This is a simple analog circuit that can be used to indicate if a minimum voltage is present at Vcc. Minimum voltage is primarily determined by the zener voltage Dz.
Overview of how it works:
When Vcc < the zener breakdown voltage of Dz, the voltage drop across R1 and R2 are close to zero. This means the voltage at base of the PNP transistor Q1,B is ~Vcc. When VEB < VEB,on, the transistor does not conduct. Therefore, current through R3 = 0, and the voltage drop across R3, defined as EN, is 0V.
Once Vcc > VZ, current starts to flow through R1. Once there is sufficient current through R1 such that VR1 > VEB,on, Q1 begins to conduct. This causes a voltage drop across R3, and EN to measure a >0 voltage (up to Vcc – VCE,sat). R2 is in place to limit the current from the base of the transistor (as part of Q1’s conducted current will flow through the base from the emitter). The sensitivity of this circuit to the specific turn-on point and the Vcc/EN curve are strongly dependent on the values of R1 and R3, and somewhat dependent on the current knee of the zener diode.
Further notes and comments:
This circuit as presented optimizes cost and provide a high signal when Vcc is above the minimum. Variations on this circuit include
- a low signal at EN when Vcc is above minimum, using an NPN and flipping the circuit’s topology
- Using a P-channel MOSFET for Q1 would eliminate the need for R2
As a note, a single LM431 (available from many manufacturers) can also perform a function similar to the “low EN” variation of this circuit with only a pair of external setpoint resistors and a pull-up resistor. Which approach you choose depends on your application and need; the above could be advantageous for reason of cost, polarity (high EN when over Vcc,min using LM431 requires an inverting component), or voltage range (Maximum operating voltage of a LM431 is 37 volts, discrete bipolar transistors can operate in the hundreds of volts).