Boost Converter Generates Three Analog Voltages

Boost Converter Generates Three Analog Voltages

The standard boost converter in FigureĀ 1 uses not only IC1, C1, L 1, D1, and C2 to generate a main 5Ā V output, but also additional small, low-cost components to provide two auxiliary supply rails ofĀ 10 and ā€“5Ā V. These auxiliary outputs are useful for analog circuitry in small handheld instruments, which often require supply voltages greater than the signal range. Input voltages ofĀ 0.8 to 5.5Ā V, which is equivalent to voltages from a battery pack of one to three cells, sustain the main regulated output of 5Ā VĀ±2%. With an input of 1.8Ā V from two flat cells, for instance, and with the other rails unloaded, the circuit can produce 25Ā mA with 80 to 90% efficiency.

The converterā€™s LX switching node drives low-cost, discrete charge pumps via ā€œflying capacitorsā€ C3 and C6 to create the ā€“5Ā V and 10Ā V outputs. The LX node switches between 0Ā V and a level-one diode drop above the 5Ā V rail, so the charge pumpsā€™ drive voltage is reasonably well-regulated. Moreover, the drop across D1 roughly compensates for diode drops in the two charge-pump outputs. IC1ā€™s internal control scheme also assists in regulating the auxiliary outputs. This ICā€™s current-limited, minimum-off-time, pulse-frequency modulation constantly adapts its switching frequency to the net load current; the frequency increases when the load increases, producing a greater transfer of energy via the flying capacitors. The result is a type of pseudo-regulation for the charge-pump outputs.

These analog supply rails can drive precision op amps, such as the MAX400 and OP-07, whose input common-mode- rejection and output-range specifications areĀ 2 to 3Ā V within the supply rails. Thus, the rails are good enough if the ā€“5Ā V output is less than ā€“3Ā V and the 10Ā V output is more than 8Ā V. Accordingly, the component choices in FigureĀ 1, such as the lossy RC output filters and silicon signal diodes in place of Schottky diodes, provide for minimal cost and ripple rather than maximum regulation. The 4.7-ĀµF capacitors, C4 and C7, can be high-ESR, commodity, multilayer ceramic types with 16Ā V ratings, a 1206 case, and a Y5V dielectric.

The output ripple varies with the supply voltage and output load. Operating with an input voltage of 1.8Ā V, the circuit produces ripple amplitudes over the load ofĀ 2 to 10Ā mV p-p for the 10Ā V rail andĀ 15 to 30Ā mV p-p for the ā€“5Ā V rail. ByĀ increasing C5 and C8 to 2.2Ā ĀµF, you can reduce these ripple levels toĀ 1 and 5Ā mV, respectively.

With no load on the auxiliary rails, the 5Ā V outputā€™s maximum available load current rises with input supply voltage (FigureĀ 2a). You can increase this available output power by replacing D1 with a lower loss Schottky diode. At an input of 1.8Ā V, the output power available for the three rails (loaded with 10Ā mA at 5Ā V, 5Ā mA at 10Ā V, and 5Ā mA at ā€“5Ā ) is somewhat less than 125Ā mW; with a 5-mA load, the 10Ā V and ā€“5Ā V outputs are approximately 9.75Ā and ā€“3.7Ā V, respectively (FigureĀ 2b). A 2.7Ā V input based on three flat cells yields around 275Ā W.

The MAX858 operates with peak inductor currents of 125Ā mA. IfĀ you need more current, you can replace this IC with related parts that have 500Ā mA and 1Ā A ratings. Note that these changes require different passive components; the inductor and main output diode ratings must match the inductorā€™s peak current. The charge pumps can remain the same if their output currents donā€™t change much.Ā You can also retain the cheap, common, commodity dual diodes D1, D2, and D3, but detail specifications vary, so look carefully at data sheets for the part you actually use.

by Tim Herklots @ EDN

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Mike is the founder and editor of Electronics-Lab.com, an electronics engineering community/news and project sharing platform. He studied Electronics and Physics and enjoys everything that has moving electrons and fun. His interests lying on solar cells, microcontrollers and switchmode power supplies. Feel free to reach him for feedback, random tips or just to say hello :-)

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