LR1130 battery

Brief analysis of the processing method of the boost circuit powered by LR1130 battery

In the DC/DC boost circuit, power is directly supplied from the input or from the output

A startup circuit is added on the basis of the original DC/DC

Portable products are generally powered by batteries. Due to cost and volume considerations, there is a trend to reduce the number and volume of batteries used in the design. If the number of batteries is reduced, the power supply voltage will be lower than the required working voltage of the device. At this time, the DC/DC boost circuit is required. In addition, due to global energy issues, the use of various types of batteries has attracted much attention, including LR1130 battery.

Generally, the minimum voltage of a single solar cell is between 0.4-0.7V. Under such a low input voltage, the following three major problems will be encountered:

Driving problem of switching devices

Current DC/DC boost circuits generally have two power supply methods, one is directly powered from the input, and the other is powered from the output. If the power supply is from the input, the high level of the NMOS is at most equal to the input voltage under normal circumstances. When the input voltage is very low, the NMOS with a low start-up voltage should be selected. If the output power supply is selected, a higher drive voltage equal to the boosted voltage can be obtained at the EXT drive end, which not only makes it easier to turn on the NMOS, but also obtains a lower Rdson to improve efficiency. Of course, these are all based on the premise that the IC starts working, but when the power supply voltage is lower than the startup voltage of the entire IC, the latter will be even more difficult to start than the former because it will pass through a diode, so it brings a question, how to start this IC?

Startup problem of boost circuit

The operating voltage of traditional DC/DC is generally above 1.0V, and if the input voltage drops below 0.6V, the internal circuit of DCDC cannot work normally. At this time, we need to consider adding a startup circuit based on the original DC/DC. This circuit should include the following important parts: an oscillator that can still work at as low as 0.3V, a charge pump voltage doubling circuit, and a voltage test comparator.

The basic working conditions are as follows: First, 0.3V is connected, the oscillator works, and then the charge pump starts to double the voltage. When the required IC drive voltage is obtained, the IC's VDD is powered. When the IC enters normal operation, the output power supply replaces the power supply of the startup circuit, and the startup circuit enters sleep mode. After the boost circuit works normally, people will be concerned about the highest voltage they can get. This leads to another question, what is the maximum duty cycle?

The problem of maximum duty cycle

For ultra-low input boost circuits, in order to obtain high output voltage, a large duty cycle must be supported.

In the continuous current mode, the calculation formula for the duty cycle (Duty) is Duty=1-Vin/Vout. According to this formula, if the input is 0.5V and the output is 5V, the maximum duty cycle is 90%, and the duty cycle of the general boost circuit is 80%~90%, which cannot fully meet the requirements.

Theoretically, the larger the duty cycle, the stronger the maximum boost capability (of course, the duty cycle cannot reach 100% like the buck), but due to non-ideal factors: inductor parasitic resistance, driver tube internal resistance, Schottky forward voltage drop, etc., when the duty cycle reaches a certain level, its boost capability will become worse. Therefore, we need to improve the above non-ideal factors and obtain a suitable high enough duty cycle to meet our boost needs.

In short, dealing with the above three problems can solve the problem of designing a boost circuit that requires ultra-low voltage input startup for single-cell solar power supply.

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