1.5A Monolithic Buck-Boost DC/DC Converter with Up to 95% Efficiency Features 2.5V–15V Input and Output Voltage Ranges Richard Cook Multicell high capacity batteries are increasingly becoming commonplace in handheld devices and industrial instruments that receive their power from a variety of sources. To maximize battery run time and support the variety of power sources, voltage regulators in multipower source systems must be able to maintain a constant output voltage even as the input voltage source resides above, below or equal to the output. This can be accomplished using two separate power converters with two controller ICs. A better solution is to use a single buck-boost DC/DC converter, which offers a smaller, simpler and more efficient design, attributes critically important in handheld devices. The LTC3111 is a monolithic buckboost converter with input and output voltage ranges of 2.5V to 15V and an output current capability of 1.5A. It allows conversion from a variety of power sources such as single or multi Li-ion cells, lead acid batteries, capacitor banks, USB cables or wall adapters. architecture, effectively eliminating jitter and EMI that can occur when crossing the boundary between step-up and step-down operation. This reduces or eliminates the need for expensive filtering or shielding for noise sensitive data conversion or RF circuitry in the system. Selectable Burst Mode® operation extends the operating time when battery-powered devices are in idle by substantially reducing the quiescent current of the power converter. In addition to its wide operating range, the LTC3111 features Linear Technology’s proprietary low noise buck-boost PWM control Figure 2. One, two and three Li-ion cells can be used in this solution with the LTC3111’s accurate RUN threshold feature 4.7µH 0.1µF VIN 3V TO 12.6V 1 TO 3-CELL Li-Ion + NUMBER R OF CELLS 274k 1 698k 2 3 1.13M SW1 SW2 BST1 BST2 VIN VOUT 10µF LTC3111 0.1µF 680pF COMP BURST PWM R 154k 24 | January 2015 : LT Journal of Analog Innovation PWM/SYNC RUN SNSGND SGND 26.1k 1M 33pF 27pF FB VCC PGND 191k 1µF 20k VOUT 5V 750mA 22µF VIN > 4V Figure 1. LTC3111-based 18W solution An accurate run threshold provides the ability to precisely program the turn‑on threshold voltage of the converter. The integrated fault protection features include: current limit, thermal shutdown and short-circuit protection, which provides robust operation in harsh environments. For applications where component size is critical, the 800kHz default switching frequency can be synchronized up to 1.5MHz. The LTC3111 based converter shown in Figure 1 can generate 18W of power with a 12V output. The solution footprint is less than 180mm2 , more compact than a controller-based buck-boost and much more efficient than a complex dual-inductor SEPIC converter design at similar power levels. The main external components are limited to the input, output filter capacitors and the power inductor. The LTC3111 design features In addition to its wide operating range, the LTC3111 features Linear Technology’s proprietary low noise buck-boost PWM control architecture, effectively eliminating jitter and EMI that can occur when crossing the boundary between step-up and step-down operation. This reduces or eliminates the need for expensive filtering or shielding for noise sensitive data conversion or RF circuitry in the system. 100 PWM 90 BURST MODE OPERATION 80 TURN ON THRESHOLD 3.3V EFFICIENCY (%) VOUT 1V/DIV TURN OFF THRESHOLD 3.0V 70 60 50 VIN 2V/DIV VIN = 3.6V VIN = 7.2V VIN = 10.8V 40 30 0.1m 100ms/DIV Figure 3. LTC3111 ramped input voltage response using the accurate run for a single Li-ion solution is offered in a thermally enhanced 16-lead 4mm × 3mm DFN or 16-lead MSOP package. ACCURATE RUN THRESHOLD WITH 1-, 2- AND 3-CELL LI-ION The LTC3111’s RUN pin can either be used to enable/disable the converter via digital select, or to set an accurate userprogrammable undervoltage lockout (UVLO) threshold—by a resistive divider from VIN to ground. The LTC3111’s RUN threshold of 1.2V (±5% over temperature) allows customization of the VIN LTC4412 GATE CTL SENSE GND STAT 0.1 1 2 is greater than 3.3V and to turn off when the input voltage drops below 3V. This technique can be applied to two or three series cell designs by changing the value of R, as shown in the table for Figure 2. The output voltage response to a slowly ramped VIN for the single cell case is shown in Figure 3. VOUT in the single cell configuration turns on when the input voltage reaches 3.3V and turns off at 3V. Similarly, this plot can be scaled for 2- and 3-cell cases, where turn-on/ Figure 2 shows an application circuit where the accurate RUN pin threshold is used to turn the LTC3111 converter on/ off when powered from a one, two or three Li-ion cell battery. For the single cell case, R is 267k, configuring the LTC3111 RUN pin to turn on when the input voltage 12V ADAPTER 10m ILOAD (A) Figure 4. 