Switching Regulator Design: Boost Converters
114
Goran Perica
1μA I Q synchronous boost converter extends battery life in portable
devices
fully charged and standing by for quick turn-on. In shutdown, the part draws less than 1μA from the input source.
Because the batteries used in portable devices are usually as small as possible, they present high internal impedance under heavy loads, especially close to the end of their dis- charge cycle. Unlike other boost converters that struggle with high source impedance at start-up, the LTC3122 prevents high surge currents at start-up.
1.8V to 5.5V input to 12V output boost regulator
The circuit in Figure 114.1 is designed for high efficiency and small size. The LTC3122 operates at 1MHz to minimize the size of the filter capacitors and boost inductor, and uses Burst Mode operation to maintain high efficiency at light loads, as shown in Figure 114.2. At heavier loads, the converter can operate in constant frequency mode, resulting in lower input and output ripple. Constant frequency operation can result in lower EMI and is easier to filter.
Efficiency can be increased by running the LTC3122 at a relatively low switching frequency. Figure 114.3 shows the results of reducing the switching frequency from 1MHz to 500kHz.
Introduction
Boost converters are regularly used in portable devices to produce higher output voltages from lower battery input voltages. Common battery configurations include two to three alkaline or NiMH cells or, increasingly, Li-Ion batteries, yielding a typical input voltage between 1.8V and 4.8V.
The 12V output converter shown in Figure 114.1 is designed to run from any typical small battery power source.
This design centers around the LTC3122 boost converter, which can efficiently generate a regulated output up to 15V from a 1.8V to 5.5V input. The LTC3122 includes a 2.5A internal switch current limit and a full complement of fea- tures to handle demanding boost applications, including switching frequency programming, undervoltage lockout, Burst Mode operation or continuous switching mode, and true output disconnect. The integrated synchronous rectifier is turned off when the inductor current approaches zero, pre- venting reverse inductor current and minimizing power loss at light loads.
This unique output disconnect feature is especially impor- tant in applications that have long periods of idle time. While idling, the part can be shut down, leaving the output capacitor
Figure 114.1 • The 1MHz Operating Frequency and Small Inductor Make This Converter Suitable for Demanding
Portable Battery-Powered Applications Figure 114.2 • The High Efficiency of the LTC3122 Boost Converter Extends Battery Life in Portable Applications Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00114-9
Efficiency can be improved further by increasing the inductor size. Figure 114.4 shows the increase in efficiency achieved by replacing the 4mm × 4mm boost inductor (XAL4030-472) with a 7mm × 7mm inductor (744-777-910 from Würth). The 90% efficiency at 10mA is 5% higher than the efficiency shown in Figure 114.3.
Battery size should be taken into account when considering inductor size. Using a relatively small inductor running at a high frequency may necessitate a correspondingly higher capacity battery to achieve the same run time at relatively lower effi- ciency. In other words, space gains achieved with a smaller inductor may be replaced by the need for a bigger battery.
Output disconnect
Typical boost converters cannot disconnect the output from the input because of the boost diode. Current always flows from the input through the inductor and boost diode to the output. Therefore the output can not be shorted or discon- nected from the input, a significant problem in many appli- cations, especially in shutdown. In contrast, the LTC3122 includes an internal switch that disconnects the boost MOS- FET body diode from the output. This also allows for inrush current limiting at turn-on, minimizing the surge currents seen by the input power source.
Figure 114.5 shows the output of the LTC3122 discon- nected in shutdown. The output voltage is pulled to zero by the load following shutdown, and the LTC3122 consumes less than 1μA of current.
rail to regulation. The input current slowly ramps up. The input current overshoot required to charge the output capaci- tor is limited to only 200mA and the input power source volt- age droop is limited to 0.5V, as shown in Figure 114.5.
Conclusion
The LTC3122 boost converter serves the needs of battery- operated applications that require low standby quiescent cur- rent and high efficiency. Unlike many other boost converters, it includes features, enabling operation from batteries near full discharge when battery ESR becomes high. Its very low quiescent and shutdown currents, combined with output dis- connect, extend battery run time in applications with long idle periods. The LTC3122 includes a complete set of fea- tures for high performance battery operated applications and comes in a small, thermally enhanced 3mm×4mm package.
Figure 114.3 • The Efficiency Is Greatly Affected by the Operating Frequency. At 100mA Load an Additional 4% Can Be Gained by Reducing the Switching Frequency from 1MHz to 500kHz.
Figure 114.4 • With a Lower Switching Frequency and a Larger Inductor, a Smaller Battery Can Be Used. Efficiency Gain Up to 30% in the 1mA to 10mA Load Range (in PWM Mode) Can Significantly Improve Applications That Operate with Light Loads.
