Simple circuit shuts off system when supply voltage is low

Posted on Feb 7, 2014

When supply voltage drops below a minimum threshold, it`s often good practice to disconnect the supply from the system to prevent poor performance or erratic operation. The circuit shown accomplishes this with minimal cost, board space, and complexity ( see the figure ). In addition, the active components are available in tiny, SOT-23  packages. IC1 is an LM4041 adjustable

Simple circuit shuts off system when supply voltage is low
Click here to download the full size of the above Circuit.

voltage reference that serves here as a programmable voltage detector. It does this by operating in an unconventional configuration. Typically, the LM4041 develops a voltage between its positive and negative terminals that forces 1. 24 V between its +  and ADJ  pins. With R3 in the circuit, however, IC1`s -  pin will be near ground when the voltage across R1 is less than 1. 24 V, and about a volt below V+ when the voltage across R1 is greater than 1. 24 V. When the supply voltage is in the normal operating range (above about 4. 6 V in the circuit shown), the voltage across R1 will be greater than 1. 24 V. This pulls the LM4041`s -  pin high, turning on the npn, which then turns on the p-channel MOSFET by pulling its gate low. In this condition, power is supplied to the load through the p-channel device. When the supply voltage drops below the normal operating range, the LM4041`s -  pin goes low, turning off the npn and the MOSFET pass device, which removes power from the load. R4 provides hysteresis to avoid supply modulation near the switching threshold. where Vlow is the low-voltage shutoff threshold. By proper choice of R1 and R2, this circuit will work well for shut-off voltages in the range of about 4. 5 to 10 V. The lower limit is determined by the pass FET`s threshold voltage and the upper limit is determined by IC1`s maximum supply-voltage rating. Higher cutoff voltages can be accommodated with minor circuit...

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