5V output efficiency from one, two and three Li-ion cells turn-on threshold voltage of the converter. Once enabled, 120mV of hysteresis is introduced at the RUN pin, requiring the source input voltage to drop 10% before disabling power conversion. Figure 5. LTC4412 PowerPath™ controller selects highest voltage input to power the LTC3111 converter 1- OR 2-SERIES Li-ION CELLS 1m 1N5819 4.7µH 0.1µF SW1 SW2 BST1 BST2 VIN VOUT 47µF LTC3111 0.1µF 1200pF COMP BURST PWM OFF ON PWM/SYNC 1M FB VCC PGND VCC 20k 33pF 27pF RUN SGND 25.5k 22µF VOUT 3.3V 1.2A MBR0520L 316k 1µF January 2015 : LT Journal of Analog Innovation | 25 The LTC3111 includes circuitry to minimize loop gain variation, resulting in improved line transient response. Regulation for VOUT = 3.3V remains within 50mV, or 1.5%, during a 20µs, 7.2V-to-12V, VIN rise and fall transition with a 22µF output capacitor and 1A load. 100 EFFICIENCY (%) 80 VOUT 100mV/DIV PWM 90 BURST MODE OPERATION 70 VIN 2V/DIV 60 50 40 30 0.1m turn-off thresholds are 6.6V/6V and 9.9V/9V, respectively. The accurate RUN feature can also be applied to sources where operation must be restricted to a minimum input operating voltage such as a bank of capacitors, lead acid or NiCd batteries. Efficiency curves for the one, two and three Li-ion cell designs operating at their typical voltages are shown in Figure 4. Peak efficiencies of greater than 90% are achieved over all three battery voltages. Note that the maximum load current capability for a 5V output decreases when the input voltage is less than 6V. The LTC3111 data sheet provides performance curves showing maximum output current capability versus input voltage in PWM and Burst Mode operation for various output voltages to aid in determining if the load can be supported over a specific input range. VIN = 7.2V VIN = 12V 1m 10m ILOAD (A) 0.1 1 2 500µs/DIV Figure 6. LTC3111 efficiency vs load current VOUT = 3.3V, VIN = 7.2V and 12V Figure 7. Line response for VOUT = 3.3V, VIN stepped from 7.2V to/from 12V MULTIPLE INPUT SOURCES Efficiency curves versus load current for the 3.3V output, based on the two input sources, are given in Figure 6. Peak efficiencies of greater than 89% are achieved. Selectable Burst Mode operation with 49µ A of typical sleep current extends high efficiency over two decades of load current. The wide operating range of the LTC3111 makes it easy to power devices from multiple input sources. Figure 5 shows an application where the LTC4412 PowerPath controller (SOT-23 package) selects from the higher of 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 switches the input of the LTC3111 to the greater of a 7.2V lithium ion battery or 12V wall adapter. 4.7µH 0.1µF VIN 5V SW1 SW2 BST1 BST2 VIN VOUT 10µF LTC3111 0.1µF 2.5V < VOUT < 10V 680pF COMP VCC PWM/SYNC RUN SNSGND SGND Figure 8. LTC3111 configured as a variable output supply 26 | January 2015 : LT Journal of Analog Innovation The LTC3111 includes circuitry to minimize loop gain variation, resulting in improved line transient response. As illustrated in Figure 7, VOUT regulation is maintained within 50mV, or 1.5%, 26.1k 20k R2 191k 0V < VCONTROL < 1.2V VCONTROL R3 162k 1µF 27pF FB VCC PGND 1µF VOUT 22µF R1 1M 33pF design features The LTC3111 provides low noise buck-boost conversion for a variety of applications requiring an extended input or output voltage range. The LTC3111’s ability to support heavy loads makes it ideal for power hungry devices. Solution size and conversion efficiency benefit from the 90mΩ internal N-channel MOSFET switches and thermally enhanced packages. 2.5 VOUT 2V/DIV IOUT (A) 2 VCONTROL 500mV/DIV 1.5 1 0.5 5ms/DIV 0 VIN = 5V INPUT CURRENT LIMIT = 2.3A 2 3 4 5 6 7 VOUT (V) 8 9 10 Figure 9. Variable output response using the LTC3111 Figure 10. Maximum output current in PWM mode vs output voltage for VIN = 5V during the 20µs rise and fall transition with a 22µ F output capacitor and 1A load in stepdown operation. Figure 9 shows the output voltage response of a 0V to 1.2V ramped control signal operating at 100Hz. The corresponding output voltage swings from 10V to 2.5V, providing an inverting gain of 6.2 from VCONTROL to VOUT. The low noise PWM control provides low distortion and high quality replication of the input signal. VARIABLE OUTPUT VOLTAGE USING THE LTC3111 For applications such as motor control, lighting or power supply margin testing, the LTC3111 can be configured as a variable voltage supply. This can be accomplished in a number of ways. Figure 8 shows one method: adding a summing resistor between the FB pin and a control voltage (VCONTROL). The programmed output voltage can be calculated using the following equation: R1 R1 VOUT = 0.8V 1+ + (0.8V − VCONTROL ) R2 R3 where R1 is the resistor connected between VOUT and FB, R2 is the resistor connected from FB and ground and R3 is the resistor connected from FB and VCONTROL . When using the LTC3111 as a variable output voltage regulator, the maximum load current capability of the LTC3111 is reduced when VOUT > VIN (i.e., when the part is in boost or step-up mode). As Figure 10 shows, the maximum output current capability is effectively reduced by the step-up ratio of the converter. For example, the output current capability when VOUT = 2VIN is roughly one half the capability when VOUT = VIN . In the example application above, a fixed 500m A load is applied to the output, which the part is capable of supplying at all output voltages. To ensure converter stability, compensation values for this application are determined at the highest boost ratio of VIN = 5V to VOUT = 10V. SUMMARY The LTC3111 provides low noise buckboost conversion for a variety of applications requiring an extended input or output voltage range. The LTC3111’s ability to efficiently support heavy load currents makes it ideal for power hungry devices. Solution size and conversion efficiency benefit from the 90mΩ internal N-channel MOSFET switches and thermally enhanced packages. Low quiescent current Burst Mode operation extends high efficiency over several decades of load current, enabling longer run times in many battery powered applications. n January 2015 : LT Journal of Analog Innovation | 27