Voltage regulators (as the name implies) regulate a certain amount of input bias to a specified voltage level. A zener diode is the most basic of all voltage regulators, though it is characterized by a significant amount of voltage drop across it. If we want a low voltage drop across our regulator, low drop-out (LDO) voltage regulators are available. They may come in a variety of packages and even offer extra features (such as the TPS7333QP which offers an extra "reset" function to shut down its load (microcontroller) in the event of an undervoltage condition).

     One important characteristic to look at in voltage regulators is their line regulation. Line regulation (as the name implies again) is a measure of how well the line voltage is maintained with respect to a varied input bias. For example, if we feed a voltage regulator with 7 V, it yields a 5 V output. But when we feed it with 13 V, line voltage/output becomes 5.3 V. Thus we can say that our sample voltage regulator has bad line regulation (compared to standards) - that is 5.3 V - 5 V = 0.3 V (neglecting current).

Let us look at some voltage regulators and compare their line regulation using NI Multisim.

     Here we have the LM109H voltage regulator, swept from 0 to 17 V and given a load of 1 mA. Observing the curve, we find that the output stabilizes at 5 V given an input of 7 V. I want to characterize its line regulation at 7.5 and 15 V. Using the cursor function, we see that the difference is 1.5186 mV.

     Now I want to compare it with another voltage regulator that regulates at 5 V too, say the MC7805BT. We simulate it with the same working conditions as the LM109H.

     Once more, by the cursor function, we find that the line regulation between 7.5 V and 15 V is 2.9216 mV. Therefore, if we are strict with line regulation in our design, we reject the MC7805BT and choose the LM109H (of course, MC7805BT may have other characteristics that are more favorable than LM109H depending on the demand of the circuit).

     On the test bench, when measuring line regulation, we sometimes place bypass capacitors at the input and output to filter out AC or high frequency components (since a shunt capacitor acts as an LPF). Also, it is important to be cautious when using a sinking current source as a load to your regulator. At undervoltage condition, the curve may be erroneous since the load would constantly be pulling the same amount of current (which is equivalent to a lower resistance). Putting a delay on your measuring instrument may make things complicated, so it is more advisable to use a passive component (a resistor) or an electronic load (if one is available).