fed 4 3.5 TO ADC 1V PER AMP 100pF 47µF 47pF FB RUN supercapacitors and the energy required for backup. The IOUT pin (Figure 6) can be monitored by an ADC to measure load current during the backup event. An important consideration in design is the maximum output current capability of the buck-boost converter. As shown in Figure 8, the LTC3112 is able to support up to 4A of load current when VIN >> VOUT. As the converter transitions from buck to boost mode, the maximum load current drops accordingly. MAXIMUM OUTPUT CURRENT (A) VOUT 5V/DIV 0.1µF 22pF 4.5 VIN 5V/DIV COMP 220µF Figure 6. Backup Supply for 5V rail runs down to VIN = 2.2V EOUT = IOUT • VOUT • t SW2 BST1 VIN VCC 1M + the RUN pin to provide a clean shutdown of VOUT. In this example, a constant 250m A load is drawn from the LTC3112 resulting in the VIN capacitors maintaining regulation for 1.7 seconds, and an average conversion efficiency of 88% including the supercapacitor losses. SW1 42.2k 10k 158k SUMMARY The LTC3112 produces low noise buckboost conversion in a range of applications requiring an extended input or output voltage range. The LTC3112’s ability to support large load currents make it ideal for handheld devices with increased processing power. Solution size and conversion efficiency benefit from 50mΩ N-Channel MOSFET switches and thermally enhanced packages. To provide longer run times, a low Burst Mode quiescent current extends high efficiency over several decades of load current. Features such as synchronized switching frequency, programmable output voltage, a load current monitor and external loop compensation allow the LTC3112 to be tailored for a specific application. n 3 2.5 2 1.5 1 0.5 0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 VIN (V) July 2012 : LT Journal of Analog Innovation | 19