115
Xiaohua Su
Ultralow power boost converters require only 8.5μA of standby quiescent current
output peak-to-peak ripple for Figure 115.1’s circuit. Output ripple voltage is less than 10mV despite the circuit’s small (0.1μF) output capacitor.
The soft-start feature is implemented by connecting an external capacitor to the VREF pin. If soft-start is not needed, the capacitor can be removed. Output voltage is set by a resis- tor divider from the VREF pin to ground with the center tap connected to the FBP pin, as shown in Figure 115.1. The FBP pin can also be biased directly by an external reference.
The SHDN pin of the LT8410/-1 can serve as an on/off switch or as an undervoltage lockout via a simple resistor divider from VCC to ground.
Ultralow quiescent current boost converter with output disconnect
Low quiescent current in standby mode and high value inte- grated feedback resistors allow the LT8410/-1 to regulate a 16V output at no load from a 3.6V input with about 30μA of average input current. Figures 115.4–115.6 show typical qui- escent and input currents in regulation with no load.
Introduction
Industrial remote monitoring systems and keep-alive cir- cuits spend most of their time in standby mode. Many of these systems also depend on battery power, so power sup- ply efficiency in standby state is very important to maximize battery life. The LT8410/-1 high efficiency boost converter is ideal for these systems, requiring only 8.5μA of quiescent current in standby mode. The device integrates high value (12.4M/0.4M) output feedback resistors, significantly reduc- ing input current when the output is in regulation with no load. Other features include an integrated 40V switch and Schottky diode, output disconnect with current limit, built in soft-start, overvoltage protection and a wide input range, all in a tiny 8-pin 2mm × 2mm DFN package.
Application example
Figure 115.1 details the LT8410 boost converter generating a 16V output from a 2.5V-to-16V input source. The LT8410/-1 controls power delivery by varying both the peak induc- tor current and switch off time. This control scheme results in low output voltage ripple as well as high efficiency over a wide load range. Figures 115.2 and 115.3 show efficiency and
Figure 115.1 • 2.5V–16V to 16V Boost Converter Figure 115.2 • Efficiency vs Load Current for Figure 115.1 Converter
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00115-0
The device also integrates an output disconnect PMOS, which blocks the output load from the input during shut- down. The maximum current through the PMOS is limited by circuitry inside the chip, allowing it to survive output shorts.
Compatible with high impedance batteries
A power source with high internal impedance, such as a coin cell battery, may show normal output on a voltmeter, but its voltage can collapse under heavy current demands. This makes it incompatible with high current DC/DC converters.
With very low switch current limits (25mA for the LT8410 and 8mA for the LT8410-1), the LT8410/-1 can operate
very efficiently from high impedance sources without causing inrush current problems. This feature also helps preserve battery life.
Conclusion
The LT8410/-1 is a smart choice for applications which require low standby quiescent current and/or require low input current, and is especially suited for power supplies with high impedance sources. The ultralow quiescent current and high value integrated feedback resistors keep average input current very low, significantly extending battery operating time. The LT8410/-1 is packed with features without com- promising performance or ease of use and is available in a tiny 8-pin 2mm × 2mm package.
Figure 115.3 • Output Peak-to-Peak Ripple vs Load
Current for Figure 115.1 Converter at 3.6V Figure 115.4 • Quiescent Current vs Temperature (Not Switching)
116
Wei Gu
Tiny dual full-bridge Piezo motor driver operates from low input voltage
The step-up converter and both Piezo drivers have their own shutdown control. Figure 116.2 shows a typical layout.
Single driver application
Each full-bridge Piezo driver can be independently enabled and disabled by controlling the SHDNA and SHDNB pins.
When held below 0.3V, SHDNA and SHDNB prevent
Introduction
Piezoelectric motors are used in digital cameras for autofocus, zooming and optical image stabilization. They are relatively small, lightweight and efficient, but they also require a complicated driving scheme. Traditionally, this challenge has been met with the use of separate circuits, including a step-up converter and an oversized generic full-bridge drive IC. The resulting high component count and large board space are espe- cially problematic in the design of cameras for ever shrinking cell phones. The LT3572 solves these problems by combining a step-up regulator and a dual full-bridge driver in a 4mm × 4mm QFN package. Figure 116.1 shows a typical LT3572 Piezo motor drive circuit. A step-up converter is used to generate 30V from a low voltage power source such as a Li-Ion battery or any input power source within the part’s wide input volt- age range of 2.7V to 10V. The high output voltage of the step- up converter, adjustable up to 40V, is available for the drivers at the VOUT pin. The drivers operate in a full-bridge fashion, where the OUTA and OUTB pins are the same polarity as the PWMA and PWMB pins, respectively, and the OUTA and OUTB pins are inverted from PWMA and PWMB, respectively.
