Richard Cook, Linear Technology
LT Journal of Analog Innovation
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 buck-boost converter with input and output voltage ranges of 2.5 V to 15 V and an output current capability of 1.5 A. 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.
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. 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.
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 800 kHz default switching frequency can be synchronized up to 1.5 MHz.
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| Figure 1. | LTC3111-based 18 W solution. |
The LTC3111 based converter shown in Figure 1 can generate 18 W of power with a 12 V output. The solution footprint is less than 180 mm2, 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 is offered in a thermally enhanced 16-lead 4 mm × 3 mm 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 user-programmable undervoltage lockout (UVLO) threshold – by a resistive divider from VIN to ground. The LTC3111’s RUN threshold of 1.2 V (±5% over temperature) allows customization of the turn-on threshold voltage of the converter. Once enabled, 120 mV of hysteresis is introduced at the RUN pin, requiring the source input voltage to drop 10% before disabling power conversion.
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| Figure 2. | One, two and three Li-ion cells can be used in this solution with the LTC3111’s accurate RUN threshold feature. |
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 is greater than 3.3 V and to turn off when the input voltage drops below 3 V.
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| Figure 3. | LTC3111 ramped input voltage response using the accurate RUN for a single Li-ion solution. |
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 3.3 V and turns off at 3 V. Similarly, this plot can be scaled for 2- and 3-cell cases, where turn-on/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.
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| Figure 4. | 5 V output efficiency from one, two and three Li-ion cells. |
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 5 V output decreases when the input voltage is less than 6 V. 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.
Multiple Input Sources
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 20 mV 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 7.2 V lithium ion battery or 12 V wall adapter.
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| Figure 5. | LTC4412 PowerPath controller selects highest voltage input to power the LTC3111 converter. |
Efficiency curves versus load current for the 3.3 V 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 49 μA of typical sleep current extends high efficiency over two decades of load current.
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| Figure 6. | LTC3111 efficiency vs load current VOUT = 3.3 V, VIN = 7.2 V and 12 V. |
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 50 mV, or 1.5%, during the 20 μs rise and fall transition with a 22 μF output capacitor and 1 A load in step-down operation.
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| Figure 7. | Line response for VOUT = 3.3 V, VIN stepped from 7.2 V to/from 12 V. |
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).
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| Figure 8. | LTC3111 configured as a variable output supply. |
The programmed output voltage can be calculated using the following equation:
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.
Figure 9 shows the output voltage response of a 0 V to 1.2 V ramped control signal operating at 100 Hz. The corresponding output voltage swings from 10 V to 2.5 V, 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.
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| Figure 9. | Variable output response using the LTC3111. |
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.
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| Figure 10. | Maximum output current in PWM mode vs output voltage for VIN = 5 V. |
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 500 mA 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 = 5 V to VOUT = 10 V.
Summary
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 efficiently support heavy load currents makes it ideal for power hungry devices. Solution size and conversion efficiency benefit from the 90 mΩ 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.
Materials on the topic
