Automatic latch circuit to save battery

Publisher:见贤思奇异果Latest update time:2012-04-27 Source: OFweek Keywords:battery Reading articles on mobile phones Scan QR code
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Although rechargeable batteries have many advantages, they can be damaged and have a shortened life if they are over-discharged. The circuit in Figure 1 can shut down a battery-powered appliance, such as an LED flash powered by a NiMH (nickel metal hydride) battery, when the battery voltage falls below a preset limit. Although this circuit targets LED flashes, it can be applied to any battery-powered device. Since there is no guarantee that the user will remove the battery to recharge it, the circuit latches off the flash when the battery voltage drops below a usable limit, which is also a strong hint that it is time to recharge.


Although a simple non-latching voltage comparator can cut power, removing the battery load causes voltage fluctuations, and the comparator forces the flash mode again when power is restored. This circuit turns off the flash light and keeps it off unless the user manually turns the light on with switch S1.

IC1, a 600mA NCP1421 PFM step-up synchronous rectification DC/DC converter from On Semiconductor, forms the heart of the circuit, but the basic circuit is applicable to other converters that provide similar functions (Reference 1). The main features of the NCP1421 include an integrated LBI/EN (low battery input/enable) and an open-drain LBO (low battery output). The circuit operates from two AA NiMH batteries and consists of a conventional step-up regulator: an inductor, input and output capacitors, and a

and the current sensing circuit on the right side of IC1. The LED forward voltage generated by the voltage divider R3 and R2, and the voltage across the current sensing resistor R1, together generate a feedback voltage that is compared with the 1.2V nominal reference voltage of the NCP1421.

On the input side, the LBI/EN pin of IC1 is connected to the battery through a voltage divider network consisting of R4, R5, and R10. When the LBI/EN voltage exceeds 1.2V, the NCP1421 remains enabled. When the LBI/EN voltage drops below 1.2V, the LBO sense pin is low, Q3 turns on, and provides current to the base of Q1. When Q1 turns on, the base of Q2 is low, latching the virtual thyristor (SCR) formed by Q1 and Q2, IC2 is an integrated dual transistor MBT3946DW1.

In addition, Q1 locks the LBI/EN pin to a low level to prevent IC1 from turning on again when the load is removed. To restart this circuit, the power supply to the circuit must be cut off with switch S1. Resistors R4, R5, and R10 set the battery voltage breakpoint for the LBO detector. R5 also sets the amount of current that flows out of the battery when the SCR is activated. When the battery voltage drops to approximately 1.3V, the circuit shuts down, and the LBI/EN pin reaches 1.2V.

Keywords:battery Reference address:Automatic latch circuit to save battery

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