Shunt regulator monitors battery voltage

A TL431 shunt regulator is a perfect choice for many applications. You can use it as a comparator with hysteresis by taking advantage of its inner voltage reference along with few additional components. You can use this comparator with hysteresis, like a Schmitt trigger, as a simple battery monitor (Figure 1). You calculate the threshold voltage, V_T+, of this comparator as V_T+=V_REF×(1+R₁/R₃), where V_REF, the internal reference voltage of shunt-regulator TL431, is 2.5V.

When the battery voltage is higher than the threshold voltage, the cathode voltage of the TL431 is at its low level of approximately 2V, and transistor Q₁ turns on, lighting LED₁. You calculate the release voltage, V_T–, of the trigger as V_T–=V_REF×(1+R₁×R₂/(R₁+R₂) ×1/R₃).

When the battery voltage is less than the release voltage, the cathode voltage of the TL431 goes to its high level—to the battery voltage. Transistor Q₁ turns off, and LED₁ does not shine. LED₁ turns on again when the battery voltage, after recharging, exceeds the threshold voltage.

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