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Figure 1: A typical DC/DC boost configuration using switching converters ADP1612/1613.
The inductor, which is a key component of the boost regulator, stores energy during the on-time of the power switch and transfers that energy to the output through the output rectifier during the off time. In an application note,¹ Analog Devices explains how to balance the trade-offs between low inductor current ripple and high efficiency. The document recommends inductance values in the 4.7 to 22 μH range.
While a lower-value inductor has a higher saturation current and a lower series resistance for a given physical size, lower inductance results in higher peak currents that can lead to reduced efficiency, higher ripple, and increased noise. Hence, to reduce the inductor size and improve stability, it is better to run the boost converter in discontinuous conduction mode, according to Analog Devices’ app note. It further argues that the peak inductor current (the maximum input current plus half the inductor ripple current) must be lower than the rated saturation current of the inductor, and the maximum DC input current to the regulator must be less than the inductor’s rms current rating.
Both these boost converters are supported by ADI’s ADIsimPower™ design toolset, which helps a designer generate a full schematic and bill of materials, as well as calculates performance in minutes. ADIsimPower can optimize designs for cost, area, efficiency, and parts count, while taking into consideration the operating conditions and limitations of the IC and all real external components.
Typical of ADI’s step up converter evaluation boards, ADP1612-BL3-EVZ offers a complete DC/DC step-up converter with all components selected to allow operation over the full range of input and load conditions for the 5 V (ADP1612) and 12 V (ADP1613) output voltages. The evaluation boards can be adjusted for different output voltages by changing R1 and R2. As per the evaluation board documentation, L1, D1, CCOMP, and RCOMP in Figure1 may also be adjusted or recalculated to ensure stable operation.
For its part, to power white LEDs used for LCD backlighting or generate LCD bias supply, Texas Instruments is offering highly integrated, low-power boost converters TPS61040/41 (Figure 2), capable of delivering output voltages up to 28 V from a dual-cell NiMH/NiCd or single-cell Li-ion battery input. The part can also be used to generate standard 12 V output from 3.3 or 5 V input.
Figure 2: Operating at frequencies up to 1 MHz, the integrated boost converters TPS61040/41 require only a few small external components.
Housed in tiny SOT23 and SON packages, the converters operate with a switching frequency up to 1 MHz. With a built-in output power MOSFET, the part requires only a few small external components. Due to high switching frequency, the output capacitors can be ceramic or tantalum. While the TPS61040 offers an internal switch current limit of 400 mA, the TPS61041 has a 250 mA switch current limit. Also, the low quiescent current (typically 28 μA), together with an optimized control scheme, allows the device to operate at very high efficiencies over the entire load current range.
Higher DC output
If your circuit requires even higher voltage, TI’s TPS61170 can be helpful. It is a monolithic, high-voltage switching regulator with integrated 1.2 A, 40 V power MOSFET. It can provide output voltages up to 38 V. The part’s datasheet presents several standard switching-regulator topologies, including boost and single-ended primary inductance converter (SEPIC). The device has a wide input-voltage range to support applications with input voltage from multi-cell batteries, or regulated 5 V or 12 V power rails.
Other semiconductor suppliers offering high output voltage boost converters include Linear Technology Corp. and Maxim Integrated, among others. Linear, for instance, has developed a current mode, step-up DC/DC converter to bias avalanche photodiodes (APDs) in optical receivers (Figure 3). Designed to generate an output voltage up to 75 V, the LT3571 features a high side fixed voltage drop APD current monitor with better than 10 percent relative accuracy over the entire temperature range. The integrated power switch, Schottky diode, and APD current monitor allow a small solution footprint and low solution cost. It combines a traditional voltage loop and a unique current loop to operate as a constant-current source or constant-voltage source.
Figure 3: LT3571-based 5 to 45-V boost power supply circuit to bias avalanche photodiodes.
Linear also has in its arsenal boost converters capable of delivering output voltages up to 40 V for applications like driving LEDs and biasing LCDs. These include the LT3494/A and LT1615. While LT3494/A is tailored for delivering output voltages up to 40 V, LT1615 is rated to provide up to 34 V output.
Similarly, Maxim’s MAX1605 can boost battery voltages as low as 0.8 V up to 30 V at the output. The converter’s integrated 0.5 A MOSFET reduces external component count, and its high switching frequency allows for tiny surface-mount components. The current limit can be set to 500, 250, or 125 mA to lower output ripple and component size in low-current applications.
In summary, step-up or boost converters delivering high output DC voltages are available from major IC manufacturers. Each part has its pluses and minuses, so depending on the requirements of the design, engineers must carefully read the datasheets for key specs before selecting a part for a given application. For more information on the parts discussed in this article, use the links provided to access product pages on the Hotenda website.