Figure 116.1 • Typical Circuit
Figure 116.2 • Typical Layout for the Figure 116.1 Converter
Figure 116.3 • Single Driver Application Circuit Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00116-2
the drivers from switching and keep the outputs in a high impedance state.
In applications where only one driver is used, the unused driver can be simply turned off without wasting any power by tying either SHDNA or SHDNB pin to the GND. Figure 116.3 shows a typical single driver application circuit where only driver A is enabled. The input pin PWMB is tied to GND.
Using external power supply
The high output voltage of the step-up converter, adjust- able up to 40V, is available for the drivers at the VOUT pin.
For some multiple Piezo motor applications with multiple LT3572s, all the full-bridge drivers are powered by an external high voltage power supply. In this case, the integrated step-up converter can be simply disabled and only the dual drivers are used. In Figure 116.4, the SHDN pin is tied to the ground so the step-up regulator is prevented from switching. The SW pin, RT pin, SS pin and PGOOD pin are left open. The VIN pin should be connected to a voltage source between 2.7V and 10V and FB pin to any voltage between 1.3V and 3V. In this example, the VIN pin and FB pin are connected together, and both drivers are fully functional while the step-up converter is not running. The VIN current is normally below 10mA.
Operating Piezo motor with long wires
In some cases, the Piezo motors are physically located far away from the driver. The parasitic inductance of the long
as shown in Figure 116.5, to slow down the driving speed and dampen the oscillation. In this example, the connecting wires are 1-foot long twisted wires and the resistor is 20Ω. The volt- age waveforms of the OUTB pin are shown in Figure 116.6 without the resistor, and Figure 116.7 with the resistor.
Conclusion
Figure 116.4 • Using External Power Supply with Integrated Step-Up Converter Disabled
Figure 116.5 • Adding a Resistor when Operating with Long Wires
Figure 116.6 • OUTB Voltage Without the Resistor. Top Trace:
OUTB Voltage (2V/Div), Bottom Trace: PWMB Voltage (2V/Div)
Figure 116.7 • OUTB Voltage with the Resistor. Top Trace:
OUTB Voltage (2V/Div), Bottom Trace: PWMB Voltage (2V/Div)
117
Dave Salerno
Tiny synchronous step-up converter starts up at 700mV
a 2mm × 2mm DFN package, the LTC3526L has a typical start-up voltage of just 700mV, with operation down to 400mV once started.1 Despite the LTC3526L’s tiny solu- tion size, it includes many advanced features, including out- put disconnect, short-circuit protection, low noise fixed frequency operation, internal compensation, soft-start, ther- mal shutdown and Burst Mode operation for high efficiency at light load. For low noise applications, the LTC3526LB offers fixed frequency operation at all load currents. With an out- put voltage range that extends down to 1.5V, the LTC3526L and LTC3526LB can even be used in applications previously requiring a boost converter followed by a buck converter.
A typical single-cell boost application is shown in Figure 117.1. In this example the LTC3526LB is used to gen- erate 1.8V for a Bluetooth radio application. The LTC3526LB was selected for its small size, minimal external component count and low-noise, fixed frequency operation at all load cur- rents. A graph of output current capability versus input volt- age is shown in Figure 117.2. Note that the converter starts up at 700mV at no load and once running, can deliver 25mA
Introduction
Alkaline batteries are convenient because they’re easy to find and relatively inexpensive, making them the power source of choice for portable instruments and devices used for outdoor recreation. Their long shelf life also makes them an excellent choice for emergency equipment that may see infrequent use but must be ready to go on a moment’s notice. It is important that the DC/DC converters in portable devices operate over the widest possible battery voltage range to extend battery run time, and thus save the user from frequent battery replacement.
Single-cell alkaline batteries, with a 1.6V to 0.9V range, present a special challenge to DC/DC converters because of their low voltage and the fact that their internal resistance increases as the battery discharges. Thus, a DC/DC converter that can both start up and operate efficiently at low input voltages is ideally suited for single-cell alkaline products.
The LTC3526L is a 1MHz, 550mA synchronous step-up (boost) converter with a wide input voltage range of 0.7V to 5V and an output voltage range of 1.5V to 5.25V. Housed in
Figure 117.1 • Single-Cell 1.8V Boost Converter for a Bluetooth Radio Application Features a Low Start-up Voltage and Uses a Monolithic Chip Inductor for a Maximum Component Height of Just 1mm
Figure 117.2 • Maximum Load Capability during and after Start-up for the Circuit in Figure 117.1
Note 1: Lower start-up devices for energy harvesting work below 100mV: see the LTC3108 data sheet.
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00117-4
of output current with an input voltage of only 400mV. The 1MHz switching frequency allows the use of small, low profile inductors, such as the monolithic chip inductor shown in this application. This provides a complete solution with a foot- print that’s just 36mm2 with a 1mm profile.
Many new battery types are available to the consumer, some of which are aimed at high-tech, high power applica- tions. One of these is the disposable lithium AA/AAA battery, which offers a significant improvement in run time over traditional alkaline batteries. Furthermore, in applications that see infrequent use, the long shelf life of lithium batteries gives them a performance edge over nickel-based rechargeable batteries, which have a high self-discharge rate.
One characteristic of the lithium battery is that its voltage can be as high as 1.8V when the battery is fresh, compared to 1.6V for a typical alkaline battery. This is a problem for 2-cell alkaline applications that use a traditional boost converter to produce a 3.3V output from an alkaline 3.2V max input. Most boost converters cannot maintain regulation when the input is higher than the output, as it is with two fresh lithium batter- ies (3.6V).
The LTC3526L solves this problem by maintaining regu- lation even when the input voltage exceeds the output volt- age. An example of a 2-cell to 3.3V boost converter using the LTC3526L is shown in Figure 117.3. A small feed-forward capacitor has been added across the upper divider resistor to reduce output ripple in Burst Mode operation. Efficiency vs load curves are shown in Figure 117.4. These curves demon- strate the high efficiency at light load made possible by the low 9μA quiescent current of Burst Mode operation. The curve in Figure 117.5 illustrates the efficiency at input volt- ages above and below the output voltage.
Conclusion
The LTC3526L is a highly integrated step-up DC/DC con- verter in a 2mm × 2mm package designed to easily fit a wide variety of battery-powered applications. Low start-up and operating voltages extend run time in single-cell applications.
It even regulates in step-down situations where the fresh bat- tery voltage (VIN) may exceed VOUT. For high efficiency at light loads, or low noise operation, it offers a choice of Burst Mode or fixed frequency operation.
Figure 117.3 • Two AA Lithium Cell to 3.3V Boost Converter with 250mA Load Capability Maintains High Efficiency Over Three Decades of Load Current and Operates with VIN≥ VOUT
Figure 117.4 • Efficiency vs Load for the Circuit in
Figure 117.3 Figure 117.5 • Efficiency vs VIN for the Circuit in Figure 117.3
(at 100mA Load Current)
118
Goran Perica
High efficiency 2-phase boost converter minimizes input and output current ripple
as well. Switching currents also increase proportional to the output-to-input voltage conversion ratio, so if the input voltage is low, the switching currents can overwhelm a simple boost converter and generate unacceptable EMI.
For example, consider Figure 118.1, a 12V input to 24V, 10A output switching converter operating at 300kHz. The currents processed by the converter in Figure 118.1 are shown in the first row of Table 118.1. The relatively high current levels in the switcher are reflected in high input and output ripple currents, which results in increased EMI.
The circuit shown in Figure 118.2 performs the same DC/DC conversion, but with greatly reduced input and
Introduction
Many automotive and industrial applications require higher voltages than is available on the input power supply rail. A simple DC/DC boost converter suffices when the power lev- els are in the 10W to 50W range, but if higher power levels are required, the limitations of a straightforward boost con- verter become quickly apparent. Boost converters convert a low input voltage to a higher output voltage by process- ing the input current with a boost inductor, power switch, output diode and output capacitor. As the output power level increases, the currents in these components increase
Figure 118.1a • Single Phase Boost Converter: Can Be Used to
Convert 12V Input to 24V, 10A Output Figure 118.1b • Single Phase Boost
Converter Output Voltage Ripple
Table 118.1 Dual Phase Boost Converter Has Lower Input and Output Ripple Currents and Voltages Than Single Phase Boost Converter INPUT RMS
CURRENT
INPUT RIPPLE CURRENT
MOSFET RMS DRAIN CURRENT
OUTPUT DIODE RMS CURRENT
OUTPUT CAPACITOR RMS CURRENT
OUTPUT CAPACITOR FREQUENCY
OUTPUT VOLTAGE RIPPLE Single Phase
Boost Converter
21.1A 4.2AP–P 15.4A 14.4A 10.5A 300kHz 212mV
Dual Phase Boost Converter
20.7A 0.17AP–P 2 × 7.4A 2 × 7.2A 1.9A 600kHz 65mV
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00118-6